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-   -   Virtual unity gain using voltage divider? (http://www.diyaudio.com/forums/chip-amps/163302-virtual-unity-gain-using-voltage-divider.html)

Artie 16th March 2010 11:34 PM

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.

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!) :cannotbe:

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 (Post 2121656)
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 (Post 2121675)
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. ;)


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