DC offset need help again :-(

[MikeB]

Yes, i'm familiar with currentsources to diffamps, but this way q17
simply passes the current from the 12k to the diffamp, at least tries
to. So without q17 the same current is fed into the diffamp.
For feeding a diffamp a simple resistor is not too bad, but introduces
ripple from psu. The impedance compared to a ccs is no big problem
here, as the voltage across the 12k does not vary much.
The AKSA for example uses a simple resistor, but has a "stabilized"
voltagesupply to the frontend.

Mike

[bocka]

Hi djdamix,

change the 12k resistor nearby Q17 to 270 ohm. Add 2 diodes 1N4148 in series from base of Q17 to the negative side of the power supply and change R36 from 1k to 12k. This will fix your problem

[djdamix]

I'have fixed the problem, The 100u capacitor was bad ... I've changed it and the offset disappear, as on my first amp about (30mV) ... strange ... Anyway as most of you say I'll change the current source (Q17) to something better.

Thanks for all your advices.
David

[Duo]

Definitely Bocka's idea is what I'd do.

With the base of Q17 going to 0V through a resistor and the collector loaded by the differential amp, the CB junction is nearly forward-biased as mentioned before.

Certainly changing in a 270ohm emitter resistor and adding the diodes forward-biased to V- and a 12k resistor will make a proper current source.