# Differential (Subtractor) op amp with low pass filter

is it possible to have a subtractor circuit with a low pass filter in it? My thought is that I would have the possibility of doing that if I add a capacitor in parallel with R3. Is it actually possible? If not, whats the best way of doing it?

• What about finding the transfer functions for the various alternatives and compare it with the genral lowpass function?
– LvW
Apr 2, 2019 at 9:31

Yes. I have done this before. You put two capacitors in; one across R3 and one across R4. But remember the impedance at the noninverting input is not the same as the inverting. I will leave it to you to determine what they are, especially as you have not given any values.

• You may simulate this, but the two capacitors do NOT have the same impact on the circuit. Even considering the impedance "seen" by the two caps (call them C3 and C4), you will never get the same bode plots. C4 is in a voltage divider producing 3 dB per octave of attenuation as high as the frequency goes. C3 is on the gain determining side of the op amp circuit, and can never make the gain less than 1. At high frequencies, the output is effectively shorted to the inverting input, which makes it a follower. If the circuit has a gain of 2, you will never seem attenuation of more than 2. Mar 4, 2023 at 19:44

In order for the differential amplifier to work correctly, the two impedances of R3 and R4 have to be the same. This is easy with resistors, but is very difficult with capacitors, in practice. Precision capacitors are very difficult to make and are expensive, if you can find them to buy. A more practical solution is to use a second op amp stage following the diff amp for low pass filtering.

Calculate the value of the required capacitor from 1/(2 * pi * fc * R3)

Then put the same value capacitor across R4 as well as R3

Make sure R1 = R2 and R3 = R4

It worked in my Circuit Wizard simulations.

is it possible to have a subtractor circuit with a low pass filter in it?

It is, but it's hard to do well. As other answers have replied, you need only put a capacitor across R3, and another across R4.

However, be aware that, while you can get resistors which are matched to high precision fairly cheaply (and the performance of the subtractor depends critically on such matching), it's much harder to get well-matched capacitors. Unless you get the capacitors almost perfectly matched, the two different impedances produced will cause different phase shifts at the two inputs of the op amp, and the subtractor will no longer work as well as you expect from its DC performance.