oscillator Related Articles

What is Hartley Oscillator Step-by-Step Build Guide

A basic Hartley oscillator is a type of electronic oscillator that produces a continuous sinusoidal wave. You often find a Hartley oscillator used in radio transmitters and receivers because it generates stable frequencies. Many electronic devices rely on what is Hartley oscillator circuits to create precise signals. You can see the Hartley design in frequency synthesizers, signal generators, and as a source for RF modulation. The Hartley circuit appears in wireless and radar systems, making it a popular choice for reliable electronic signal creation.What is Hartley OscillatorBasic Hartley OscillatorYou may wonder, what is Hartley oscillator and why do so many people use it in electronics? The basic Hartley oscillator is a simple electronic circuit that creates a repeating signal, usually a sine wave. Ralph Vinton Lyon Hartley invented this circuit in 1915 while working at Western Electric Company. He wanted to help with radio receiver development for the Bell System’s transatlantic radiotelephone tests. His invention became a key part of radio technology and is still important in RF circuit design today.The basic Hartley oscillator stands out because of its unique tank circuit. This tank circuit uses two inductors (or a tapped coil) and one capacitor. You connect the inductors in series and place the capacitor in parallel with them. The feedback comes from a tap on the inductor, which is different from other oscillator circuits like the Colpitts oscillator that uses capacitive feedback. This design makes the Hartley oscillator easy to tune and reliable for generating radio frequencies.Here are some features that make the basic Hartley oscillator special:Uses a tank circuit with two inductors and one capacitor.Feedback comes from the center tap of the coil.Generates stable sinusoidal signals at radio frequencies.Simple design and easy to adjust for different frequencies.How It WorksThe working of Hartley oscillator depends on its clever feedback system. When you power the circuit, current flows and charges the capacitor. The capacitor then discharges through the inductors, starting the oscillation. The signal from one part of the inductor goes to the amplifier, which could be a transistor. The amplifier boosts the signal and sends it back to the tank circuit.The working explanation of the Hartley oscillator involves two main phase shifts. The tank circuit gives a 180° phase shift, and the amplifier adds another 180°. This total 360° phase shift creates positive feedback, which keeps the oscillations going. You can control the frequency by changing the values of the inductors or the capacitor. The formula for the frequency is:f = 1 / (2π√(LC))where L is the total inductance and C is the capacitance. This simple formula helps you design oscillator circuits for many uses.Tip: The Hartley oscillator is popular because you can easily adjust its frequency and it works well in RF applications.Hartley Oscillator CircuitImage Source: pexelsComponents NeededTo build a hartley oscillator, you need a few basic electronic parts. You can find these components in most electronics stores. Here is a list of what you need for a typical hartley oscillator circuit:NPN Transistor (such as BC107, BC108, or AC127): These are small signal transistors that work well for beginners.Resistors:R1: 47 kΩR2: 10 kΩR3: 1 kΩInductors:L1: 10 μHL2: 100 μHYou can use a center-tapped coil or two separate inductors in series.Capacitors:C1 (Tuning Capacitor): 100 pF to 470 pF (variable for tuning)C2 (Coupling Capacitor): 10 nFC3 (Emitter Bypass Capacitor): 100 nFRadio Frequency Choke (RFC): 1 mHPower Supply: 6V DC battery or adapterBreadboard or PCB (for mounting components)Connecting WiresSoldering Kit (if using PCB)Tip: Use silicon NPN transistors for easy handling and reliable performance. These transistors are widely available and perfect for your first hartley oscillator circuit.You can adjust the values of the inductors and capacitors to change the frequency. The table below shows some common frequency ranges you can achieve with different component values:Frequency RangeInductor Values (L1, L2)Capacitor (C1)Notes500 kHz to 2 MHzL1 = 10 μH, L2 = 100 μHVariable C1Frequency depends on component valuesAround 750 kHzL1 = 1 μH, L2 = 10 μHCalculated C1Example frequency near 750 kHz5 MHzL ≈ 5.7 μH≈ 177 pFNarrow tuning ratio, higher frequencyBelow 20 kHzLarge inductors requiredLarge C1Not practical due to inductor sizeCircuit DiagramThe hartley oscillator circuit uses a simple layout. You connect the tank circuit (L1, L2, and C1) between the collector and base of the transistor. The RFC connects to the power supply and collector. The resistors set the bias for the transistor. The emitter resistor and bypass capacitor help stabilize the circuit.Here is a basic diagram of the hartley oscillator circuit: +6V | RFC | C (Collector) | L1 | L2 | GND | | C1 | GNDTransistor:- Base connects to the tap between L1 and L2 through a coupling capacitor (C2) and bias resistors (R1, R2).- Emitter connects to ground through R3 and bypass capacitor (C3).The NPN transistor sits in a common emitter setup.The tank circuit (L1, L2, C1) sets the frequency.The feedback comes from the tap between L1 and L2.The RFC keeps AC signals in the tank circuit and blocks DC.The output signal can be taken from the collector or across the tank circuit.Note: The layout of the hartley oscillator circuit is important. Keep the wires short and the components close together to reduce unwanted capacitance and inductance.Assembly StepsYou can build a hartley oscillator by following these step-by-step instructions:Prepare Your WorkspaceSet up a clean, dry area. Gather all your components and tools. Make sure your power supply is off before starting.Mount the ComponentsPlace the transistor, resistors, inductors, and capacitors on your breadboard or PCB. If you use a PCB, solder the parts carefully. For RF circuits like the hartley oscillator, avoid using solderless breadboards. They can add stray capacitance and inductance, which may cause the circuit to malfunction.Connect the Tank CircuitWire L1 and L2 in series. Connect one end to the collector and the other to ground. Attach C1 in parallel with L1 and L2. This forms the tank circuit that controls the frequency.Set Up the Biasing NetworkConnect R1 and R2 to the base of the transistor. These resistors set the operating point. Use a coupling capacitor (C2) between the tank circuit and the base.Add the RFC and Power SupplyConnect the RFC between the collector and the positive terminal of your 6V power supply. This helps keep the AC signal in the tank circuit.Complete the Emitter CircuitAttach R3 and C3 between the emitter and ground. This stabilizes the gain and bias.Check ConnectionsDouble-check all wiring. Make sure there are no loose connections or solder bridges.Power Up and TestTurn on the power supply. Use an oscilloscope or frequency counter to check for oscillation at the output.Safety Precautions:Always assume the circuit is live until you confirm it is safe.Never work alone if you use high voltage.Use only one hand when adjusting the circuit to avoid current passing through your heart.Never bypass fuses or safety interlocks.Discharge all capacitors before touching the circuit.Keep your workspace clean and free of flammable materials.Use the correct fire extinguisher for electrical fires.Stop and think before making changes to the circuit.Tip: For best results, use Manhattan-style or "ugly" construction on a copper-clad board. Solder components directly to the ground plane. This reduces stray capacitance and inductance, making your hartley oscillator circuit more stable.If you follow these steps, you will build a hartley oscillator that works well for learning and experimentation. The hartley oscillator circuit is a great project for beginners who want to understand how an oscillator circuit works.Designing a Hartley OscillatorImage Source: pexelsChoosing Inductors and CapacitorsWhen you start designing a hartley oscillator, you need to pick the right inductors and capacitors. The tank circuit, which includes two inductors and one capacitor, sets the frequency of your oscillator. You can change the frequency by adjusting these parts. For most hartley circuits, you use a single tuning capacitor in parallel with the inductors. This setup makes tuning easy and lets you pick the frequency you want.You should know that the values of the inductors and the tuning capacitor directly control the frequency. If you want to build a circuit for higher frequencies, you can use smaller inductors and capacitors. Lower