Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
How To Improve PSRR Of This Gain Stage
« on: October 07, 2005, 11:50:17 AM »
Hi

Working on this gain stage (for a transformerless mic pre): [removed]

Almost everything simulates well except negative PSRR - about +25 dB instead of the -70 dB I'd like... Seems to enter mostly at Q7 emitter, and a bit at Q6 emitter.

Any quick fix for this? RC filtering is unlucky as this would need impossible sized caps.

Oh, and if your solution would include a (push-pull?) VAS which removes the slew-rate constraint set by the current topology, that would be great!

Thanks for your help!

Samuel


bcarso

Re: How To Improve PSRR Of This Gain Stage
« Reply #1 on: October 07, 2005, 03:52:30 PM »
Quote from: "Samuel Groner"
Hi

Working on this gain stage (for a transformerless mic pre): SGA-SEA-1_r3.gif

Almost everything simulates well except negative PSRR - about +25 dB instead of the -70 dB I'd like... Seems to enter mostly at Q7 emitter, and a bit at Q6 emitter.

Any quick fix for this? RC filtering is unlucky as this would need impossible sized caps.

Oh, and if your solution would include a (push-pull?) VAS which removes the slew-rate constraint set by the current topology, that would be great!

Thanks for your help!

Samuel


The topology isn't really suited for high neg rail PSRR---but then most single-ended input amps are not.  The Q6-7 stage is amplifying based on the difference between the signal at Q6's base and the negative rail.  But that signal is coming from the collectors of Q1-3 and R1, with R1 from a lowpass that you are referencing at ac to ground.

Getting rid of C1 will help, to start with.  If the output Z at Q1-3's collectors was infinite you would be getting there.

If you were to lift the whole shebang (Q1-3, Q6, Q7) above the neg rail a bit and bypass/regulate that relative to ground you will get still further, while sacrificing output swing some.  Then playing with some strategically located cancelling impedances could null over a limited freq range.

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
How To Improve PSRR Of This Gain Stage
« Reply #2 on: October 08, 2005, 01:52:05 AM »
Thanks for the suggestions, Brad.

I thought about using a single-ended PSU (+30 V only) and ground the -15 V side - would this solve the PSRR problem or am I misleaded by thinking that ground is ideal? I'd use a ground plane anyway.

Samuel

PRR

How To Improve PSRR Of This Gain Stage
« Reply #3 on: October 08, 2005, 03:55:40 AM »
C1 is going the wrong way. That is why you have GAIN from the rail. Rail crap is applied through Q7 Q6 and R1, then to ground via C1: a very high gain input. It should reference Q7 emitter (and in that case, C1 does no good at all).

It should work nice single-ended. At gain of 10, you could short out the gF (gF???) feedback cap and bias the input around 1V.

Slew should be like 50V/uS. If it isn't, you probably have some wild current swing that will also clip audio signals. Gain-bandwidth is like 40MHz?

With a shorted load, Q7 will die fast, and may cascade into other deaths. You didn't ask that, and I know you won't produce and warranty it yet, but just be aware in bench-testing.

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
How To Improve PSRR Of This Gain Stage
« Reply #4 on: October 08, 2005, 05:33:42 AM »
Thanks for the explanation - I think I understud the mechanism now. Without C1, PSRR is in fact good enough for bipolar supplies (perhaps with some additional RC filtering).

Yes, slew rate is high, I just thought if there is a simple solution, why not (very likely there is none).

This stage simply needs to drive the feedback network and does not interface to the outside world, so the transistors shouldn't die except through my imprudence.

Samuel


 

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