# Voltage controlled resistor at non inverting input of OP-AMP

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)

• 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
– G36
Aug 7, 2023 at 20:15
• 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. Aug 7, 2023 at 20:18
• Why do you think that it is a voltage-controlled resistor? It is nothing more than an ordinary trimpot potentiometer.
– G36
Aug 7, 2023 at 20:24
• Okay, I was confused by pin 2 being connected to V- Aug 7, 2023 at 20:26

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.