I am studying a schematics where the non inverting input of an op amp controls what looks to be a voltage controlled resistor (VR202). Without this component, I think the op amp would work as an integrator or an amplifier with a certain AC bandwidth, but I am not sure what this additional component does. To me it looks like it changes the gain of the amplifier so that the output of the op-amp is regulated faster , am I remotely correct ?

Additional information: On the manual related to the device I am studying (an analog isolator with variable gain SIM 984 from SRS), one can read :"The overall AC gain is trimmed using VR202 at the factory, and should not require user adjustment." What is trimmed here if the component is voltage controlled ?

The full schematics can be found on (https://xdevs.com/doc/Stanford_Research_Systems/SIM900/sim984_sch.pdf)

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  • \$\begingroup\$ From the SM "The overall AC gain is trimmed using VR202 at the factory, and should not require user adjustment". thinksrs.com/downloads/pdfs/manuals/SIM984m.pdf \$\endgroup\$
    – G36
    Aug 7, 2023 at 20:15
  • \$\begingroup\$ Thanks @G36 , I am not looking to trim it, I am trying to understand what it is they trim since it looks to be voltage controlled, and why it should be voltage controlled if a simple varistor could work. also that's the sentence I've written in my question. \$\endgroup\$ Aug 7, 2023 at 20:18
  • \$\begingroup\$ Why do you think that it is a voltage-controlled resistor? It is nothing more than an ordinary trimpot potentiometer. \$\endgroup\$
    – G36
    Aug 7, 2023 at 20:24
  • \$\begingroup\$ Okay, I was confused by pin 2 being connected to V- \$\endgroup\$ Aug 7, 2023 at 20:26

1 Answer 1


That's not a voltage controlled resistor, it's a variable resistor. It's probably a trim-pot. It's a potentiometer wired as two wire variable resistor by connecting the wiper to one end and is in series with the input.

Since the gain of an inverting amplifier is $$ A=\frac{R_f}{R_{in}} $$ and the resistor is part of \$R_{in}\$, varying it adjusts the gain.

Looking at the full schematic there is a 1.62k resistor in series with it, so the DC gain will vary from approximately 10/2.62 to 10/1.62, roughly 4 to 6. There are capacitors switched in and out of the circuit that will vary the AC gain and frequency response.

  • \$\begingroup\$ Thanks @GodJihyo, I was just confused by the wiring \$\endgroup\$ Aug 7, 2023 at 20:29

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