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[SOLVED] On amplifier circuit with closed loop gain less than 1

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hyleeinhit

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Hi,

Is there anyone who ever built such a kind of circuit with amplifier:
The closed loop gain is lower than 1 (ex:0.1 or 0.01).

How to select the amplifier phase margin? What is the specific relation between the closed gain and phase margin? Thanks.

I know that for closed loop gain larger or equal to 1, the phase margin should be 60 or 45 degree.
 

Inverting or non-inverting?
* Inverting: No problem with A=-R2/R1, phase margin will be smaller of course (critical)
* Non-inverting: Why not simply use a resistive voltage divider and a unity gain buffer in series?

- - - Updated - - -

What is the specific relation between the closed gain and phase margin?

There is no general formula that contains this relation as it strongly depends on the particular opamp characteristic.
 
LvW said:
Inverting or non-inverting?
* Inverting: No problem with A=-R2/R1, phase margin will be smaller of course (critical)
* Non-inverting: Why not simply use a resistive voltage divider and a unity gain buffer in series?

- - - Updated - - -

What is the specific relation between the closed gain and phase margin?

There is no general formula that contains this relation as it strongly depends on the particular opamp characteristic.
Click to expand...

You gave a really good point with a non-inverting structure.

Actually, I set up a simple test with LM741 operated under 10V. The closed loop gain was at a very small value and input amplitude is 3V. The output can be as low as 5mV regardless of the noise without oscillation. It turns out that the phase margin of LM741 is enough for low closed gain operation although I have no idea about the exact value (datasheet does not show it).
 

hyleeinhit said:
It turns out that the phase margin of LM741 is enough for low closed gain operation although I have no idea about the exact value (datasheet does not show it).
Click to expand...

The "exact value" (as exact as possible using the macro models available) can be found via loop gain simulation.
 
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