CFB amplifiers at high gain

Sep 09, 2019 2 Replies

CFB op-amps are famous for not loosing their wide -3dB amplifier bandwidth as you increase gain, as do VFB op-amps. Many of us know this aspect degrades at high gains, but not as well as we should. There's a deadly parameter, Rb, which is the input resistance of the "-" input. This ranges from 8 ohms to 500 ohms. For low Rb, transistors have to run at high currents.



Rb competes with low Rg for feedback current, forcing one to use high Rg and Rf values to get high gain. But high Rf means poor slewing and bandwidth. Most CFB op-amp formulas don't mention or include this critical Rb parameter. If you force Rg too far below Rb, oscillation or severe peaking and ringing can occur.



This can mean that if one needs a stage gain of 20 to 100x, a VFB op-amp can compete with a CFB type for the job. Today I updated my op-amp spreadsheets, taking Rb into account. Only a fraction of manufacturers give us the information we need; most content themselves describing their op-amps for use at G=1 or 2. But much can be surmised by studying curves.



Thanks, - Win

Are you making a distinction between inverting and non- inverting configurations?

It's true that the available gain starts to roll off at higher frequencies for whatever (parasitic?) reasons, but I've never run into a case where reducing Rg by itself causes peaking or instability. (I've also never built an inverting amp with these parts, or tried to go beyond Av=20 in a single stage.)

Some informal measurements:

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Other parameters like the output swing capability get much worse at high frequencies, but I'd assume that has more to do with the output stage than anything happening at the + or - inputs.

-- john, KE5FX

The issue is pretty much the same for inverting or non-inverting gain, but for high gains with low Rg, it's a pain to drive a super-low Rg inverting input, so non-inverting is usually the way to go.

Your THS3491 is the fastest high-voltage part in my table, with Rb = 15 ohms, so you're less likely to run into the issue. You'd probably need Rg below 10 ohms to see it.

Thanks, - Win

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