frequencies need larger inductors, which can make your design bulky. Most hartley oscillator circuits work best between 20 kHz and 30 MHz. When you select components, look for stable parts with good quality to keep your frequency steady during tuning.Tip: Always use a variable tuning capacitor if you want to adjust the frequency after building your hartley oscillator.Calculating FrequencyYou can find the frequency of your hartley oscillator using a simple formula. The tank circuit’s total inductance and capacitance set the frequency. The formula is:f = 1 / (2π√(L_eq * C))Here, L_eq is the sum of both inductors and twice their mutual inductance. C is the value of your tuning capacitor. Let’s see how you can use this formula:Suppose you have C = 150 μF, L1 = 1.5 mH, L2 = 1.5 mH, and mutual inductance M = 0.746 mH.Add the inductors and mutual inductance: L_eq = L1 + L2 + 2M.Plug these values into the formula to get L_eq.Use the frequency formula to find the oscillation frequency.This step-by-step method helps you with tuning and makes your hartley design more accurate.Biasing and StabilityProper biasing is very important in any hartley oscillator. You need to set the transistor’s operating point so the circuit starts and keeps oscillating. The emitter resistor helps control the amplitude and keeps the output stable. If you do not set the bias correctly, your oscillator may not start or could produce unstable signals.To keep your hartley oscillator stable over time, you should also think about thermal stability. Here are some ways to improve it:Use stable components, like capacitors with low temperature drift.Place your oscillator in a shielded, stable enclosure.Keep the inductors grounded to reduce interference.Use separate power supplies or filters to avoid unwanted coupling.These steps help your hartley oscillator stay on frequency, even when the temperature changes or when you use it for a long time.Hartley Oscillator OperationFeedback MechanismThe feedback mechanism keeps your Hartley oscillator circuit running. When you power the circuit, the tank circuit starts to lose energy because of resistance. The feedback path sends a portion of the output back to the input. This feedback replaces the lost energy and keeps the oscillations going.Feedback must have the right amount and phase to work well.The feedback network uses the inductors and capacitor to send energy back at the right time.If you picture a swing, feedback is like giving it a push at just the right moment to keep it moving.The amplitude of the signal grows until it reaches a steady level. Nonlinear parts in the circuit or automatic gain control stop the amplitude from getting too high.The ratio of the inductors (L1 and L2) sets how much feedback you get. If you make L2 smaller, you increase the feedback. You must balance this carefully to avoid distortion or weak oscillations.Output TestingYou need to test your oscillator circuits to make sure they work as expected. An oscilloscope is the best tool for this job. Here is how you can test your Hartley oscillator:Set up your circuit and connect the oscilloscope probe to the output.Turn on the power supply.Look at the oscilloscope screen. You should see a smooth sine wave if the circuit is working.Measure the frequency and compare it to your calculated value using the formula f = 1 / (2π√(LtC)).Record the peak-to-peak voltage and frequency.A well-built Hartley oscillator circuit gives you a clean sine wave, usually between 500 kHz and 2 MHz. The output voltage can be over 10 volts peak-to-peak. The frequency depends on your inductor and capacitor values.Tip: If you do not have an oscilloscope, you can use a frequency counter or even a simple radio receiver to check for signals.TroubleshootingSometimes, oscillator circuits do not work right away. Here are common problems and how you can fix them:Frequency changes with temperature or environment.Output power is too low.The circuit is sensitive to small changes in component values.You see extra signals or harmonics.Tuning is hard or the circuit does not start.To diagnose these issues:Check all connections and make sure you used the right parts.Confirm the power supply voltage.Use an oscilloscope to look at the waveform and frequency.Look for sources of interference.If your oscillator does not start, try these steps:Make sure the feedback path is correct and the inductor tap is grounded.Adjust the values of L1 and L2 to change the feedback ratio.Check the transistor bias and emitter resistor.Replace any suspect components.Try a different transistor or op-amp if needed.You can use your Hartley oscillator in many projects. It works well in radio transmitters, receivers, alarms, and as a signal generator for testing. You can also use it for educational experiments, such as testing conductors and insulators or as a continuity tester.You have learned how to build a Hartley oscillator from start to finish. This project helps you understand feedback, frequency generation, and hands-on circuit assembly. As you grow your skills, try exploring other oscillator circuits like:Colpitts OscillatorWien Bridge OscillatorPhase Shift OscillatorMultivibrators555 Timer OscillatorEach new circuit will teach you more about electronics and signal generation. Building a Hartley oscillator gives you a strong foundation for future experiments.FAQWhat is the main use of a Hartley oscillator?You use a Hartley oscillator to generate radio frequency signals. Many people use it in radio transmitters, receivers, and signal generators. It helps you create stable and adjustable frequencies for different electronic projects.Can I build a Hartley oscillator without a breadboard?Yes, you can. You may use a printed circuit board (PCB) or even solder components directly onto a copper-clad board. This method often gives you better performance for high-frequency circuits.Why does my Hartley oscillator not start oscillating?Check your connections and component values. Make sure the feedback coil tap is correct. The transistor must have proper bias. If you still see no output, try changing the transistor or adjusting the inductor values.How do I change the frequency of my Hartley oscillator?You can change the frequency by adjusting the values of the inductors or the tuning capacitor in the tank circuit. Use a variable capacitor for easy tuning. Smaller inductors or capacitors give you higher frequencies.Is the Hartley oscillator safe for beginners?Yes! The Hartley oscillator uses low voltage and simple parts. You can build and test it safely if you follow basic safety rules. Always check your circuit before turning on the power.

Oscillators

A brief introduction of flicker noise

 Overview of flicker noiseFlicker noise in oscillatorsFlicker Noise in SemiconductorFlicker Noise in op AmpHow to eliminate the flicker noise in op AmpThe working mechanism of flicker noiseEquation of flicker noiseThermal Noise vs. Flicker NoisePros of the flicker noiseCons of flicker noiseApplications of flicker noiseFlicker Noise FAQ Overview of flicker noiseElectronic noise known as flicker noise or 1/f noise happens naturally in almost all electronic parts. It can also result from contaminants in conductive channels, creation and recombination noise inside transistors due to base current, and other factors. Pink noise or 1/f noise are common names for this noise. All electrical devices commonly experience this noise, which has a variety of origins but is typically correlated with direct current flow. It is important in a variety of electronic fields and is important for oscillators used as RF sources.Because the power spectral density of this noise increases with frequency, it is sometimes referred to as low-frequency noise. Below a few KHz, this noise is generally visible. The flicker noise bandwidth ranges from 10 MHz to 10 Hz.Figure 1: The relationship between noise voltage and frequency Flicker noise in oscillatorsFlicker noise is inversely proportional to frequency, or 1/f, and in many applications, such as RF oscillators, there are parts where flicker noise, or 1/f noise, dominates, and other regions where white noise from sources like shot noise and thermal noise, or both, dominate. Within the oscillator the flicker noise expresses itself as sidebands that are near to the carrier, the other kinds of noise stretching away from the carrier with a smoother spectrum, however fading the larger the offset from the carrier.As a result, there is a corner frequency, fc, between the regions where the various types of noise predominate. It is typically discovered that the noise outside of the region where flicker noise predominates is phase noise for a system