I'm working on a 3-stage circuit involving cascading op-amps. I was wondering if there was a general consensus on where the gain should be?
My three stages involve:
Stage 1 - Input : Basic Passive Filtering, no op-amps here
Stage 2 - Sum/Difference here
Stage 3 - Sum/Difference here, output
Stages 2 and 3 use op-amps, and I want my output to have a certain gain involving the inputs. Question is, do I place the gain at Stage 2 or Stage 3, or is it better to split the gain?
Stage 1 filters the inputs to a certain cutoff frequency. Working with op-amps, the max bandwidth for a closed loop circuit is the GBW, provided that gain = 1. As gain increases, the bandwidth decreases linearly. Assuming an inverting op-amp configuration, you can put a small capacitor in parallel with the feedback network to further filter signals. Keeping that in mind, I see some pros and cons to both approaches.
If I put the gain in stage 3, I can use the max bandwidth in stage 2, keeping with a gain of 1 at that stage. Furthermore, with a feedback capacitor, that would also filter out the signals even further, ensuring that stage 3 sees signals that can fall within the op-amp bandwidth at a set gain. So, assuming that all signals are within the bandwidth of the op-amp, stage 3 should have no problems with the signals.
On the other hand, if I put the gain in stage 2, it will also filter out certain frequencies even without the feedback capacitor (as the closed loop gain is only good up to the corner frequency found using the GBW and gain). However, here, I have to make sure that the reduced bandwidth as a result of having a gain here will be fine with the frequencies filtered from stage 1. After that, stage 3 can utilize its max bandwidth since it doesn't need to have gain here.
Both methods seem valid, but I was wondering what more experienced engineers think?