I am currently tasked with improving the design of an electrometer unit. It was NOT designed by an electrical, and there are some questionable design practices. My question is concerning the first amplification stage. A much simplified schematic follows: First stage amplification

In this schematic, R1 and C1 are set from a network of relays (not shown). R2 is constant, and in all cases much much less than R1. My question is, what is the purpose of R2? I have never seen this type of feedback loop. It seems to me that this is just going to make the opamp have to work a lot harder to maintain stability. I'm tempted to just remove R2 outright.

  • 3
    \$\begingroup\$ R2 is not in the feedback loop, it's just a load resistor on the output of the op-amp. Without more context it's hard to say why it's there. \$\endgroup\$ – John D Nov 5 '19 at 22:07
  • \$\begingroup\$ You did not mention the opamp type or resistance values or anything, not even where else the output goes. Depending on opamp type it may need load or else it performs poorly, so you can't just start removing things. \$\endgroup\$ – Justme Nov 5 '19 at 22:09

R2 is not part of the feedback loop, it's just a load. Generally, it won't affect your stability, unless it draws enough current to reach the limit of your opamp output current. In that case you'll start losing gain and that might compromise stability, but it is hard to say what would happen and you'd have to experiment.

Let me just be clear that this sounds more like a mode of failure of the circuit than something you should plan out for during correct (intended) operation. If you're concerned about too much current then increase load resistance or pick a better opamp.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.