I'm designing a differential amplifier to remove noise from a signal, and I'm trying to verify my design by simulating a noisy input, but I'm not sure how to do this. The diagram below is roughly what I'm trying to achieve. Signal 1 and 2 have different frequencies. Signal 1 is present on both inputs of the amplifier, while signal 2 is only present on one. The amplifier should isolate signal 2, and amplify it at the output Vo.

I've already designed the amplifier, but I'm just not sure what the combining circuit should be? What's the best way to combine the two signals, making sure that they are both still present in the input V2?

enter image description here

  • \$\begingroup\$ Do you want to do this in a simulation or on the bench? \$\endgroup\$ Dec 13, 2020 at 14:14
  • \$\begingroup\$ Its a simulation using LTspice \$\endgroup\$
    – MendelumS
    Dec 13, 2020 at 14:20

1 Answer 1


This is how I would do it: -


simulate this circuit – Schematic created using CircuitLab

Given that Signal 2 is a voltage source it has zero impedance and therefore also "carries" the unwanted signal (Signal 1) to appear on Vin- and, equal in level to that on Vin+.

  • \$\begingroup\$ Oh ok thanks, I guess I was massively overcomplicating it. I'd assumed there was a bit more to it than that \$\endgroup\$
    – MendelumS
    Dec 13, 2020 at 14:53
  • 1
    \$\begingroup\$ @MendelumS I've always done it that way because it fully separates wanted signal from unwanted signal. Not my invention but it is also the conventional way of doing it in many analyses. \$\endgroup\$
    – Andy aka
    Dec 13, 2020 at 14:56

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