like an oscillator. As the offset from the carrier increases, this decays until flat white noise takes over.MOSFETs have a greater fc (which can reach GHz levels) than JFETs or bipolar transistors, whose fc is typically below 2 kHz. When building RF oscillators, flicker noise, or 1/f noise, is a crucial type of noise. Although it is frequently disregarded, its influence can be reduced by selecting the right gadget.Figure 2: Flicker noise in ocillators Flicker Noise in SemiconductorThe nature of semiconductor noise and how it is specified in semiconductor devices are covered in the section that follows. Since the origin of each semiconductor noise source is a random process, the noise's instantaneous amplitude is unpredictable. The distribution of the amplitude is Gaussian (normal).Figure 3: Flicker Noise in SemiconductorRemember that the RMS value of noise (Vn) equals the standard deviation (σ) of the noise distribution. A random noise source's RMS and peak voltages have the following relationship: VnP-P = 6.6 VnRMS. The crest factor of any signal is the ratio of peak-to-peak to RMS voltage (VnP-P/VnRMS). Because a Gaussian noise source statistically delivers peak-to-peak voltages that are 6.6 times the RMS voltage or higher 0.10% of the time, the crest factor in Equation 1 is 6.6. The likelihood of surpassing 3.3s is 0.001 in this shaded area under the noise voltage density curve in Figure 2. It's crucial to keep in mind that while random signals (like noise) multiply geometrically in a root sum square (RSS) way, associated signals add linearly. Flicker Noise in op AmpSince flicker noise occurs in addition to the thermal noise present in carbon composition resistors, it is frequently referred to as excess noise there. In varied degrees, other resistor types also show flicker noise, with wire coiled having the least. The type of resistor used will not impact the noise in the circuit because flicker noise is proportional to the DC current in the device, thus if the current is kept low enough, thermal noise will predominate. Scaling up resistors to minimize power consumption in an op amp circuit may result in a reduction in 1/f noise at the expense of an increase in thermal noise. Below is the formula to calculate the flicker noise:Figure 4: Flick noise formulaWhere Ke and Ki are proportionality constants (volts or amps) representing En and In at 1 Hz. fMAX and fMIN are the minimum and maximum frequencies in hertz. How to eliminate the flicker noise in op AmpWhat is the best way to deal with this loud, low-frequency noise? With the limited bandwidth, it is almost impossible to try and filter out this noise without changing the important signal. There is yet some hope, though. Although an amplifier's inherent 1/f noise is beyond the control of a system designer, this noise source can be reduced by choosing the right amplifier for the job. The best option is a zero-drift amplifier if 1/f noise is a major problem. Figure 5: zero-drift op amp chartAny amplifier that uses a constantly self-correcting architecture is referred to as "zero-drift" in the industry, regardless of whether it uses an auto-zero topology, a chopper-stabilized topology, or a combination of the two. No matter the specific architecture used, the objective of zero-drift amplifiers is to reduce offset and offset drift. Other dc features, such common-mode and power supply rejection, are also significantly enhanced during the procedure. The fact that the 1/f noise is eliminated during the offset correction procedure is another significant advantage of these self-correcting designs. This noise source occurs at the input and is relatively slow moving, hence it looks to be a component of the amplifiers offset and gets adjusted accordingly.  The working mechanism of flicker noiseBy raising the overall noise level above the thermal noise level, which exists in all resistors, flicker noise is produced. In contrast, wire-wound resistors have the least amount of flicker noise. This noise is merely present in thick-film and carbon-composition resistors, where it is referred to as surplus noise. Charge carriers that are sporadically trapped and released between the interfaces of two materials may be the source of this noise. Because instrumentation amplifiers use semiconductors to record electrical signals, this phenomena is common in those materials.This noise is merely inversely proportional to the frequency. There are various areas in many applications, such as RF oscillators, where noise predominates, and other areas where white noise from sources like shot noise & thermal noise predominates. A correctly constructed system is typically dominated by this low-frequency noise. Equation of flicker noiseSimply put, nearly all electronic components produce flicker noise. In light of this, the noise is discussed in respect to semiconductor devices, notably MOSFET devices. The formula for this noise is S(f) = K/f. Thermal Noise vs. Flicker NoiseThermal NoiseFlicker NoiseIn order to use SAR data both quantitatively and qualitatively, thermal noise must be eliminated by normalizing the backscatter signal throughout the whole SAR image.Several methods, like ac excitation and chopping, can be used to reduce this noise.The lower parasitic resistance components will result in a reduction in the intensity of thermal noise.Wherever the offset voltage of the amplifier is reduced, this noise intensity will be reduced using a chopper or chopper stabilization approach.Anytime current passes through a resistor, thermal noise results.Semiconductors used in instrumentation amplifiers to record various electrical signals typically experience this noise.Johnson noise, Nyquist noise, and Johnson-Nyquist noise are further names for this sound.1/f noise is another name for this noise.Thermal noise is the noise caused by the equilibrium thermal agitation of the electrons in an electrical conductor.Flicker noise is the sound produced by randomly trapped and released charge carriers at the interfaces of two materials. Pros of the flicker noiseAs the noise is low frequency, it will become quieter if the frequency increases.It is an innate noise present in semiconductor devices that is caused by their physics and manufacturing process.The effects are typically seen in electrical components at low frequencies. Cons of flicker noisePerformance can be hampered by this noise in any precision DC signal chain.In all varieties of resistors, the overall noise level can be raised above the thermal noise level.It is frequency dependant. Applications of flicker noiseCertain passive devices and all active electronic components contain this noise.This phenomena typically happens in semiconductors, which are primarily used to store electrical signals in instrumentation amplifiers.The amplifying capabilities of the device are limited by this noise in BJTs.In resistors made of carbon, this noise is present.This noise typically appears in active gadgets because the charge conveys unpredictable behavior. Flicker Noise FAQFlicker noise is measured in what ways?Similar to other types of noise measurement, flicker noise in current or voltage can be measured. The sampling spectrum analyzer instrument extracts a discrete sample from the noise and uses the FFT method to produce the Fourier transform. Low frequencies are beyond the capability of these sensors to accurately measure this noise. Thus, sampling equipment is wideband and has a high noise level. They can reduce the noise by averaging many sample traces. Due to its narrow-band acquisition, conventional-type spectrum analyzer equipment nonetheless have a higher SNR. What should I do to stop the flickering noise?By a chopper stabilization technique that lowers the amplifier's offset voltage, this noise can be effectively eliminated. Flicker Noise: Why Is It Pink?Pink noise, which has a spectral power density reduction of 3 dB per octave, is also known as flicker noise. As a result, the frequency has an inverse relationship with the pink noise band power. Lower power is produced at higher frequencies. Why is flickering called pink noise?One of the most frequently seen signals in biological systems is pink noise. The term originates from the pink appearance of visible light with this power range. White noise, on the other hand, has an equal strength throughout all frequency ranges. How is flicker noise measured?Flicker noise is proportional to the inverse of the frequency, i.e. 1/f and in many applications such as within RF oscillators there are sections in which the flicker noise, 1/f noise dominates and other regions where the white noise from sources such as shot noise and thermal noise dominate.

