Virtual unity gain using voltage divider?

Artie · 16th March 2010, 11:34 PM

This is something I've been wondering for some time: I notice that some op-amps are not specified to be unity gain stable. Couldn't you just set the gain to maybe 5 or 10, and then run the output to a matching 5 or 10 resistive voltage divider for a "virtual" unity gain effect?

Seems too simple. I'm probably missing something.

Steve Eddy · 16th March 2010, 11:38 PM

Sure.

se

Artie · 16th March 2010, 11:47 PM

No downside?

Dr.EM · 16th March 2010, 11:49 PM

You could, but of course you'll increase the effective output impedence by doing this, making systems like active filters troublesome to design. In fact, most systems/stages requiring a unity gain are going to be severely restricted by having this increased output impedence, the fix for which is a unity gain buffer following it (making it totally irrelevant!)

I should ask specifically what it is you are trying to do, what this unity gain application is?

Steve Eddy · 16th March 2010, 11:50 PM

Well, you'll get a bit of additional noise due to the excess gain, and the voltage divider (just as with a regular attenuator) will result in the chip being driven from a bit higher source impedance which will increase distortion a bit.

Don't know that I'd lose much sleep over it though.

Why not use a unity gain stable chip like the OPA-549?

se

cliffforrest · 16th March 2010, 11:52 PM

Quote:

Originally Posted by Artie

This is something I've been wondering for some time: I notice that some op-amps are not specified to be unity gain stable. Couldn't you just set the gain to maybe 5 or 10, and then run the output to a matching 5 or 10 resistive voltage divider for a "virtual" unity gain effect?

Seems too simple. I'm probably missing something.

Yes, but why bother?

It is not the amp design itself which has a specific stability, but how much internal compensation is applied.

I am not sure there are any op-amps available which do not have a version which is unity gain stable.

So use that one.

Artie · 16th March 2010, 11:56 PM

Quote:

Originally Posted by Dr.EM

I should ask specifically what it is you are trying to do, what this unity gain application is?

Nothing specific. I started thinking about it partly because I have some older, high-power, non-unity gain stable opamps in my "junkbox".

Thanks guys for the responses. You brought up some points I hadn't considered.

jcx · 17th March 2010, 12:03 AM

"noise gain compensation"
Google

Artie · 17th March 2010, 12:20 AM

Good reading. One of the first links listed took me to app note AN-20 from Apex Corp. One of the chips I was wondering about is an Apex PA02 high-power opamp. (5A output capability.)

Thanks.

16th March 2010, 11:34 PM	#1
Artie diyAudio Member Join Date: Jul 2007	Virtual unity gain using voltage divider? This is something I've been wondering for some time: I notice that some op-amps are not specified to be unity gain stable. Couldn't you just set the gain to maybe 5 or 10, and then run the output to a matching 5 or 10 resistive voltage divider for a "virtual" unity gain effect? Seems too simple. I'm probably missing something.

16th March 2010, 11:49 PM	#4
Dr.EM diyAudio Member Join Date: Oct 2006 Location: Swindon	You could, but of course you'll increase the effective output impedence by doing this, making systems like active filters troublesome to design. In fact, most systems/stages requiring a unity gain are going to be severely restricted by having this increased output impedence, the fix for which is a unity gain buffer following it (making it totally irrelevant!) I should ask specifically what it is you are trying to do, what this unity gain application is? __________________ My music page 3-Way system build thread

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16th March 2010, 11:38 PM	#2
Steve Eddy diyAudio Member Join Date: Sep 2002 Location: Sacramento, CA	Sure. se

16th March 2010, 11:47 PM	#3
Artie diyAudio Member Join Date: Jul 2007	No downside?

16th March 2010, 11:50 PM	#5
Steve Eddy diyAudio Member Join Date: Sep 2002 Location: Sacramento, CA	Well, you'll get a bit of additional noise due to the excess gain, and the voltage divider (just as with a regular attenuator) will result in the chip being driven from a bit higher source impedance which will increase distortion a bit. Don't know that I'd lose much sleep over it though. Why not use a unity gain stable chip like the OPA-549? se

17th March 2010, 12:03 AM	#8
jcx diyAudio Member Join Date: Feb 2003 Location: ..	"noise gain compensation" Google

17th March 2010, 12:20 AM	#9
Artie diyAudio Member Join Date: Jul 2007	Good reading. One of the first links listed took me to app note AN-20 from Apex Corp. One of the chips I was wondering about is an Apex PA02 high-power opamp. (5A output capability.) Thanks.

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