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Introduction

The LM386 is a power amplifier designed for use in low voltage consumer applications. The gain is internally set to 20 to keep the external part count low, but the addition of an external resistor and capacitor between pins 1 and 8 will increase the gain to any value from 20 to 200. The inputs are ground referenced while the output automatically biases to one-half the supply voltage. The quiescent power drain is only 24 mW when operating from a 6 V supply, making the LM386 ideal for battery operation.

Catalog

Introduction

Catalog

I How does LM386 Internal Circuit Work?

1.1 Input stage

1.2 Voltage Amplifier stage

1.3 Output stage

1.4 Feedback Network

II LM386 Application Circuit

2.1 Circuit of Infrared alarm

2.2 Circuit of Automobile Voice Horn

2.3 Circuit of Microcomputer Stereo Power Amplifier

FAQ

Ordering & Quantity

I How does LM386 Internal Circuit Work?

The principle of the LM386 internal circuit is shown in the figure. The internal circuit is based on a typical audio power amplifier configuration, often referred to as Lin topology. LM386 internal circuit is divided into input stage, voltage amplifier stage  (VAS), output stage (OPS), and feedback network.

Figure 1. LM386 Internal Circuit

1.1 Input stage

The first module is the PNP emitter follower amplifier  (Q1, Q3). It sets the input impedance and defines the DC operating point to raise the input voltage from the ground, so the circuit will accept the negative input signal to - 0.4V. Both 50K input resistors  (R 1, R 3) have established paths to the base current ground, and the inputs need to be coupled to avoid interfering with the internal bias. So the input impedance is determined by these resistors and is set to 50K.

Analysis of voltage gain: The gain of the long-tail pair (Q2, Q4) of the differential amplifier is regulated by two gain setting resistors 1.35k +150 Ω (R5 + R5). External pins 1 and 8 can adjust the gain from 20 (minimum) to 200 (maximum).

The voltage gain can be calculated under static conditions (without input signal applied) as follows:

Figure 2. Analysis of LM386 voltage gain

The voltage (vdiff) at both ends of R4 and R5 is only the differential input voltage  (VIN) because the base-emitter voltage drop of PNP transistors (Q1, Q2, Q3, and Q4) on both sides of LTP is the same. 

The current mirror formed by Q5 and Q6 produces equal current on both sides of the LTP. The current is marked as "I".

Due to the current mirror, the current intensity through R8 is equal to 2I, while ignoring the current (i7) through the two 15K resistors (R6, R7). these two resistors have a larger impedance compared to the rest of the circuit, thereby:

In the figure above, it is easy to see that if i7 = 0, then:

The formula can also be rewritten in a more general way:

Z 1-5 and Z 1-8 are the impedance between the corresponding pins.

Without any external components, the gain is GV = 2x15k / (150 + 1350) = 20 (26 dB).

Use a capacitor (or shortcut) between pin 1 and 8, then its gain of GV = 2x15k / 150 = 200 (46db).

1.2 Voltage Amplifier stage

The common emitter amplifier (Q7) amplifies the low amplitude input signal to the appropriate level directly coupled to the output stage.

1.3 Output stage

It is a class AB power amplifier, that is, push-pull configuration. Each transistor amplifies its corresponding half-wave. Because the gain difference between Q9 and Q10 of PNP transistors is in a compound PNP transistor configuration, β TOTAL =β Q9 X β Q10.

Divider compensation: Diodes D 1 and D 2 are used to compensate for cross distortion.

In fact, in a push-pull topology, the transistor does not start conducting until the input signal begins to exceed its forward voltage (Vbe). The forward voltage (VBE) is the voltage at the base emitter junction (usually about ±0.6 V).

In order to offset the minimum conduction limit (Vbe) of the transistors, they need to be biased so that their idling voltage will never be lower than the forward voltage (Vbe). A certain amount of current (called a bias current) will continuously feed into the base of the transistor to ensure that they maintain the sacrificial efficiency of conduction.

It is proved that using diodes is one of the best solutions. It provides a temperature-dependent pressure drop. And by matching the thermal coefficient to the transistor, the bias current can be kept fairly stable. If accurate heat tracing is required, install the diodes on the same heat sink as the power transistor. Since one diode is usually not enough, amplifiers usually use multiple diode junctions

1.4 Feedback Network

Negative feedback is applied from the output to the emitter Q4 via the resistor R8. The function of the DC feedback is to stabilize the output DC bias voltage to half of the supply voltage.

The functions of DC feedback are as follows: If for some reason VO increases, the corresponding current increment will flow through R8 and into the emitter of Q4. Therefore, the increase of collector current of Q4 leads to the positive increase of base voltage of Q7. This leads to an increase in the collector current of Q7, which reduces the base voltage of Q7, thus reducing Vo.

