I'm having a brain hiccup and can't figure this out. I've tried using simple summing amplifiers where I add my wave to an offset of 2.5V and then High-pass the offset.

However, I need to be able to vary the gain with a pot and in summin amplifiers circuits it seems like changing the gain on one of the inputs will also change the other one, moving the 2.5V offset I'm introducing to the system.

Is there a way to do what I'm trying to do with a single op-amp?

  • \$\begingroup\$ Don't change the feed back resistor, but the resistor from your signal source. \$\endgroup\$ – George Herold Feb 25 '15 at 17:01
  • \$\begingroup\$ Your only going to see the positive side of the AC waveform...Is that OK? \$\endgroup\$ – tman Feb 25 '15 at 17:08
  • \$\begingroup\$ No, the point is for me to be able to see the entire waveform. \$\endgroup\$ – Sanuuu Feb 25 '15 at 17:33
  • \$\begingroup\$ Do you have bipolar supplies? \$\endgroup\$ – Spehro Pefhany Feb 25 '15 at 18:07
  • \$\begingroup\$ You can have only the positive potion of the wave form or you can have the entire waveform with an offset... From your title I am not sure if having a DC offset is OK on the output. \$\endgroup\$ – tman Feb 25 '15 at 18:16

To 'sum' your AC wave with a 2.5V offset, simply couple the wave through a capacitor. This provides high pass filtering with a cutoff frequency determined by the capacitance and resistance at the junction.

Similarly you can vary the AC gain of your amplifier (while the DC gain remains fixed) by AC coupling the feedback pot. Here is an example circuit:-


simulate this circuit – Schematic created using CircuitLab

In this circuit R1 and R2 provide the 2.5V bias, and the AC waveform is mixed into it via C1. R3 couples the output of the op amp back into the inverting input to provide a DC gain of 1. R4 and R5 increase the AC gain to between ~+2 and +21 (205k/105k to 105k/5k) while C2 blocks the effect of R4 and R5 at DC.

With this feedback arrangement the pot is more sensitive at high gain, and the minimum gain is just under +2. There are other ways to connect the pot which can provide different adjustment ranges, eg. pot across R3, or one pot end to the output, pot wiper to the - input, and the other pot end to R5. In all cases the feedback path to ground must be coupled through a capacitor to avoid upsetting the DC gain and output offset.

  • \$\begingroup\$ This is precisely the kind of thing I was looking for. Thank you! \$\endgroup\$ – Sanuuu Feb 25 '15 at 21:29

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