Phone

    00852-6915 1330

What Are Application Circuits of Differential Amplifier?

  • Contents

The differential amplifier circuit is also called the differential circuit.

It can not only effectively amplify the AC signal, but also effectively reduce the zero drift caused by the power supply fluctuation and the temperature change of the transistor, so it has been widely used. Especially, it is widely used in integrated operational amplifier circuits, and it is often used as the pre-stage of multi-stage amplifiers.

Starting from actual production design, this blog discusses the shortcomings of discrete resistors, filtering, AC common mode rejection, and high noise gain.

What a Differential Amplifier is along with the Derivation

of the Equation Relating Input to Output

Catalog

I Classic Four-resistor Differential Amplifier

II CMRR

III Low Tolerance Resistance

IV Another Low-end Detection Application

V High Noise Gain

VI Single Capacitor Roll-off

VII Capacitance between Input Terminals of Op Amp

VIII Conclusion

I Classic Four-resistor Differential Amplifier

Figure 1 shows the classic four-resistor differential amplifier is very useful.

Classical differential amplifier

Figure 1. Classical differential amplifier

The transfer function of this amplifier is:

If R1 = R3 and R2 = R4, then Equation 1 is simplified to:

This simplified theory works, but it cannot be done in reality. Because the resistance can never be exactly equal. In addition, other changes in the basic circuit can produce unexpected behavior. Although the following example is simplified to show the essence of the problem, it is derived from actual application problems.

II CMRR

An important function of the differential amplifier is to suppress the common mode signal of the two inputs. As shown in Figure 1, assuming that V2 is 5 V and V1 is 3 V, then 4V is the common-mode input. V2 is 1 V higher than the common mode voltage, and V1 is 1 V lower. The difference between the two is 2 V, so the "ideal" gain of R2/R1 is applied to 2 V.

If the resistance is not ideal, part of the common-mode voltage will be amplified by the differential amplifier and appear at VOUT as the effective voltage difference between V1 and V2, which cannot be distinguished from the real signal. The ability of the differential amplifier to suppress this part of the voltage is called  Common Mode Rejection  (CMR). This parameter can be expressed as a ratio (CMRR) or converted into decibels (dB).

In an article published in 1991, Ramón Pallás-Areny and John Webster pointed out that assuming that the op amp is an ideal op amp, the common mode rejection can be expressed as:

Among them, Ad is the gain of the difference amplifier, t is the resistance tolerance. therefore:

  • In the case of unity gain and 1% resistance, CMRR is equal to 50 V/V (or about 34 dB);
  • In the case of 0.1% resistance, CMRR is equal to 500 V/V (or approximately 54 dB);

It is even assumed that the operational amplifier is an ideal device with unlimited common-mode rejection.

If the common-mode rejection of the operational amplifier is sufficiently high, the total  CMRR is limited by resistance matching. Some low-cost op-amps have a minimum CMRR of 60 dB to 70 dB, making calculations more complicated.

III Low Tolerance Resistance

The first suboptimal design is shown in Figure 2.

This design is a low-end current detection application using OP291. R1 to R4 are discrete 0.5% resistors. According to the formula in the article by Pallás-Areny, the optimal CMR is 64 dB.

Fortunately, the common-mode voltage is very close to the ground, so CMR is not the main source of error in this application. A current sense resistor with a tolerance of 1% will produce a 1% error, but the initial tolerance can be calibrated or adjusted.

However, since the operating range exceeds 80°C, the temperature coefficient of resistance must be considered.

Low-end detection with high noise gain

Figure 2. Low-end detection with high noise gain

For extremely low shunt resistance values, a 4-pin Kelvin sense resistor should be used. Using a high-precision 0.1 Ω resistor and directly connecting the resistor with a PCB trace of a few tenths of an inch can easily increase 10 mΩ, resulting in an error of more than 10%. But the error will be greater because the temperature coefficient of the copper traces on the PCB exceeds 3000 ppm.

The shunt resistance value must be carefully selected. A higher value produces a larger signal. This is a good thing, but the power consumption (I2R) will also increase, possibly up to several watts. With a smaller value (mΩ level), the parasitic resistance of the line and PCB trace may cause a larger error. Generally, Kelvin detection can be used to reduce these errors.

We can use a special four-terminal resistor (such as Ohmite LVK series) or optimize the PCB layout to use standard resistors. If the value is extremely small, PCB traces can be used, but this will not be very accurate.

Commercial four-terminal resistors (such as Ohmite or  Vishay's products) may cost several dollars or more to provide 0.1% tolerance and extremely low temperature coefficient. A complete error budget analysis can show how to improve accuracy with the minimal cost increase.

Regarding the problem of a large offset (31mV) with no current flowing through the sense resistor, it is caused by the "rail-to-rail" op amp being unable to swing all the way to the negative power rail (ground).

However, the term "rail-to-rail" can be misleading: the output will be close to the power rail—much closer than the output stage of a classic emitter follower—but never actually reach the power rail. Rail-to-rail operational amplifiers have a minimum output voltage VOL, which is equal to VCE(SAT) or RDS(ON) × ILOAD.

If the offset voltage is equal to 1.25 mV and the noise gain is equal to 30, the output is equal to: 1.25 mV × 30 = ±37.5 mV (35 mV due to the presence of VOS and VOL). Depending on the polarity of VOS, the output may be as high as 72.5 mV without load current.

