Inverting amp + Class A bias. is it possible or not? - diyAudio
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Old 31st July 2007, 03:59 PM   #1
Archwn is offline Archwn  United Kingdom
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Default Inverting amp + Class A bias. is it possible or not?

As we know the non inverting amp can be biassed by putting out some current via a resistor(the easiest way) from the output to V-

I've been searching for a while(and I admit that my skill is so decent ) and can not find any information on biasing the inverting amp into (more) class A. So I'm afraid that this is doable or not? please help
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Old 31st July 2007, 09:57 PM   #2
KP11520 is offline KP11520  United States
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Hey Archwn,

What you are looking to do (the best way) is called a "JFET Cascode." You can do a search right here on this site and find some audio only related info and even more but not only audio on Google.

Good Luck!

Regards//Keith

BTW, I have spent weeks researching this subject and .....
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Old 31st July 2007, 10:07 PM   #3
KP11520 is offline KP11520  United States
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Does this help: http://home.no.net/andiha/articles/audio/cascamp.htm
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Old 31st July 2007, 11:03 PM   #4
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Default Re: Inverting amp + Class A bias. is it possible or not?

Quote:
Originally posted by Archwn
As we know the non inverting amp can be biassed by putting out some current via a resistor(the easiest way) from the output to V-

I've been searching for a while(and I admit that my skill is so decent ) and can not find any information on biasing the inverting amp into (more) class A. So I'm afraid that this is doable or not? please help

You can use the same trick on both inverting or non-inv. to force a class A operation. I have no idea what are the others talking about...
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Old 1st August 2007, 09:16 AM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
Looking at it, the resistor or CCS pulls bias current through the upper half of the quasi output stage, shutting down the lower half.

Whether the output is in phase or out of phase with the input should not make any difference. The output is still working single ended.
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Old 1st August 2007, 12:39 PM   #6
KP11520 is offline KP11520  United States
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Hey Archwn,

It looks like I wasn't giving any useful advice. I was confusing op amp biasing with your situation. Obviously, I am still learning. I am sorry about that.

I am glad Andrew and darkfenriz came to help! Good luck!

Regards//Keith
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Old 1st August 2007, 02:53 PM   #7
Archwn is offline Archwn  United Kingdom
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Thanks Keith, darkfenriz and Andrew

Keith: no worries mate. at least I know your point was on the better way to bias the opamp rather than using a resistor. I'm still learning too. I'm now looking into CCS using fet or LM317/LT1085 regs and every comments are at most welcome.

Now I dare to do it. Even the mechanism Andrew explained is far from me to understand, I would take the result then.

I will report if there is or is not a problem.

Cheers,

Archwn
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