Inverting amp + Class A bias. is it possible or not?

Archwn · 31st July 2007, 04:59 PM

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

KP11520 · 31st July 2007, 10:57 PM

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 .....

KP11520 · 31st July 2007, 11:07 PM

Does this help: http://home.no.net/andiha/articles/audio/cascamp.htm

darkfenriz · 1st August 2007, 12:03 AM

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...

AndrewT · 1st August 2007, 10:16 AM

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.

KP11520 · 1st August 2007, 01:39 PM

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

Archwn · 1st August 2007, 03:53 PM

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

31st July 2007, 04:59 PM	#1
Archwn diyAudio Member Join Date: Oct 2006	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

31st July 2007, 10:57 PM	#2
KP11520 diyAudio Member Join Date: Apr 2007	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 ..... __________________ If Wile E. Coyote had enough money to buy all that ACME ****, why didn't he just buy dinner?

31st July 2007, 11:07 PM	#3
KP11520 diyAudio Member Join Date: Apr 2007	Does this help: http://home.no.net/andiha/articles/audio/cascamp.htm __________________ If Wile E. Coyote had enough money to buy all that ACME ****, why didn't he just buy dinner?

1st August 2007, 10:16 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

1st August 2007, 01:39 PM	#6
KP11520 diyAudio Member Join Date: Apr 2007	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 __________________ If Wile E. Coyote had enough money to buy all that ACME ****, why didn't he just buy dinner?


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1st August 2007, 03:53 PM	#7
Archwn diyAudio Member Join Date: Oct 2006	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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