Op amp feedback resistor selection problem

模拟链新手

Op amp feedback resistor selection problem [Copy link]

When amplifying a 500mVpp, 100Mhz square wave signal using opa843 simulation, the resistor shown in Figure 1 can be amplified normally, but when it is changed to Figure 2, there will be oscillation. Does anyone know why?

maychang

100MHz is a very high frequency, not to mention a square wave. This op amp is not an ordinary op amp. To understand why the waveform changes, you must read the manual of this model of op amp carefully.

模拟链新手

maychang posted on 2022-11-15 18:04 100MHz is a very high frequency, not to mention a square wave. This op amp is not an ordinary op amp. To understand why the waveform changes, you must read this...

OK, thank you. My understanding is that the resistance increases and the phase changes, resulting in insufficient phase margin and oscillation.

maychang

Simulation Chain Newbie posted on 2022-11-15 19:51 OK, thank you. My understanding is that the resistance becomes larger and the phase changes, resulting in insufficient phase margin and oscillation.

It is hard to say that this waveform is an oscillation.

However, changes in resistance will definitely affect the open-loop phase, that's for sure.

maychang

Simulation Chain Newbie posted on 2022-11-15 19:51 OK, thank you. My understanding is that the resistance becomes larger and the phase changes, resulting in insufficient phase margin and oscillation.

Your waveform seems to be simulated.

You may want to ask teacher gmchen how specific resistance affects the phase.