# Feedback resistors and op-amp instability

I am a building a circuit which has a differential amplifier part. To keep things simple I have simplified the circuit to only show the relevant part below.

One thing from op amps which is still difficult for me is the selection of the feedback elements, that is, in what range should you choose the resistors. I have seen answers such as: high value resistors will cause issues due to bias current, while low value resistors require higher currents which lead to dissipation, etc.

In the simulation below all the resistors have the same value and are swept from 1k to 100k in steps of 20k. The op-amp becomes unstable as R increases. Even at 10k! You already see some oscillations.

I expect that this is related to the input capacitance of the op-amp. Is this the case? Or are there other factors here? What op-amp parameter should I look for in the datasheet that could give a stable output with high resistance values? And how can one predict based on the input signal what value might lead to instability?

The input of the system: source with 3 µs rise and fall time.  where x = swept from 1k - 100k with 20k steps with x = 10k

• Opamps are limited in the output current; I'd not go above 10 milliAmps. Thus a 10 volt output requires Feedback of MORE than 1Kohms. Resistors (as do opamps) produce a random (thermal agitation) voltage noise; Thus I'd keep your resistors below 100Kohm, which produces 40 nanoVolts RMS/rtHz and in a 10,000Hz bandwith will become 40nV * swrt(10,000) = 40nV * 100 = 4 microVolts random noise spread uniformly over the 10,000Hz bandwidth. Having be beaten around head and shoulders by these limits (Ishort_circuit_output, and thermal noise) for decades, my default opamp resistor is 10K ohms. – analogsystemsrf Nov 21 '19 at 21:08