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Old 16th December 2003, 12:39 AM   #1
matjans is offline matjans  Netherlands
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Default biasing opamps in class a

hi guys,

with my limited knowledge of electronics i haven't really been able to find out whether it's possible to bias an opamp into class a without using a buffer.

If it's not possible: o well, i'll let'er switch. If it is; can anybody give me an example?
I'm building a opa2132 / 627 / 637 based linestage (without a buffer, obviously).
if i get it to work nicely i might mod an old arcam dac...
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Old 16th December 2003, 01:27 AM   #2
Ilianh is offline Ilianh  Canada
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hum... of course.

Looking at a inverting topology, having the input on end of the feedback resistors and the non inverting input is usualy to ground..

Consider that the opamp is trying to keep a difference of potential of 0 volts across its inputs, that means the out equal :

Out = -((((input - 0v)*R2)/R1)- 0V)

So, what happens to the formula if you apply a voltage to the Non inverting input?
5v for eg;

Out = -((((input - 5v)*R2)/R1)-5V)

The signal is inverted and 10v above 0v.

Heres a drawing.
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Old 16th December 2003, 01:50 AM   #3
matjans is offline matjans  Netherlands
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ah. That would mean a complete redesign of my amp. as i'm (basically) using this design.

so how does it work here ?
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Old 16th December 2003, 01:56 AM   #4
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rather than tying R1 to ground, tie it to a voltage source then you are done!

I am not convinced on the other hand you will hear any difference,
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Old 16th December 2003, 03:58 AM   #5
GregGC is offline GregGC  Canada
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Quote:
Originally posted by Ilianh
hum... of course.

Looking at a inverting topology, having the input on end of the feedback resistors and the non inverting input is usualy to ground..

Consider that the opamp is trying to keep a difference of potential of 0 volts across its inputs, that means the out equal :

Out = -((((input - 0v)*R2)/R1)- 0V)

So, what happens to the formula if you apply a voltage to the Non inverting input?
5v for eg;

Out = -((((input - 5v)*R2)/R1)-5V)

The signal is inverted and 10v above 0v.

Heres a drawing.

I think he has in mind adding a resistor between Out and -V, to bias the output stage of the opamp in Class A.

If you do bias one of the inputs, you'll get large DC at the output (if the gain is valid for DC it'll even saturate and nothing's going to come out of that opamp) and I'm not sure that's what you want (DC at the output doesn't mean that the output stage works inclass A).
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Old 16th December 2003, 04:16 AM   #6
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This sort of stuff?
http://tangentsoft.net/audio/opamp-bias.html

I'm biasing the buffer into class A too. Works fine.


JF
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Old 16th December 2003, 09:03 AM   #7
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The easy way is with a resistor from output to the - rail.
Depending on the op-amp used, you may or may not hear any difference.

matjants, are you gonna use those op-amps at +/- 9 volts?
Double the voltage, and double the pleasure.
At least +/- 15 volts.
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Old 16th December 2003, 09:19 AM   #8
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Default Re: biasing opamps in class a

Quote:
Originally posted by matjans
hi guys,

with my limited knowledge of electronics i haven't really been able to find out whether it's possible to bias an opamp into class a without using a buffer.
... yes, but you can't get more than 1-3 mA or so as max output current. This technique is best if you use it together WITH a buffer. Without this OPA627 has 0.00003% dist!
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Old 16th December 2003, 09:59 AM   #9
matjans is offline matjans  Netherlands
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i think i'll just give it a try and bias it at 0.4 ma or something. we'll see.

carlosfm: what do you mean double the pleasure ?
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Old 16th December 2003, 11:24 AM   #10
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Quote:
Originally posted by matjans
i think i'll just give it a try and bias it at 0.4 ma or something. we'll see.

carlosfm: what do you mean double the pleasure ?
This means that the load can't take more than 0.35 mA or so.
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