Oscillators

What are Oscillator Types? Example with Diagrams

Introduction An oscillator is an electronic component used to generate an oscillating signal. The circuit composed of it is called an oscillating circuit, which can convert direct current into an electronic circuit or device with a certain frequency of alternating current signal. It is widely used in electronics industry, medical treatment, scientific research, etc. Catalog Introduction Ⅰ Oscillator Basics 1.1 Oscillator Meaning 1.2 Classification Rules Ⅱ Examples: RC Oscillator, LC Oscillator and Crystal Oscillator 2.1 RC Oscillator 2.2 LC Oscillator 2.3 Crystal Oscillator Ⅲ Selection Rules Ⅳ FAQ Ⅰ Oscillator Basics 1.1 Oscillator Meaning The oscillator is simply a frequency source and generally used in a phase-locked loop. In detail, it is a device that can convert DC power into AC power without external signal excitation. Generally divided into two types: positive feedback and negative resistance. The so-called oscillation, its meaning alludes to AC, and the oscillator includes a process and function starting from scratch. In other words, it can complete the conversion from DC power to AC power, such a device can be called an oscillator. 1.2 Classification Rules Oscillators are widely used, and there are many types:According to the oscillation frequency: high frequency oscillator, and low frequency oscillator.According to the oscillation waveform: sine wave oscillator, and non-sine wave oscillator.According to the oscillation feedback: positive feedback oscillator, and negative resistance oscillator.   Ⅱ Examples: RC Oscillator, LC Oscillator and Crystal Oscillator Electronic Oscillators || RC, LC, Crystal 2.1 RC Oscillator In a resistance-capacitance oscillator or short for RC oscillator, by using RC components in the feedback branch, a phase shift occurs between the input of the RC network and the output from the same network. The input is again moved through the second inverting stage, giving a phase shift, which is the same as providing the required positive feedback. It is suitable for low frequency oscillation, and is generally used to generate low frequency signals of 1Hz to 1MHz. The circuit is composed of four parts: amplifying circuit, frequency selection network, positive feedback network, and amplitude stabilization. The main advantages of it are simple structure, economic and convenient, and belong to the audio frequency oscillator. Figure 1. RC Oscillator Circuit (1) Vibration ProcessWhen the power is just turned on, there are various electrical disturbances in the circuit, and a relatively large feedback voltage is generated through feedback through the frequency selection network. Passing through the continuous loop of linear amplification and feedback, the oscillation voltage will continue to increase.(2) Oscillation FrequencyThe oscillation frequency is determined by the phase balance condition., Only meets the phase balance condition at f0, the oscillation frequency is .Changing R and C can change the oscillation frequency.(3) Conditions for Start-up and Stable OscillationTaking into account the starting conditions of AuF>1, generally Rt should be selected slightly larger than 2R1. If this value is too large, it will cause serious distortion of the oscillation waveform.The RC series-parallel sine-wave oscillator circuit composed of an op amp does not rely on the transistor inside the op amp to enter the nonlinear region to stabilize the amplitude, but to achieve the purpose of amplitude stabilization by introducing negative feedback from the outside.(4) Stable AmplitudeThe growth process of the oscillation amplitude cannot continue forever, when the amplifier gradually enters the saturation or cut-off zone from the amplification zone. Working in a non-linear state, its gain gradually decreases. When the amplifier gain decreases and the loop gain decreases to 1, the amplitude increase process will stop and the oscillator will reach equilibrium.For the RC oscillator circuit, increasing the resistance can reduce the oscillation frequency, and it does not need to increase the cost. The frequency of the sine wave generated by the commonly used LC oscillation circuit is relatively high. If a low frequency sine oscillation is to be generated, the oscillation circuit must have a larger inductance and capacitance. This will not only cause the components to be bulky, heavy, and inconvenient to install, but also difficult to manufacture with high cost. Therefore, the sinusoidal oscillation circuit below 200kHz generally adopts an RC oscillation circuit with a lower oscillation frequency.   2.2 LC Oscillator LC oscillator is also called LC oscillating circuit, resonance circuit, tank circuit or tuning circuit. It consists of a capacitor and a parallel coil. The circuit has an inductor L and a capacitor C. Capacitors store energy in the form of electrostatic fields and generate potential on their plates, while inductance coils store energy in the form of electromagnetic fields. By placing the switch in a specific position, the capacitor is charged to the DC supply voltage. When the capacitor is fully charged, the switch is switched to a certain position, and the charged capacitor is connected in parallel to the inductor coil, so the capacitor starts to discharge itself through the coil. Figure 2. LC Oscillator Circuit The LC circuit is not only used to generate a specific frequency signal, but also used to separate a specific frequency signal from a more complex signal. They are key components in many electronic equipment, especially radio equipment, used in oscillators, filters, tuners and mixer circuits.The inductive circuit is an idealized model because it assumes that there is no energy dissipated due to resistance. The actual realization of any LC circuit will include the loss caused by the small but non-zero resistance of the components and connecting wires. The purpose of an LC circuit is usually to minimize oscillations, so the resistance is made as small as possible. Although there is no lossless circuit in practice, studying the ideal form of this circuit is beneficial to study physical phenomenon.When electromagnetic oscillation occurs in an oscillating circuit, if there is no energy loss and no external influences, the period and frequency of it at this time are called the natural frequency and natural period of the oscillating circuit. The natural period can be obtained by the following formula: Where, the time constant is L/R. What are LC Oscillations? 2.3 Crystal Oscillator Some electronic devices require an AC signal with a highly stable frequency, but the LC oscillator has poor stability and the frequency is easy to drift (that is, the frequency of the generated AC signal is easy to change). A special component-quartz crystal is used in the oscillator, which can generate a highly stable signal. This kind of oscillator that uses a quartz crystal is called a crystal oscillator. It is mainly composed of a crystal and peripheral components. In a crystal oscillator, the main frequency determining element is a quartz crystal. Due to the inherent characteristics of the quartz crystal oscillator, it has extremely high frequency stability. Temperature compensation may be related to the crystal oscillator to improve the thermal stability. Because it is a fixed frequency oscillator, stability and accuracy are the basic considerations when use it. Figure 3. Crystal Oscillator Circuit The crystal oscillator has a piezoelectric effect, that is, the crystal will deform when a voltage is applied to the two poles of the wafer. Conversely, if an external force deforms the wafer, the metal sheets on the two poles will generate voltage. If an appropriate alternating voltage is applied to the chip, the chip will resonate (the resonance frequency is related to the tilt angle of the quartz slope, etc., and the frequency is constant). The crystal oscillator uses a crystal that can convert electrical energy and mechanical energy into each other. It can provide stable and accurate single-frequency oscillation when working in a resonance state. Under normal working conditions, the absolute accuracy of ordinary crystal oscillator frequencies can reach 50 parts per million. Using this feature, the crystal oscillator can provide a more stable pulse, which is widely used in the clock circuit of the microchip. In addition, the wafers are mostly quartz semiconductor materials, and the shell is encapsulated with metal.The main parameters of the crystal oscillator include nominal frequency, load capacitance, frequency accuracy, frequency stability, etc. These parameters determine the quality and performance of the crystal oscillator. Therefore, in practical applications, an appropriate crystal oscillator should be selected according to specific requirements. For example, systems such as communication networks and wireless data transmission require high-precision crystal oscillators. However, since the higher the performance of the crystal oscillator is, the more expensive it is, so you can choose a crystal that meets the requirements when buying.   