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How should I check whether the given configuration has a negative or positive feedback. I am stuck because, I know that as Vin increases the drain current will increase but after that I don't know how to proceed.

So please explain how should I proceed with this?

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2 Answers 2

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There are three sets of negative feedback in this circuit; one associated with M3 (i.e. the connection from drain to gate), one associated with M2 via R1 and R2 and a subtler one associated with M1 and M2 sharing a common source connection.

How you would recognize them is a matter of experience and learning.

The most basic method is to think about the MOSFETs as having an inverting voltage gain between gate and drain. This then explains M3's local feedback and M2's local feedback.

As for M1's feedback, Vout increases due to Vin increasing and this in turn causes M2 to try and conduct more (due to its local neg feedback via R1 and R2). This then has the effect of lifting both M1 and M2 sources and reducing the drain current of M1 and this of course is effectively a reduction in the real gate-source voltage of M1 and hence is equivalent to Vin reducing. It's is negative feedback.

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The upper part functions as a current mirror, and therefore as Vin increases so does the drain current of M3 and M4.

Now consider the lower part. As the drain current of M1 increases, that of M2 decreases, since they need to add up to ISS.

The current going through R1 and R2 is determined as the current drain current of M4 subtracting that of M2. Now that the former is increasing and the latter is decreasing, the current through the output node increases, hence Vout and Vx.

If Vx is raised, the drain current of M2 also rises, which is in opposite direction as the influence of Vin. So the feedback is negative.

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