# Find the differences between two signals with an opamp

I have been trying to find an answer to this question and I am sure I probably just don't know what the circuit is called so I am unable to find it. Basically I want to take two input signals that for example are exactly the same (ex. 1Vp-p @ 1hz) and put it through an op amp to only get the differences. Image below of what I am kind of thinking:

So for the above circuit the two buffer opamp's create 2 signals that are the exact same. Then the second opamp stage add's a gain of 2 to each individual signal. The thought on the third stage is to take the the two signals and cancel each other out to have a 0Vp-p output signal. To do this I thought just putting each signal to the input of an opamp with no gain would give this result since the voltage on each input terminal would be the exact same. But when I simulate it with the circuit above it only acts like a buffer. The through is that I can then add noise to one (or both) of the signal paths and only have the 3'rd opamp output the noise and no signal, which then I could sum the two similar signals together and remove the non-common noise.

Sorry for the question I am not an electronic engineer (just a hobbyist) and I am just trying to learn things by trying and then looking up/asking questions.

• What you are looking for is a "differential amplifier".
– user16324
Commented Aug 3, 2020 at 17:36

But when I simulate it with the circuit above it only acts like a buffer

Try adding the resistor in the green box below: -

You need to make the right hand op-amp a proper differential amplifier as used inside such devices as Instrumentation Amplifiers: -