Ⅲ Selection Rules Oscillators are used in many electronic products. In order to ensure the normal operation of electronic products, the selection of oscillators is important. The following summarizes the five selection rules for reference.1) Appearance InspectionBy checking the appearance of the product, whether the marking text is clear and standard, whether there are cracks on the surface of the appearance, and whether the pins have been soldered. If the product is found to be imperfect on the outside, it should not be used.2) FrequencyChoose the appropriate frequency according to the actual product requirements. The frequency is the most important, and it cannot be replaced casually. Negotiations must be conducted after passing the qualification verification or professional test. If the frequency required by the actual circuit is 5MHZ, do not replace it with a similar frequency without any original replacement.3) Output ModeWhen choosing a oscillator, consider the type of oscillator output required by the circuit, which generally divided into level output and differential output. As for level output, CMOS is the most commonly used type, and in terms of differential output, LVPECL(Low Voltage Positive Emitter-Couple Logic) and LVDS (Low-Voltage Differential Signaling) are commonly used differential output type. Different output types cannot be changed randomly, especially differential and ordinary oscillators.4) ModelTo use the oscillator, you must see the model mark of the shell. The model number indicates its multiple parameters. According to the product requirements, the corresponding product parameters can be found, and the same model can be found later. If the crystal oscillator model is not selected properly, it will cause errors in the application.5) ReplacementIf an oscillator is damaged, it should be replaced by the original model in principle. When the original model is not available, it is best to consider replacing it with another model or other type of oscillator after testing.   Ⅳ FAQ 1. What is oscillator and its types?An oscillator is a type of circuit that controls the repetitive discharge of a signal, and there are two main types of oscillator; a relaxation, or an harmonic oscillator. This signal is often used in devices that require a measured, continual motion that can be used for some other purpose. 2. What are the types of oscillator in electronics?There are two main types of electronic oscillator – the linear or harmonic oscillator and the nonlinear or relaxation oscillator. 3. How many types of oscillations are there?There are 3 main types of Oscillation – Free, damped, and forced oscillation. When a body vibrates with its own frequency, it is called a free oscillation. 4. What is RC and LC oscillator?The oscillation frequency is proportional to the inverse of the capacitance or resistance, whereas in an LC oscillator the frequency is proportional to inverse square root of the capacitance or inductance. So a much wider frequency range can be covered by a given variable capacitor in an RC oscillator. 5. What is the principle of oscillator?There are many types of electronic oscillators, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback. 6. What are the three types of oscillator?The main types of Oscillators include: Wien Bridge Oscillator. RC Phase Shift Oscillator. Hartley Oscillator. 7. What is the use of LC oscillator?LC oscillators are used in heating with high-frequency, RF generators, radios, TV receivers, etc. These types of oscillators use tank circuits including the components like a capacitor (C) and an inductor (L). 8. What does RC oscillator do?RC oscillators are a type of feedback oscillator; they consist of an amplifying device, a transistor, vacuum tube, or op-amp, with some of its output energy fed back into its input through a network of resistors and capacitors, an RC network, to achieve positive feedback, causing it to generate an oscillating. 9. What is RC phase oscillator?RC phase-shift oscillators use resistor-capacitor (RC) network to provide the phase-shift required by the feedback signal. They have excellent frequency stability and can yield a pure sine wave for a wide range of loads. ... Further, the circuit also shows three RC networks employed in the feedback path. 10. What are the advantages of RC oscillator?The RC phase shift oscillator gives good Frequency stability. The output of this circuit is sinusoidal that is quite distortion free.. It is suitable for lower frequencies and this lower limit exists in as low as 1Hz. RC phase shift oscillators don't require any negative feedback and stabilization arrangements. 11. What is a crystal oscillator used for?A crystal oscillator is an electronic oscillator circuit that is used for the mechanical resonance of a vibrating crystal of piezoelectric material. It will create an electrical signal with a given frequency. 12. What are the advantages of crystal oscillator?The Advantages of a Crystal OscillatorStability. Stability is one of the most important requirements of any oscillator.High Q. The Q factor or quality factor describes how 'underdamped' oscillators are.Frequency Customization and Range.Low Phase Noise.A Crystal Oscillator Is Compact and Inexpensive. 13. What is crystal oscillator explain?A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a constant frequency. ... Quartz crystals are manufactured for frequencies from a few tens of kilohertz to hundreds of megahertz.

Amplifiers

Operational Amplifier Oscillation Analysis with Circuits

Introduction Operational amplifiers will oscillate in many practical applications. For example, there are many kinds of loads that will cause them to oscillate. A feedback network that is not properly designed can cause them to become unstable. Insufficient power supply bypass capacitors may also make them unstable. Even the input and output may oscillate into a single-port system. This article will tell some common causes that cause the op amp to oscillate and the corresponding countermeasures. Catalog Introduction Ⅰ Basic Op Amp Circuits Ⅱ Example: LTC6268 Amplifier Ⅲ Decompensated Amplifiers Ⅳ Feedback Network Ⅴ Load Problem Ⅵ Strange Impedance Ⅶ Power Ⅷ Conclusion Ⅸ FAQ Ⅰ Basic Op Amp Circuits Figure 1. shows a block diagram of a non-rail-to-rail amplifier. The input controls the gm box, which drives the gain node and is buffered at the output. The compensation capacitor Cc is the main frequency response component. The return pin of Cc should be grounded, if there is such a pin and the op amp is not grounded, the capacitor current will return to one or two power supplies. Figure 1. Block Diagram of a Non-Rail-to-Rail Amplifier Figure 2. is a block diagram of a rail-to-rail output amplifier. The output current of the input box gm is sent through a current coupler, which divides the current into two parts and supplies them to the output transistor. The frequency response is determined by two Cc/2s, which are actually connected in parallel. Figure 2. Block Diagram of a Rail-to-Rail Output Amplifier Figure 3. shows the frequency response of the ideal amplifier. Although the electrical principles of the two circuits are different, the behavior is similar. The single pole compensation formed by gm and Cc provides a unity gain bandwidth product frequency of GBF = gm/(2πCc). In the vicinity of GBF/Avol, the phase lag of these amplifiers changes from -180° to -270°, where Avol is the open-loop DC gain of the amplifier. When the frequency is much higher than this low frequency, the phase stays at –270°. This is the well-known "dominant pole compensation", where the Cc dominates the frequency response, hiding the various frequency limitations of the active circuit. Figure 3. Frequency Response of the Ideal Amplifier   Ⅱ Example: LTC6268 Amplifier Figure 4. shows the open-loop gain and phase response of the LTC6268 amplifier with frequency. The LTC6268 is a small and low-noise 500MHz amplifier with rail-to-rail output and only 3fA bias current. It can be used as a good example to illustrate the performance of real amplifiers. The -90° phase lag of the dominant pole compensation starts from about 0.1MHz, reaches -270° around 8MHz, and moves down by more than -270° when it exceeds 30MHz. In fact, all amplifiers have high frequency phase lag, except for the basic dominant compensation lag caused by the additional gain stage and output stage. Generally, the starting point of the additional phase lag is around GBF/10. Figure 4. Open-Loop Gain and Phase Response of the LTC6268 Amplifier with Frequency The stability of the feedback is a matter of loop gain and phase, or Avol multiplied by the feedback coefficient, which is the loop gain. If we connect the LTC6268 in a unity gain configuration, 100% of the output voltage is fed back. At very low frequencies, the output is the negative value of the "–" input, or the phase lags by -180°. Compensation adds a -90° hysteresis through the amplifier, introducing a –270° hysteresis from the "–" input to the output. When the loop phase lag increases to ±360° or its multiples, oscillation will occur, and the loop gain is at least 1V/V or 0dB. The phase margin is a measure of how much the phase lag differs from 360° when the gain is 1V/V or 0dB. Figure 4. shows that the phase margin is about 70° (10pF red curve) at 130MHz, and the phase margin as low as about 35° is feasible.A topic that is not often mentioned is gain margin, although it is an equally important parameter. When it is reduced to zero at some higher frequencies, the amplifier will oscillate if the gain is at least 1V/V or 0dB. As shown in Figure 4, when the phase drops to 0° (or a multiple of 360°, or –180° as shown in the figure), the gain is about –24dB around 1GHz. This is a very low gain and no oscillations will occur at this frequency. In fact, people want the gain margin to be at least 4dB.   Ⅲ Decompensated Amplifiers Although the LTC6268 is fairly stable at unity gain, there are still unstable op amps. By designing the amplifier compensation to be stable only at higher closed-loop gains, the design trade-off can provide a higher conversion rate, wider GBF, and lower input noise than the unity gain compensation scheme. Figure 5. shows the open loop gain and phase of the LTC6230-10. The amplifier is intended to be used with a feedback gain of 10 or greater, so the feedback network will attenuate the output by at least 10 times. Through this feedback network, you can find the frequency when the open-loop gain is 10V/V or 20dB, and find that the phase margin is 58° at 50MHz (±5V power supply). At unity gain, the phase margin is only about 0°, so the amplifier oscillates. Figure 5. LT6230-10 Gain and Phase Change with Frequency It is observed that when the closed-loop gain is higher than the minimum stable gain, all amplifiers will be more stable. Even a gain of 1.5 will make a unity gain stable amplifier much more stable.   