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Old 1st December 2004, 12:32 PM   #21
Electrons are yellow and more is better!
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Which side that is the best is determined by the whole design, not just the output stage.
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Old 1st December 2004, 12:33 PM   #22
uli is offline uli  Austria
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Quote:
Originally posted by classd4sure



Hi,

To what end?

I'm just guessing you would do that with the intention of using the NPN in class A and biasing off the PNP, however it doesn't seem to make sense if the NPN has -20dB less PSRR.

You should bias the follower stage to class A, whatever kind of transistor it is.

If you have 20dB difference in PSRR, you should work on that problem first although implementing a ccs would be a good solution for that problem too.

Uli

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Old 1st December 2004, 12:40 PM   #23
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Quote:
Originally posted by uli



You should bias the follower stage to class A, whatever kind of transistor it is.

If you have 20dB difference in PSRR, you should work on that problem first although implementing a ccs would be a good solution for that problem too.

Uli


Fair enough, but you say "..if you have..work on that problem first" as though it's unusual to have -20dB on one rail, from what I've seen, it's almost a given for any op amp.

CCS all around?
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Old 1st December 2004, 12:50 PM   #24
uli is offline uli  Austria
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Default if your

psu is an issue in the circuit I would work on that.

For example the OP2134 (which I like very much) has no different specs for plus and minus rails.

Uli

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Old 1st December 2004, 12:54 PM   #25
mandat is offline mandat  Poland
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Try to follow the thread "The OPA 627 really sings"
The OPA627 really sings
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Old 1st December 2004, 12:54 PM   #26
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It does if you're looking in the right place, to my shock even the uncompensated opa637 has this symptom.

http://focus.ti.com/lit/ds/symlink/opa2134.pdf

page 5 "POWER SUPPLY AND COMMON-MODE REJECTION

vs FREQUENCY"
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Old 1st December 2004, 12:56 PM   #27
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Quote:
Originally posted by mandat
Try to follow the thread "The OPA 627 really sings"
The OPA627 really sings

Good thread for sure but it was more of a debate on if it was worth doing or not, "just tie a resistor to the negative rail", this thread has already gone into advanced detail about how to do it properly.
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Old 1st December 2004, 01:15 PM   #28
Electrons are yellow and more is better!
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Default Re: if your

Quote:
Originally posted by uli
psu is an issue in the circuit I would work on that.

For example the OP2134 (which I like very much) has no different specs for plus and minus rails.

Uli

uli, I'm afraid the difference is 30dB! between V- and V+..... but what does these figures mean if your power supply voltage has a good quality? I think the whole debate forgot about this? I think we never got a real answer on this issue. I think also that this class A is a bit overrated and is a typcial internet rumour more than an important circuit.
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Old 1st December 2004, 01:55 PM   #29
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Good point.

If you took the measures of a very ..... stiff? /clean supply, then I guess you could just concentrate on the best output device to use, and EF would make sense, but in the case of complimentary, NPN.

Never any simple answers now are there?

Is it overrated? Good question, maybe not for a 741 Maybe for a high quality audio op amp.
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Old 1st December 2004, 03:05 PM   #30
uli is offline uli  Austria
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Default Re: Re: if your

Quote:
Originally posted by classd4sure
It does if you're looking in the right place, to my shock even the uncompensated opa637 has this symptom.

http://focus.ti.com/lit/ds/symlink/opa2134.pdf

page 5 "POWER SUPPLY AND COMMON-MODE REJECTION

vs FREQUENCY"
oops!
youre right!


Quote:
Originally posted by peranders

uli, I'm afraid the difference is 30dB! between V- and V+..... but what does these figures mean if your power supply voltage has a good quality? I think the whole debate forgot about this? I think we never got a real answer on this issue. I think also that this class A is a bit overrated and is a typcial internet rumour more than an important circuit.

Again I agree, biasing class A is just icing the cake when everything else is perfect (->PSRR etc.)

Uli

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