If the maximum value of VOS is 30μV and the maximum value of VOL is 8 mV, modern zero-drift amplifiers (such as AD8539) can reduce the total error to the level mainly caused by the sense resistor.

IV Another Low-end Detection Application

Another example is shown in Figure 3. This example has low noise gain, but it uses a low precision four-channel op amp with 3 mV offset, 10-μV/°C offset drift, and 79 dB CMR,  And in the range of 0 A to 3.6 A, an accuracy of ±5 mA is required. If a ±0.5% detection resistor is used, the required ±0.14% accuracy cannot be achieved. If a 100 mΩ resistor is used, a ±5 mA current can produce a ±500 μV voltage drop.

Unfortunately, the offset voltage of an op amp with temperature is ten times greater than the measured value. Even if VOS is adjusted to zero, a temperature change of 50°C will exhaust the entire error budget. If the noise gain is 13, any change in VOS will be expanded by 13 times. To improve performance, zero-drift operational amplifiers (such as AD8638, ADA4051, or ADA4528), thin-film resistor arrays, and higher-precision sense resistors should be used.

Low-end detection

Figure 3. Low-end detection

V High Noise Gain

The design in Figure 4 is used to measure high-side current, and its noise gain is 250. The maximum VOS rating of the OP07C operational amplifier is 150 μV. The maximum error is 150 μV × 250 = 37.5 mV. To improve performance, and ADA4638 zero-drift operational amplifier is used. The device has a nominal offset voltage of 12.5 μV over the temperature range of -40°C to +125°C.

However, due to the high noise gain, the common-mode voltage will be very close to the voltage across the sense resistor. The input voltage range (IVR) of OP07C is 2 V, which means that the input voltage must be at least 2 V below the positive rail. For the ADA4638, IVR = 3 V.

High-side current detection

Figure 4. High-side current detection

VI Single Capacitor Roll-off

The example in Figure 5 is slightly more complicated. So far, all the equations are for resistance. But it is more accurate than they should take impedance into account. In the case of adding capacitance (whether it is deliberately added capacitance or parasitic capacitance), the AC CMRR depends on the impedance ratio at the target frequency. To roll off the frequency response in this example, you can add a capacitor C2 across the feedback resistor, as you would normally do in an inverting op-amp configuration.

Try to create a low pass response

Figure 5. Try to create a low pass response

If you need to match the impedance ratio Z1 = Z3 and Z2 = Z4, you must add capacitor C4. It is easy to buy 0.1% or better resistors on the market, but even 0.5% capacitors cost more than $1. The impedance at very low frequencies may not matter, but capacitance tolerance or a 0.5 pF difference between the two op amp inputs due to PCB layout can cause the AC CMR  to drop by 6 dB at 10 kHz. This is particularly important when using a switching regulator.

Single-chip difference amplifiers (such as AD8271, AD8274 or AD8276) have much better AC CMRR performance. Because the two inputs of the operational amplifier are in a controlled environment on the chip, and the price is usually cheaper than a discrete operational amplifier and four precision resistors.

VII Capacitance between Input Terminals of Op Amp

In order to roll off the response of the differential amplifier, some designers will try to add a capacitor C1 between the two op amp inputs to form a differential filter, as shown in Figure 6.

This is feasible for instrumentation amplifiers, but not feasible for operational amplifiers. VOUT will move up and down through R2, forming a closed loop. At DC, this does not cause any problems, and the circuit behaves as described in Equation 2.

As the frequency increases, the reactance of C1 decreases. The feedback into the input of the op amp decreases, causing the gain to increase. Eventually, the op amp will work in an open-loop state because the capacitor shorts the input.

Input capacitance reduces high frequency feedback

Figure 6. Input capacitance reduces high frequency feedback

On the Bode plot, the open-loop gain of the operational amplifier drops at -20dB/dec, but the noise gain increases at +20dB/dec, forming a -40dB/dec crossover. As we know, it must oscillate. In general, never use capacitors between the inputs of an operational amplifier (except in rare cases, but this blog will not discuss it here).

VIII Conclusion

Whether it is a discrete or single chip, the four-resistor differential amplifier is widely used. In order to obtain a stable and production-worthy design, the noise gain, input voltage range, impedance ratio, and offset voltage specifications should be carefully considered.

 

FAQ

  • What is the differential amplifier circuit able to do?

Reduce the zero drift

 

  • What is the differential amplifier circuit often used as?

Multi-stage amplifiers

 

  • What is one of important functions of the differential amplifier?

Suppress the common mode signal of the two inputs.

Kynix

Kynix was founded in 2008, specializing in the electronic components distribution business. We adhere to honesty and ethics as our business philosophy and have gradually established an excellent reputation and credibility in our international business. With the accurate quotation, excellent credit, reasonable price, reliable quality, fast delivery, and authentic service, we have won the praise of the majority of customers.

Join our mailing list!

Be the first to know about new products, special offers, and more.

Leave a Reply

We'd love to hear from you! Feel free to share your thoughts and comments below. Rest assured, your email address will remain private.

Name *
Email *
Captcha *
Rating:

Kynix

  • How to purchase

  • Order
  • Search & Inquiry
  • Shipping & Tracking
  • Payment Methods
  • Contact Us

  • Tel: 00852-6915 1330
  • Email: info@kynix.com
  • Follow Us

authentication

Kynix

© 2008-2026 kynix.com all rights reserved.