ExtremA, class-A strikes back?

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LineSource

LineSource

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Joined 2003
Hi Sanders,

I would appreciate your design thoughts on the DC offset circuit from T25-T26 diff amp controlling the cascode pull-up bias.

1) How well does this circuit work at holding 0V offset?

2) Did you try other circuits like input diff pair current control, or the traditional summing an offset voltage at the negative diff input? Does your circuit function as well in terms of true 0V offset control?

3) your circuit includes a RC filter on the output bias, but does not include the traditional integrator capacitor on the diff-amp to filter the output signal. How stable is your offset control at high output swings?

Thanks for the education.
 
SSassen

SSassen

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LineSource,

1) How well does this circuit work at holding 0V offset?
Click to expand...

Very well, provided you give the amplifier a chance to warm up prior to setting the offset. Overall, even with a non-matched input pair, DC offset should never be more than about 15mV anyway.

2) Did you try other circuits like input diff pair current control, or the traditional summing an offset voltage at the negative diff input? Does your circuit function as well in terms of true 0V offset control?
Click to expand...

Yes, we looked at a few solutions, this just works fine, mostly due to the fact than when warmed up the delta-T in the various stages will not change much.

3) your circuit includes a RC filter on the output bias, but does not include the traditional integrator capacitor on the diff-amp to filter the output signal. How stable is your offset control at high output swings?
Click to expand...

DC offset at high output swings is not an issue, remember we're driving both the frontend and the output stages from two different power supplies. The output stage draws a constant current which will only affect the rail voltage when you're pushing it beyond class-A into class-B territory.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
SSassen

SSassen

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Unclejed163,

i'm guessing that since this design resembles an op amp, and has global feedback, that this amp (unlike many class A designs) has a pretty good damping factor.
Click to expand...

Output impedance is about 20milli-ohms up to 200kHz. Because damping factor is calculated by dividing the load resistance by the output impedance we arrive at about 8/0.02 = 400.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
SSassen

SSassen

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Pasi P,

I suppose it can be used unbalanced as well?
Click to expand...

Well yes, but that negates some of the benefits of using a balanced setup in the first place.

How about input impedance, slew rate and power bandwidth?
Click to expand...

As per the schematic both inputs are driven into a virtual ground, as is normally the case with any balanced opamp (remember this is basically a power balanced opamp by design). Hence input impedance would be 2K2 per input. As for amplifier slew rate, gain is flat out to 200kHz, so I'll leave for you to calculate the slew-rate, same for power bandwidth.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
SSassen

SSassen

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Joined 2005
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Mike?

DC protection not shown in detail.
Click to expand...

I'm sorry? It is, but obviously the relays and the circuit driving them are on the power supply PCB and hence they're on the power supply schematic as well. But I'm sure you've figured that out by now?

Also, it is NOT a DC offset protection, but a DC protection, in case of failure of one of the output transistors etc. to safeguard the attached loudspeakers. If you need a foolproof solution to prevent DC from entering the amplifier at the input I suggest you mount coupling caps or reduce the value of the zeners at the DC protection circuit to a value that's more to your liking.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
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