Newbie question. Is there any difference between opamp in unity gain configuration (buffer) and opamp in non-inverting configuration with gain of 2? Is there output impedance difference etc?

 

Many opamps are not stable in a unity gain/voltage follower configuration; you need to read the datasheet (s) for the opamp(s) in question - so no; they are not the same.

 

The gain of 2 circuit bandwidth will be 1/2 the unity gain configuration (equal to 1/2 the GBWP of the op amp). There may also be some peaking in the frequency response from stray capacitance at the inverting input node.

The closed-loop output impedance will be higher than in the unity gain configuration, but will still be very low.

Any op amp input offset will be multiplied by a factor of 2.

I believe those are the primary differences.

Edit: The above assumes the op amp is stable in those configurations, as SW noted.

 

Yes, the input impedance is increased by a factor of the closed loop gain

Rin = Rin*Acl

And output impedance is decreased by the same factor:

Rout = Rout/Acl

 

Yes, the input impedance is increased by a factor of the closed loop gain

Rin = Rin*Acl

And output impedance is decreased by the same factor:

Rout = Rout/Acl

Might be worth double checking that.....

 

I remembered it from the class I learned it in not that long ago, and it's verified here:

http://opencourseware.kfupm.edu.sa/colleges/ces/ee/ee303/files\3-Lesson_Notes_Lecture_11.pdf

Pages 5-7. I Googled "non ideal op amp" to find it.

 

The input impedance of a typical op amp is already so large that any increase due to the closed loop gain in a non-inverting amp configuration is generally of little practical significance.

 

Hi Veracohr,

What does Acl mean in your earlier post?

 

Hi Veracohr,

What does Acl mean in your earlier post?

I assume Close Loop Gain.

 

I assume Close Loop Gain.

Most likely. The issue is with the input / output resistance relationships with respect to that closed loop gain - as quoted by Veracohr in post #4.

 

I assume Close Loop Gain.

Yes that's what I meant.

Most likely. The issue is with the input / output resistance relationships with respect to that closed loop gain - as quoted by Veracohr in post #4.

I see now. Not the closed loop gain, but the loop gain (1+Aβ), where A is the op amp's open loop gain and β is the feedback network factor R1/(R1+R2). Correct? Sorry for giving out misinformation.

 

Yes that's what I meant.



I see now. Not the closed loop gain, but the loop gain (1+Aβ), where A is the op amp's open loop gain and β is the feedback network factor R1/(R1+R2). Correct? Sorry for giving out misinformation.

No need to apologize. Simple mistake. BTW the loop gain is usually taken to be Aβ. Some people include a negative sign which probably pre-supposes negative feedback is being applied.

 

No need to apologize. Simple mistake. BTW the loop gain is usually taken to be Aβ. Some people include a negative sign which probably pre-supposes negative feedback is being applied.

Perhaps not too important in the context of this thread - nevertheless I like to mention that the loop gain is the gain of the COMPLETE loop, that means: including any sign inversion within the loop. Remember that there are circuits with another active inverting element within the loop - and in this case, the loop is closed using the non-invering input node of the main amplifier.
Thus, the loop gain usually should be taken to be (-Aβ).

I think, this consideration is important because very often this is the background of some misunderstandings in finding stability margins:
Is it necessary to investigate the loop gain at a the frequency with -180deg or with -360deg phase shift ?

 

Thanks guys for answers. The pratical appliacion for the question was about buffer before Linkwitz-Riley 24db/oct filter stages. Do I need add an extra unity gain buffer before subsonic filter stage or do balanced input stage will manage right output impedance etc? Schematic attached.

 

That stage is fed by an op amp, I don't see why a buffer would be necessary. The op amp has sufficiently low output impedance.