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loop gain phase margin question

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mike_bihan

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when we are simulating loop-gain to find the phase-margin of the closed loop, we are actually

simulating a weakened loop.

Since the op-amp in the loop tweaked its output to reach the loop stable condition, its gain will probably be reduced significantly. The loop looks much more stable. Is this real?

How we approach this problem?

Thanks
 

The gain of op-amp wont be reduced even in a loop. Whether the loop is stable or not is decided by the stable condition (amplitude & phase of loop gain), not just the gain of the op-amp.
 

loop gain will weak some gain when we test the phase margin.

As far as a op-amp is concerned, its product of Gain and Bandwidth is
determined by the circuit.
 

lunren said:
The gain of op-amp wont be reduced even in a loop. Whether the loop is stable or not is decided by the stable condition (amplitude & phase of loop gain), not just the gain of the op-amp.
Click to expand...

True.

But, I am thinking about that when the loop is disturbed a lot far from the stable point, and, the op-amp's gain increased due to the change of the Operating Point. The stability of the loop is hence reduced significantly. Small signal simulation never shows it.
 

Perhaps the output voltage is not into the ICMR,
some MOS are working at linear region when you put the output as positive feedback, the OP-AMP's open loop gain will reduce.
 

Hi mike_bihan,

when a feedback circuit is analyzed, the load effect of the feedback network must be included in the calculation of the open loop response. The feedback network affects both the input and the output of the open loop transfer function. This is readily considered suppressing the feedback at the input for looking at its load effect at the output, and conversely for the input.
This is well explained in an oooold books of the SEEC series (I guess in volume 5) and in the Gray-Searle book, but certainly there must be also in other bibliography. Look for the four topologies of feedback circuits.
Regards

Z
 

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