Ⅳ Feedback Network The feedback network itself may also cause oscillations. In Figure 6, put a parasitic capacitor in parallel with the feedback divider resistor. It is inevitable that each terminal of each component on the circuit board has a capacitance of about 0.5pF to the ground, and there is also a wiring capacitance. Figure 6. Parasitic Capacitance In fact, the minimum capacitance of the node is 2pF, and there is about 2pF of wiring capacitance per inch of trace. The accumulated parasitic capacitance can easily reach 5pF. Using LTC6268, in order to reduce the power, we set the values of Rf and Rg to a very high 10kΩ. When Cpar = 4pF, the feedback network has a pole at 1/(2π*Rf||Rg*Cpar) or 8MHz. The phase lag of the feedback network is -atan(f/8MHz), we can estimate that the loop will have a phase lag of 360° around 35MHz. At this time, the phase lag of the amplifier is -261°, and the feedback network lags about -79°. At this phase and frequency, the amplifier still has a gain of 22dB, and the gain of the voltage divider is .At the 0° phase, the amplifier's 22dB multiplied by the feedback divider's –19dB produces a +3dB loop gain, and the circuit oscillates. In order to operate normally in the presence of parasitic capacitance, we must reduce the value of the feedback resistor so that the feedback pole can far exceed the unity gain frequency of the loop. That is, the ratio of the pole to the GBF should be at least 6 times.The input end of the op amp itself may also have a considerable capacitance, the same as Cpar. In particular, low noise and low Vos amplifiers have large input transistors and may have larger input capacitance than other types of amplifiers, and the input capacitance is loaded on the amplifier's feedback network. We need to consult the data sheet to understand how much capacitance will be connected in parallel with Cpar. Fortunately, the LT6268 has only 0.45pF capacitance, which is already very low for such a low noise amplifier. The macro model running on LTspice® provided free of charge by ADI can be used to simulate a circuit with parasitic capacitance. Figure 7. shows how to improve the capacitor tolerance of the voltage divider. Figure 7(a) shows a non-negative output amplifier configuration with Rin. Assuming that Vin is a low impedance source (<Rin), Rin will effectively attenuate the feedback signal without changing the closed-loop gain. And it will also reduce the impedance of the voltage divider and increase the feedback pole frequency, which is expected to far exceed GBF. In addition, Rin reduces the bandwidth around the loop and amplifies the input offset and noise.Figure 7(b) shows a negative output configuration. Rg still performs loop attenuation without changing the closed loop gain. In this case, the input impedance is not affected by Rg, but the noise, offset and bandwidth parameters will deteriorate.Figure 7(c) shows the preferred method of compensating Cpar in a non-inverting amplifier. If we set Cf* Rf = Cpar * Rg, then we have a "compensation attenuator", so that the feedback divider now has the same attenuation at all frequencies and solves the Cpar problem. The mismatch in the product will cause "bumps" in the passband of the amplifier and "shelf" in the response curve (At this time, the low-frequency response is flat, but becomes straight near f = 1/2 * Cpar * Rg.).Figure 7(d) shows the equivalent Cpar compensation for the negative output amplifier. The frequency response must be analyzed to find a correct Cf, and the bandwidth of the amplifier is part of the analysis.Here are some comments on current feedback amplifiers (CFA) in turn. If the amplifier in Figure 7(a) is a CFA, then "Rin" has little effect on changing the frequency response, because the negative input is very low impedance and actively copies the positive input. The noise index will degrade slightly, and the additional negative input bias current will actually appear in the form of Vos/Rin. Similarly, in terms of frequency response, the circuit in Figure (b) is not changed by "Rg". The inverting input is not just a virtual ground, it is a real ground with low impedance, and Cpar has been tolerated (only in negative output mode). The DC error is similar to the situation shown in (a), (c) and (d) may be the preferred solution for voltage input op amps, but CFA can't tolerate a direct feedback capacitor without oscillation at all.   Ⅴ Load Problem Just as the feedback capacitor can damage the phase margin, the load capacitor can do the same. Figure 8 shows the change in LTC6268 output impedance with frequency in the case of several gain settings. Note that the unity gain output impedance is lower than the output impedance at higher gains. Full feedback enables the open-loop gain to reduce the inherent output impedance of the amplifier. Therefore, in Figure 8, the output impedance at a gain of 10 is generally 10 times the output impedance at unity gain. Since the feedback attenuator reduces the loop gain, the gain around the loop is 1/10, otherwise it will reduce the closed-loop output impedance. The open-loop output impedance is about 30, which is obvious in the high-frequency flat region of the curve with a gain of 100. In this area, from around gain bandwidth frequency (about 100) to gain bandwidth frequency, there is not enough loop gain to reduce the open loop output impedance. Figure 8. Impedance and Frequency of LTC6268 Under Three Gain Conditions The capacitor load will cause the phase lag and amplitude attenuation of the open-loop output impedance. For example, a 50pF load and our LTC6268 output impedance form another pole at 106MHz, where the output has a –45° phase lag and –3dB attenuation. At this frequency, the amplifier has a phase of -295° and a gain of 10dB. Assuming unity gain feedback is used, we have not fully realized the oscillation because the phase is not brought to ±360° (at 106MHz). However, at 150MHz, the amplifier has 305° phase lag and 5dB gain. The phase of the output pole is –atan(150MHz/106MHz) = -55°, and the gain is .Multiplying the gain cyclically, we get a 360° phase and +0.2dB gain, which is another oscillator. 50pF seems to be the minimum load capacitance that will force the LTC6268 to oscillate.The most common way to prevent oscillations caused by the load capacitor is to simply connect a small resistor in series to the capacitor after the feedback connection. The resistance value of 10Ω to 50Ω will limit the phase lag that may be caused by the capacitive load and isolate the amplifier and low capacitive impedance when the speed is very high. Disadvantages include DC and low frequency errors that vary with load resistance characteristics, capacitive load frequency response is limited, and signal distortion caused if the load capacitance is not constant when the voltage changes.Increasing the closed-loop gain of the amplifier can often prevent the oscillation caused by the load capacitance. Operating the amplifier with a higher closed-loop gain means that at frequencies where the loop phase is ±360°, the feedback attenuator also attenuates the loop gain. For example, if we use the LTC6268, its closed-loop gain is +10, then we will see that the amplifier has a gain of 10V/V or 20dB at 40MHz and a phase lag of 285°. To ignite the oscillation, an output pole is required, causing an additional 75° hysteresis. By -75° =-atan(40MHz/Fpole) →Fpole =10.6MHz, we can find the output pole. This pole frequency comes from a load capacitance of 500pF and an output impedance of 30Ω. The output pole gain is .When the unloaded open-loop gain is 10, the loop gain at the oscillation frequency point is 0.26, so there is no oscillation this time, at least no oscillation caused by the simple output pole. In this way, we increased the tolerable load capacitance from 50pF to 500pF by increasing the closed-loop gain.In addition, unterminated transmission lines are also very bad loads because they will cause "runaway" impedance and phase changes that repeat with frequency (See the impedance of an unterminated 9-foot cable in Figure 9).If your amplifier can safely drive the cable under certain low-frequency resonance conditions, it is likely to oscillate at a higher frequency because its own phase margin is reduced. If the cable must be unterminated, a "back-match" resistor in series with the output can isolate the cable's extreme impedance changes. In addition, even if the transient reflection from the this end of the cable just recoils back to the amplifier, if the resistance of the backward matching resistor matches the characteristic impedance of the cable, the resistor can properly absorb this energy. If the backward resistor does not match the cable impedance, some energy will be reflected from the amplifier and terminals, and back to the unterminated end. When the energy reaches this end, it is quickly reflected back to the amplifier. As a result, there is a series of pulses bouncing back and forth, but attenuate each time. Figure 9. Impedance and Phase of the Unterminated Coaxial Cable Figure 9 shows a more complete output impedance model. The ROUT is the same as what we discussed in the LTC6268, and it is also 30Ω, in addition, add the Lout item. This is a combination of physical inductance and electronic equivalent inductance. The physical package, bonding wire, and external inductance add up to 5nH to 15nH. The smaller the package, the smaller the total value. Figure 10. Inductive Component of Amplifier Output Impedance In