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Old 16th March 2010, 10:34 PM   #1
Artie is offline Artie  United States
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Default 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.
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Old 16th March 2010, 10:38 PM   #2
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Sure.

se
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Old 16th March 2010, 10:47 PM   #3
Artie is offline Artie  United States
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No downside?
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Old 16th March 2010, 10:49 PM   #4
Dr_EM is offline Dr_EM  United Kingdom
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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?
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Old 16th March 2010, 10:50 PM   #5
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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
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Old 16th March 2010, 10:52 PM   #6
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Quote:
Originally Posted by Artie View Post
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.
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Old 16th March 2010, 10:56 PM   #7
Artie is offline Artie  United States
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Quote:
Originally Posted by Dr.EM View Post
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.
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Old 16th March 2010, 11:03 PM   #8
jcx is online now jcx  United States
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"noise gain compensation"
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Old 16th March 2010, 11:20 PM   #9
Artie is offline Artie  United States
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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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