Looking at the negative feedback loop in a lot of opamp gain stages (even
unity gain), there is often a capacitor from a few pF to around 100 pF in
parallel with the feedback resistor. What's the idea behind this? I'm
guessing it effectively forms some kind of low-pass filter, as the higher
frequencies passed by the cap will be attenuated. Is this supposed to
stabilize the circuit from some kind of HF oscillation?

I'm particularly looking at the output stages of various mic preamp
circuits. Some don't have this capacitor in the negative feedback loop and
some do. Any pearls of wisdom out there?

Bill.

"Bill Ruys" wrote
in message
Looking at the negative feedback loop in a lot of opamp
gain stages (even unity gain), there is often a capacitor
from a few pF to around 100 pF in parallel with the
feedback resistor. What's the idea behind this?

Puts in a high frequency roll-off.

I'm guessing it effectively forms some kind of low-pass
filter, as the higher frequencies passed by the cap will
be attenuated.

Right.

Is this supposed to stabilize the circuit
from some kind of HF oscillation?

Reduces HF noise from the input.

I'm particularly looking at the output stages of various
mic preamp circuits. Some don't have this capacitor in
the negative feedback loop and some do. Any pearls of
wisdom out there?

Most well-designed audio gear has a pole in the 50 KHz - 100 KHz range.
Grants some immunity to EMI, etc.

Arny Krueger wrote:




Most well-designed audio gear has a pole in the 50 KHz - 100 KHz range.
Grants some immunity to EMI, etc.


Glad to see the audio industry in keeping our former eastern block Polish
people employed. See ya on the s plane.........

Bob

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