addition, any amplifier has an electrical inductance of 20nH to 70nH, especially bipolar devices. The finite Ft of the device turns the parasitic base resistance of the output transistor into an inductance. The harm is that Lout and CL may interact to form a series resonant circuit, then the same problem comes again. If there is no greater phase lag in the loop, the impedance of the series resonant circuit may drop to a level that Rout cannot drive. This may cause oscillations. For example, set Lout = 60nH and CL = 50pF. Resonant frequency is .Just within the passband of the LTC6268. In fact, this series resonant circuit is loaded to the output terminal during resonance, which changes the phase of the loop greatly near the resonant frequency. Unfortunately, Lout is not mentioned in the amplifier's data sheet, but its effect can sometimes be seen on the open-loop output impedance circuit. In short, for amplifiers with a bandwidth of less than 50MHz, this effect is not important.One solution is shown in Figure 10. Rsnub and Csnub form a so-called "shock absorber" whose purpose is to reduce the Q value of the resonant circuit so that the resonant circuit does not have a very low resonant impedance to the output of the amplifier. The value of Rsnub is usually estimated as the reactance of CL to reduce the Q value of the output resonance circuit to about 1. Adjust the size of Csnub to fully insert Rsnub into the output resonance frequency, that is, the reactance of Csnub <Cl. Csnub = 10 * CL is practical. Csnub unloads the amplifier at intermediate and low frequencies, especially at DC. If it is very large, Rsnub will put a heavy load on the amplifier at intermediate frequency, which will affect the low frequency, gain accuracy, closed-loop bandwidth and distortion. However, after a little fine-tuning, shock absorbers are often useful for controlling reactive loads, but shock absorbers must be adjusted through experiments. Figure 11: Using an Output Shock Absorber The negative input of the current feedback amplifier is actually a buffer output and will also have the series characteristics shown in Figure 8. Therefore, it may oscillate under the action of Cpar, just like the output terminal. You should try to reduce Cpar and any related inductance. Unfortunately, the damper on the negative input terminal modifies the relationship between closed-loop gain and frequency, so it is not very useful.   Ⅵ Strange Impedance Many amplifiers have an abnormal input impedance at high frequencies. This is most true for amplifiers with two input transistors in series, such as the Darlington configuration. Many amplifiers have PNP/NPN transistor pairs at the input, and their behavior changes with frequency similar to the Darlington configuration. The real part of the input impedance will become negative at some frequencies (generally much higher than GBF). Inductive source impedance will resonate with the input and circuit board capacitance, and negative real components may provoke oscillations. When driving with unterminated cables, this can also cause oscillations at many repetition frequencies. If it is inevitable to use a long inductive wire at the input, you can disconnect the wire with several series-connected resistors that can absorb energy, or install a medium-impedance shock absorber (about 300Ω) on the input lead of the amplifier.   Ⅶ Power The last source of oscillation to consider is power supply bypass. Figure 10 shows part of the output circuit. LVS+ and LVS– are the unavoidable packaging, IC bond wires, the physical length of the bypass capacitor (inductive like any conductor), and the series inductance of the circuit board traces. It also includes the external inductance that connects the local bypass component to the rest of the power bus (if not the power plane). Although 3nH to 10nH may seem small, at 200MHz, it is 3.8 to 12Ω. If the output transistor conducts a large high-frequency output current, there will be a voltage drop across the power inductor. Figure 12. Power Supply Bypass Capacitor Details The rest of the amplifier needs a noise-free power supply, because these parts cannot suppress power supply noise as the frequency changes. In Figure 13 we can see the power supply rejection ratio (PSRR) of the LTC6268 with frequency. In all operational amplifiers, because there is no ground pin, the compensation capacitor is connected to the power supply, which will couple power supply noise into the amplifier, and gm must cancel this noise. Due to the compensation, PSRR decreases with 1/f, in addition, the power supply rejection actually increases after 130MHz. Figure 13. LTC6268 Power Supply Rejection with Frequency Variation At 200MHz, due to the increase of PSRR, the output current may interfere with the power supply voltage inside the LVs inductor. Through the amplification of PSRR, the interference becomes a strong amplifier signal, driving the output current, generating internal power signals, etc., causing the amplifier to oscillate. This is why the power supplies of all amplifiers must be carefully bypassed with traces and components with very small inductance. In addition, the power supply bypass capacitor must be much larger than any load capacitor.If consider the frequency around 500MHz, then the range 3nH to 10nH becomes 9.4Ω to 31.4Ω. This is enough for the output transistor to generate self-oscillation by its inductance and IC component capacitance, especially when the output current is large (transistor gm and bandwidth increase). Because the bandwidth of transistors is very large, special attention needs to be paid, especially at high output currents.   Ⅷ Conclusion In short, the designer needs to consider the parasitic capacitance and inductance associated with each op amp terminal and the natural characteristics of the load. Usually the designed amplifier is very stable in the nominal environment, but each application needs to analyze it by itself.   Ⅸ FAQ 1. Does your op amp oscillate?Well, it shouldn't. We analog designers take great pains to make our amplifiers stable when we design them, but there are many situations that cause them to oscillate in the real world. ... Improperly designed feedback networks can cause instability. Insufficient supply bypassing can offend. 2. What is oscillator in op amp?An oscillator is an electronic circuit that produces a periodic signal. ... The feedback network takes a part of the output of amplifier as an input to it and produces a voltage signal. This voltage signal is applied as an input to the amplifier. 3. What causes an amplifier to oscillate?Causes of parasitic oscillationParasitic oscillation in an amplifier stage occurs when part of the output energy is coupled into the input, with the correct phase and amplitude to provide positive feedback at some frequency. ... Similarly, impedance in the power supply can couple input to output and cause oscillation. 4. How do you compensate an op amp?Another effective compensation technique is the miller compensation technique and it is an in-loop compensation technique where a simple capacitor is used with or without load isolation resistor (Nulling resistor). That means a capacitor is connected in the feedback loop to compensate the op-amp frequency response. 5. How can an op amp improve stability?To ensure stability, the value of RX should be such that the added zero (fZ) is at least a decade below the closed loop bandwidth of the op amp circuit. With the addition of RX,circuit performance will not suffer the increased output noise of the first method, but the output impedance as seen by the load will increase. 6. What are the requirements of oscillations in an amplifier?Oscillations around the 3dB bandwidth of the amplifier are usually due to input/output feedback. Higher frequency oscillations may only be visible on a spectrum analyzer. They may cause waveform distortion and be affected by touching the amplifier on power and signal cables. 7. How do you stop an oscillating op amp?If the op-amp still oscillates, try these things, in this order:1) Add a small resistor to the op-amp's output, either inside or outside the feedback loop. ...2) Do the same as in the previous step, except use a ferrite bead or chip ferrite instead of the resistor. ...3) Raise the amp's gain a bit. 8. How do you increase the gain margin of an op amp?You can increase the phase margin by making a dominant pole nearer to the zero frequency origin. This is accomplished by compensating the op amp through adding a shunting capacitor in the highest impedance node of the amplifier. This is a very well known technique which is used commonly to increase the phase margin. 9. Why the gain of op amp deteriorate with frequency?All opamps have a limit on upper frequency. In a LPF, at low frequencies, the output amplitude is equal to input. But as the frequency increases, the capacitive reactance decreases and the output amplitude starts to decrease. 10. What is used to avoid or minimize instability in amplifiers?It is often desirable to use capacitance to ground from an amplifier's active input terminals to reduce high-frequency interference, RFI and EMI. This filter capacitor has a similar effect on op amp dynamics as increased stray capacitance. 11. Why op amps oscillate an intuitive look at two frequent causes?With delay in the loop, the amplifier does not immediately detect its progress toward the final value. ... It overreacts by racing too quickly toward the proper output voltage. Note the faster initial ramp rate with delayed feedback. 12. How does an op-amp oscillator work?The Op-amp Multivibrator is an astable oscillator circuit that generates a rectangular output waveform using an RC timing network connected to the inverting input of the operational amplifier and a voltage divider network connected to the other non-inverting input.

