Electronics > Beginners
AC Coupling and OP Amp Stages
eev_carl:
Hi,
I have a single supply. If I'm connecting several op amps together for an audio guitar effects pedal, should I AC couple each stage? I have an input buffer, clipping stage, filter, and output buffer implemented with a 4-op amp IC. I remember reading somewhere that if you don't AC couple everything, you'll get more noise as the when DC is passed along and amplified. Do only gain-producing stages need this treatment?
Thanks,
Carl
Benta:
There's no reason to AC couple each stage, input and output is enough.
With a single supply, you have the issue of setting a noise-free DC bias for the first stage. With a dual supply you have a nice quiet ground as bias.
eev_carl:
I was breadboarding the attached circuit but don't get similar results unless I put a non-polarized coupling capacitor at the NPN's emitter. I re-implemented this with a pair of op amps on a 1458 with the first op amp serving as the input buffer and got the same results on the scope.
Are there any situations where I'd need to AC couple op amp stages? I'm planning on chaining together an input buffer > clipping circuit > filter circuit (tone control) -> output buffer with a single supply. I've seen coupling and de-coupling between stages in some schematics I've been reviewing but don't have a feel for the rule of thumb.
Thanks,
Carl
MiDi:
AC coupling is used if there is the need for a specific high pass filter and/or if the DC part is out of common mode input range of amplifier (e.g. high voltage gain stages suffer from input offset voltage)
eev_carl:
--- Quote from: MiDi on November 20, 2018, 04:37:43 pm ---AC coupling is used if there is the need for a specific high pass filter and/or if the DC part is out of common mode input range of amplifier (e.g. high voltage gain stages suffer from input offset voltage)
--- End quote ---
But my circuit doesn't meet either criteria (audio frequencies, low input levels, and modest gain (x5)).
If I had to guess, I think I need the caps for the same reason I put a cap between the source voltage and the circuit. DC is flowing back to the input and interfering with the preceding signal. At least that's what it seems from my A/B testing of the cap and cap-less circuits.
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