Author Topic: Op Amp Unity Gain Stability  (Read 1604 times)

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Offline EteslaTopic starter

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Op Amp Unity Gain Stability
« on: May 31, 2019, 04:53:16 pm »
I want to use an opamp as a non inverting buffer. I know that a lot of op amps are not 'unity gain stable'. Is using an op amp as a non inverting buffer (gain of +1) considered using it at unity gain, or does unity gain in the context of 'unity gain stability' only refer to an inverting buffer where R1 and R2 are the same value? Thanks.
 

Offline Benta

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Re: Op Amp Unity Gain Stability
« Reply #1 on: May 31, 2019, 05:25:40 pm »
It refers to both +1 and -1 gain.
 
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Offline Wimberleytech

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Re: Op Amp Unity Gain Stability
« Reply #2 on: May 31, 2019, 05:40:28 pm »
^^^^ ditto what he said

See my comment below modifying this answer.
« Last Edit: May 31, 2019, 11:02:05 pm by Wimberleytech »
 

Offline magic

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Re: Op Amp Unity Gain Stability
« Reply #3 on: May 31, 2019, 09:49:15 pm »
I thought unity gain means exactly +1 and nothing else?

Gain of -1 doesn't actually require a unity gain stable amp, but one stable at 2 or more. It's noise gain that matters.
 

Online Zero999

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Re: Op Amp Unity Gain Stability
« Reply #4 on: May 31, 2019, 10:15:36 pm »
I thought unity gain means exactly +1 and nothing else?

Gain of -1 doesn't actually require a unity gain stable amp, but one stable at 2 or more. It's noise gain that matters.
Yes, that's true. It's the noise gain which matters. An op-amp which is not unity gain stable can easily be used as a buffer, by setting the noise gain to a high enough value. It does increase noise, but this can be mitigate somewhat by only having a high noise gain, at high frequencies, which cause oscillation. Noise is also less of an issue for a buffer, as the signal level is generally very high.

An application of this is connecting two audio amplifier ICs together to make a bridged output. A gain of -1 is required to make one side inverting, but many audio amplifier ICs need a much higher gain to be stable. The solution is to set the noise gain high enough to be stable, even through the signal gain is unity.

See the thread linked below for more information.
https://www.eevblog.com/forum/beginners/op-amp-spec-noise-gain-configuration/msg2401983/#msg2401983
« Last Edit: June 01, 2019, 07:55:27 am by Zero999 »
 

Offline Wimberleytech

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Re: Op Amp Unity Gain Stability
« Reply #5 on: May 31, 2019, 10:30:42 pm »
I thought unity gain means exactly +1 and nothing else?

Gain of -1 doesn't actually require a unity gain stable amp, but one stable at 2 or more. It's noise gain that matters.

Yes, you are correct.  Feedback factor (beta, or b) for an inverting amp with a gain of -1 is 1/2.  Thus when 1+Ab=0, A is 2 for b=1/2.
 


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