II LM386 Application Circuit

2.1 Circuit of Infrared alarm

Figure 3. Circuit of Infrared alarm

Figure 3 shows the circuit of the infrared alarm. The circuit is composed of an AND gate circuit, a monostable delay circuit, a four-way infrared transmitting and receiving circuit, a trigger and a two-color light-emitting circuit, and an audio alarm circuit. Four pairs of warning lines for transmission and reception are composed of HF1~HF4 (infrared light-emitting diodes) and BG1~BG4 (infrared receiving pairing tubes). If someone crosses the warning line and the infrared beam is blocked, the corresponding matching tube will be cut off. And the input of the corresponding NAND gate is high level. It makes the trigger terminal pin2 of IC3 (555) obtain a negative differential pulse, that is, the output terminal of the AND gate composed of D1~D4. Therefore, the 555 is set, and the high level is output from pin 3 which makes BG5 saturated and turned on. The chip IC4 (KD-9562) is powered on and an alarm sound is given. The delay width of the IC3 monostable circuit td=1.1Rw1R3 determines the sound time. The delay alarm time corresponding to the parameters in the figure is about 100 seconds. At the same time, the corresponding time can be changed by changing the value of W and C according to the specific situation. The voltage stabilizing tube DW adopts 2cw7 or 2cw10, and the voltage range is about 3V to protect the music integration KD-9562, so as to prevent it from burning due to over-voltage (high voltage). The chip KD-9562 is an eight analog sound integrated circuit, which can select the corresponding music according to the use situation and purpose. LM386 is a single power audio power amplifier integrated circuit, which is used to expand the range of alarm sound. F1-1~F1-6, F2-1~F2-6 use six inverter CD4069. Two-color light-emitting diodes LED1~LED4 adopt 2EF303. Under normal conditions, they emit green light, and when someone crosses the warning line, they send out a red warning light.

2.2 Circuit of Automobile Voice Horn

Figure 4. Circuit of Automobile Voice Horn

The left side of the dotted line in Figure 4 is the original circuit diagram of the vehicle's electric horn. SL is the electric horn button switch on the steering wheel. S2 is a newly added SPDT Switch, which is used for switching between the electric horn and voice horn of the original vehicle. When the switch S2 is set to "2", press the switch Sl, the capacitor Cl is charged, the transistors VTl, VT2 turn on, the relay Jl pulls in. J1-1 is closed and held for 15 seconds to supply power to the circuit. ICl is a dedicated voice integrated circuit HL-169A. Since its working time is 2.8 seconds, the self-excited multivibrator is composed of transistors VT3 and VT4. Every 3 seconds, a high level is output as a trigger signal, so that ICl outputs a voice signal every 3 seconds, and sends it to IC2 for audio power amplification, and the speaker BL emits voice.

2.3 Circuit of Microcomputer Stereo Power Amplifier

Because the audio signal amplitude of the "line output" of the sound card is too large, the LM386 can be driven to push the speaker. In addition, due to the interference of video signal when playing VCD, it is not good to connect the VCD according to the typical circuit, so it is necessary to add some components and debug the component values. The schematic diagram is shown in Figure 5.

Figure 5. Circuit of Microcomputer Stereo Power Amplifier

LM386 has two input terminals, 3 pin in-phase input and 2 pin inverting input. The input signal can be input from any end and the other input terminal can be grounded. The input end is connected with capacitor C4 in order to filter out the video interference when playing VCD. The value can be increased appropriately, but it can be used with or without C4 when playing the CD. The pin1 and pin 8 are gain control terminals, which are composed of C2 and W2. The smaller the resistance, the higher the gain. It is more appropriate to adjust the gain of W2 to about 150Ω. If the gain is too high, it will easily cause self-excitation. Pin 7 is connected to a 10μ capacitor. The high frequency component attenuation circuit is composed of R2 and C6 to eliminate the crash sound from the loudspeaker. The capacity of C6 can be adjusted according to the actual effect. Pin 6 is connected to the ground with a 0.1μ capacitor, which acts as a filter to eliminate the static hum of the amplifier. Pin 5 is connected to the coupling capacitor C3. If only one cone speaker is connected to one channel, C3 capacity should not exceed 470 μ, otherwise, the loudspeaker will be blocked when playing low music. If high-frequency and low-frequency crossover technology is adopted, the capacity of C3 can be increased to make the bass fully reflected. W1 is used to adjust the output volume, which is particularly convenient when playing games or listening to CDs.


FAQ

  • How much gain is the LM386 internally set to?

20

  • What is the LM386 internal circuit often referred to as?

Lin topology

  • What are the two input terminals of LM386?

3 pin in-phase input and 2 pin inverting input

  • How does an LM386 work?

The Lm386 integrated chip is a low power audio frequency amplifier, which uses low level power supply like batteries in electronic circuits. It is designed as 8 pin mini DIP package. This provides voltage amplification of 20. By using external parts voltage gain can be raised up to 200.

  • Is lm386 an op amp?

The LM386 is a type of operational amplifier (Op-Amp). In an amplifier circuit, the LM386 takes an audio input signal and increases its potential anywhere from 20 to 200 times. That amplification is what's known as the voltage gain.

  • What is lm386 IC?

The LM386 is an integrated circuit containing a low voltage audio power amplifier. It is suitable for battery-powered devices such as radios, guitar amplifiers, and hobby electronics projects.

  • How do you calculate lm386 gain?

Voltage Gain Analysis:
Without any external components, it has a gain of Gv = 2x15K/(150+1350) = 20 (26 dB).
With a capacitor (or shortcutting) between pins 1 and 8 , it has a gain of Gv = 2x15K/150 =200 (46dB).

  • Which IC is used in audio amplifier?

The IC LM386 is a low-power audio amplifier, and it utilizes low power supply like batteries in electrical and electronic circuits. This IC is available in the package of mini 8-pin DIP.

  • What are some projects that use the LM386 audio amplifier circuit?

LM 386 is an integrated class AB amp and is good for beginners small audio amplifier applications…for example in a RF receiver,small Stereo system,cheap low voltage amplifier etc…drawbacks is that it cannot handle much power and hence creates distortion when you crank up the volume too much.. So other ICs are used in practical.

  • How to make an LM386 audio amplifier circuit?

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