Disagreed. Referring to your latest test circuit in post #13, Middlebrooks method is applied correctly, except for a wrongly calculated loop gain which should be Vfeedback/Vfb rather than Vout/Vfb. The complete Middlebrook circuit must be applied, if Vfb is significantly loaded, which is obviously not the case here.The phase margin test circuit (middlebrook method) that I am using now is still incorrect.
I have not included the test current injection to be included in the middlebrook AC analysis (loop gain and phase margin)
Middlebrooks method is applied correctly, except for a wrongly calculated loop gain which should be Vfeedback/Vfb rather than Vout/Vfb.
The complete Middlebrook circuit must be applied, if Vfb is significantly loaded
Because you wan't to measure loop gain, read Middlebrook method explanations or compare the #13 schematic with the Middlebrook circuits you have posted in your other thread. For the voltage injection method, loop gain is the ratio of voltages measured at both sides of the injection source.Why Vfeedback/Vfb ?
Your posts give the impression that have thoroughly read about loop gain measurement methods. It seems to me however that you still have problems to grasp the basic idea behind it. To quote the original paper R. D. Middlebrook, Measurement of loop gain in feedback systems:Why so ?
For the voltage injection form of the loop-gain measurement method to give the correct result, it is necessary that the injection be performed at a point in the loop at which the impedance Z2 looking 'backward' from the injection point is sufficiently smaller than the impedance Z1 looking 'forward' from the injection point. The opposite condition, Z2>>Z1 is necessary for the current injection technique to give a correct result. Since in a practical system it may not be possible to find an inject.ion point that satisfies either of these extreme conditions, at least over the entire frequency range of interest, it is desirable to extend the loop-gain-measurement method to be applicable at an injection point where the impedance ratio Z2/Z1 is arbitrary.
Problem of this circuit isn't wrong loop gain analysis but unsuitable amplifier topology with excessive poles in the first stage.
Middlebrooks method is applied correctly, except for a wrongly calculated loop gain which should be Vfeedback/Vfb rather than Vout/Vfb.
See my previous post #11.Which exact mosfet or circuit node in the first stage contributes to excessive poles ?
However, I do not know why LTSpice does not allow the syntax "v(Vfeedback)/v(Vfb)" to be used ?
I don't see zeros but a relative low frequent pole in the input stage resulting in insufficient phase margin. Reason is the low current mirror transistors M11 and M17.
LTSpice does not allow the syntax "V(Vfeedback)/V(Vfb)" to be used
Look at the bias current e.g. of M11 compared to M12. I presume it's not intentionally that low.What do you exactly mean by low current mirror transistors ?
M11/M17 bias current becomes even smaller then.If I make M11, M12, M16 and M17 to have the same m=8 , then see the following bode waveform :
For the voltage injection form of the loop-gain measurement method to give the correct result, it is necessary that the injection be performed at a point in the loop at which the impedance Z2 looking 'backward' from the injection point is sufficiently smaller than the impedance Z1 looking 'forward' from the injection point. The opposite condition, Z2>>Z1 is necessary for the current injection technique to give a correct result. Since in a practical system it may not be possible to find an inject.ion point that satisfies either of these extreme conditions, at least over the entire frequency range of interest, it is desirable to extend the loop-gain-measurement method to be applicable at an injection point where the impedance ratio Z2/Z1 is arbitrary.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?