The datasheet SNOSB00K – provides guidance to use Rf between 500 ohms & 1kOhms on page 22/ Figure 68.
Is there a problem with using a higher resistance in the amplifier such as ~ 49.9K ot 75K for a gain of ~ -60 to - 75 inverting amplifier application? (Rin = 1k, Rf=60.4K, Cf = 220pF)
Hello Steve,
This should be fine. The section of the datasheet that discusses the recommended Rf value does not account for introducing feedback capacitance into the circuit. The zero produced by Cf and Cin should maintain the stability of your amplifier without limiting your bandwidth. You can see this in the attached TINA model:
Best,
Hasan Babiker
I would not be quite so sanguine Hasan,
the point I was trying make is this RR out model may not be emulating the higher F resonance we see in those stages and more recent models - here is what the LMH6611 model produces - might be right, but most parts have what is shown below for the OPA837 model, I got that Zol from the designer and tuned the model in to produce with an RLC network on the output.
Looking at the LMH6611 there is a closed loop plot and a funny error on that page - probably should be Av = 1V/V not not equal,
here is what the OPA837 zol looks like in the model - ideally, you would at this point ask a designer to recover the design files and run an open loop zout to see if there a need for model update - not likely I know, but that resonance in the Zout for the OPA837 is what causes the phase margin to dip with a feedback C. The precision labs LG sim approach misses that while the one I use breaking the loop at the summing node does not - described in that part 5 article attached.
