What does this cap/resistor do in the input stage?

[Artie]

What does this series cap and resistor do in the differential input stage?

Edit: I meant to add: Sometimes I see it in schematics, and some times I don't. Even with the same manufacturer, but different models. Apparently, it isn't absolutely neccessary.

[AndrewT]

it loads the two halves of the LTP and thus selectively reduces the LTP gain.
Reduced gain is usually used to add phase margin and add gain margin to the amplifier.

[Artie]

Thanks Andrew, although I'm surprised by that answer. (Not doubting you.) I thought that the degeneration resistors did that. That cap/resistor looks more like a high-pass filter.

[jan.didden]

Originally Posted by Artie

Thanks Andrew, although I'm surprised by that answer. (Not doubting you.) I thought that the degeneration resistors did that. That cap/resistor looks more like a high-pass filter.

It's kind of both. Andrew is right, it starts to roll off the gain above a certain frequency for stability. That's the purpose. The way it does it is by introducing a freq dependent load, which looks like (and is) a hpf.

jd

[Artie]

Ah . . . thanks for that clarification. So, let me ask this: would it be considered a "mod", or improvement to add this to a circuit that didn't include it already? Was this something omitted as a cost cutting measure?

[wahab]

Originally Posted by Artie

Ah . . . thanks for that clarification. So, let me ask this: would it be considered a "mod", or improvement to add this to a circuit that didn't include it already? Was this something omitted as a cost cutting measure?

It s not an improvement , it s used because the circuit
is surely instable without it..
If you add this RC circuit to an amp that doesn t need it,
it will undoubtly degrade the performances.

[Artie]

Could I impose on you to expand on that a bit? The reason I ask is because I see it more on higher end units than lower. Its generally located between the differential input pair and a current mirror or CCS. Units that omit it also omit the CM or CCS. If I were to mod a unit, I'd do the full monty, (so to speak), with CM or CCS.

[wahab]

"High end " units are generaly high power, and as such , they
need more gain to reach full power using a standard input voltage
in the vicinity of 1V, so there will be less negative feedback
available to reduce distorsion.

That, with the high output voltage that ask for more gain/bandwith
product, will mandate a higher open loop gain to reach the same
perfs as a say 50Wamp.

More open loop gain is synonym of more instability issues, therefore
these kind of arrangements to reduce the open loop gain at very high
frequencies , where the instabilities occur...

[AndrewT]

there are many places that compensation can be added.
I suspect they all sound a bit different.
The trick is finding those that are needed and sound right.

[Artie]

Ok. Thanks again guys. This something I just need to read up on some more. That circuit segment, by the way, is from a Yamaha CA-400. Its a good amp, but not a powerful one.