Oscillators

Oscillator Basics with 5 Circuit Examples

Ⅰ IntroductionOscillators are the heartbeat of modern electronics. From the quartz watch on your wrist to the 5G smartphone in your pocket, these components play a critical role in generating timekeeping signals and carrier waves. While early applications included simple AM radios and metal detectors, today's oscillators are foundational to IoT devices, advanced computing, and high-speed data transmission.To understand how electronic oscillators function in 2025, it helps to look at physical analogies and fundamental circuit designs. This guide covers the core concepts, modern classifications, and practical examples of oscillators in electronics.Ⅱ What is an Oscillator?An oscillator is an electronic circuit that converts direct current (DC) from a power supply into an alternating current (AC) signal—typically a sine wave, square wave, or triangle wave. They are ubiquitous in technology, found in everything from microcontrollers and music synthesizers to GPS receivers.Every oscillator contains at least one active device (such as a transistor or Op-Amp) that acts as an amplifier. The core operating principle relies on a feedback loop: an oscillator employs a sensitive amplifier where a portion of the output signal is fed back into the input in phase. This process, known as positive feedback, allows the signal to regenerate and sustain itself indefinitely, provided there is a power source.Ⅲ The Working Principle of an OscillatorFor an oscillator to sustain a frequency, energy must oscillate between two forms. The simplest way to visualize this is through a Tank Circuit, created by connecting a capacitor and an inductor in parallel.The Energy Cycle:Storage: Capacitors store energy in an electrostatic field, while inductors store energy in a magnetic field.Discharge: When a charged capacitor discharges through an inductor, the current creates a magnetic field around the inductor coil.Collapse & Recharge: As the capacitor fully discharges, the inductor's magnetic field collapses, inducing a current that recharges the capacitor (with opposite polarity).Oscillation: This back-and-forth transfer of energy creates an oscillation. In a perfect world, this would continue forever. In reality, internal resistance dissipates energy (damping), so an active component (amplifier) is required to inject energy and keep the oscillation going.Ⅳ Types of Oscillators4.1 General ClassificationWhile there are countless variations, oscillators generally fall into two primary categories:Harmonic (Linear) Oscillators: Energy flows from active to passive components to generate a purely sinusoidal waveform. The frequency is determined by a feedback path. These are crucial for radio frequencies (RF) and audio applications.Relaxation Oscillators: These operate by exchanging energy between active and passive components through charging and discharging phases. They produce non-sinusoidal shapes like square, saw-tooth, or triangular waves, commonly used in digital timing and signal processing.4.2 The 5 Basic TypesRC and LC Oscillators: Basic circuits using resistors/capacitors or inductors/capacitors to determine frequency.Crystal Oscillators: Use vibrating quartz crystals (and increasingly MEMS technology) for high-precision stability.Sinewave Oscillators: Circuits optimized to produce low-distortion sine outputs (e.g., Wien Bridge).Square Wave Oscillators: Circuits like the Multivibrator or 555 Timer used for clock pulses.Voltage Controlled Oscillators (VCO): The frequency output can be tuned by varying the input voltage.Ⅴ Details and Circuit Examples5.1 LC OscillatorsLC oscillators combine inductors and capacitors (a tank circuit) to generate high-frequency sine waves. They are preferred in Radio Frequency (RF) applications because they offer good phase noise performance and are easy to tune. In 2025, advanced LC tank circuits are still relevant in communication hardware, though they are often integrated into silicon chips.Figure 1: Basic LC Oscillator ConfigurationExample: Gated LC Phase Shift OscillatorThis circuit allows the oscillation to be turned on or off via a logic input. When the input is high (e.g., 5V), the oscillator runs; when grounded, it stops. This "burst" mode capability is useful in digital communication protocols.Figure 2: Gated LC Phase Shift Oscillator5.2 RC (or CR) OscillatorsAt low frequencies (like the audio range of 20Hz - 20kHz), inductors become large and impractical. Engineers solve this by using Resistors and Capacitors (RC) to set the frequency. While creating a pure sine wave with RC circuits is challenging, they are cost-effective and compact for audio signal generation.Figure 3: Basic RC OscillatorExample: CMOS 555 Timer & Schmitt TriggerEven decades after its invention, the 555 timer remains a staple in electronics. The modern CMOS versions consume less power and offer cleaner switching, making them ideal for battery-operated IoT sensor polling.Figure 4: 555 Timer based RC Oscillator5.3 Crystal OscillatorsCrystal oscillators utilize the piezoelectric effect of quartz to generate a frequency with immense stability. They act as the "heartbeat" for microprocessors. In recent years, MEMS (Micro-Electro-Mechanical Systems) oscillators have begun to replace quartz in some high-vibration environments, but quartz remains the standard for precision.Figure 5: Crystal Oscillator SchematicFor High-Frequency (HF) applications, a transistor like the 2N2222A (or modern surface-mount equivalents) is typically used. The tuned circuit matches the impedance, often loading at nominally 50 ohms. Modern designs frequently include a buffer amplifier stage to prevent the load from pulling the crystal off-frequency.5.4 Sinewave OscillatorsThe Wien Bridge Oscillator is a specific type of RC oscillator capable of generating very low-distortion sine waves. It is famous for being the first product designed by Hewlett-Packard (HP).Figure 6: Practical Wien Bridge Oscillator using a light bulb for stabilizationHistorical Note: The schematic above uses an incandescent light bulb for gain stabilization. As the bulb heats up, its resistance increases, stabilizing the feedback loop. In modern 2025 circuitry, this bulb is typically replaced by JFETs or automatic gain control (AGC) ICs for higher reliability and lower power consumption, though the bulb method remains an excellent educational example of negative feedback.5.5 Square Wave OscillatorsAlso known as Astable Multivibrators, these generate a digital on/off signal without external input. They are fundamental to digital logic clocks and PWM (Pulse Width Modulation) controllers.Figure 7: Multi-frequency Square Wave Generator using 555 Timer5.6 Voltage Controlled Oscillator (VCO)A VCO allows the frequency to be tuned dynamically by changing a control voltage. This is the core component of Phase Locked Loops (PLLs) used in Wi-Fi, Bluetooth, and cellular radios to lock onto specific frequencies.In the circuit below, a Varactor Diode is used. When reverse-biased, a diode acts like a capacitor; varying the voltage changes the capacitance, thus tuning the oscillator circuit without moving parts.Figure 8: Hartley Oscillator configuration for VCO applicationsⅥ Frequently Asked Questions (FAQ)1. What is the primary function of an oscillator?Oscillators convert a steady DC supply into a periodic AC signal. They provide the timing signals (clock) for computers, generate carrier waves for wireless transmission, and produce audio signals for synthesizers and alarms.2. How do you calculate oscillation frequency?For a simple pendulum or mechanical system, the formula is T = 2π√(m/k). In electronics (LC circuit), the resonant frequency is calculated as f = 1 / (2π√(LC)), where L is inductance and C is capacitance.3. What are the core components of an oscillator circuit?Most oscillators require three elements: 1. Tank Circuit/Network: Passive components (Inductors/Capacitors or Crystals) to set the frequency. 2. Amplifier: An active device (Transistor, Op-Amp) to gain power. 3. Feedback Loop: A positive feedback path to sustain the oscillation.4. What is the difference between an oscillator and an alternator?While both generate AC, an alternator is a mechanical device that converts mechanical energy into electrical energy (usually at low frequencies like 50/60Hz). An electronic oscillator is a solid-state circuit that converts DC electrical energy into high-frequency AC signals without moving parts.