MOS Amplifier Circuits with Feedback
Basic question: What is the meaning of the term "feedback"?

Answer: We have feedback when (a part of) the output signal is coupled back to the input. Therefore, at first we need a definition of the output signal. 

(a) Answer to (a) :
Well, the answer is not too evident - as far as the given example (first circuit) is concerned. For a decision - feedback current-controlled or voltage-controlled - it is necessary to know if the output signal is defined  as (a) a voltage across a certain load which is connected between the output node and ground or (b) as a current through a certain load.  This seems not to be 100% clear with the shown circuit (because the output quantity is bot mentioned explicitely), however, we can see a resistor named RL (load?) - and this is an indication that the output quantity is a CURRENT (Io) resp. the corresponding voltage across this resistor (and NOT the collector voltage of the pnp transistor, see also the small circled nodes across this resistor RL). 

This observation allows the conclusion that the output current Io (which is nearly independend on the value of RF (BJT is acting as a voltage-controlled current source) is "sampled" - that means: A voltage proportional to the output current Io is fed back to the input of the whole circuit.

I think, it is not a problem to verify that we have, indeed, negative feedback (output current-controlled voltage feedback). 

In the second circuit, it is clear that the output quantity is the voltage Vo and that we have, therefore, voltage-controlled current feedback. 

(b) Answer to (b): A "mixing" of two currents (input and feedback current) is performed when two currents meet in a common node. I think, it is not too difficult to see how this happensat the input node of the two last circuits.

(c) I do not understand this question. Either a current (or a voltage derived from this current) is sampled - or it is mixed with another current. A combination of these two effects does not take place.



COMMENT 1: A decision if we have current- or voltage-controlled feedback is possible also applying the rules for calculating input or output resustances.

(1) for a voltage (or current) controlled feedback (sampling the output voltage or the output current) the output resistance goes down (or high); 

(2) for a combination of voltages (or currents) at the input the input resistance goes high (or low).     

COMMENT 2: For improving the undestanding of the different feedback schemes, it is helpful to realize the following: For the first circuit, without any change of the parts arrangements, we would have voltage-controlled feedback (output voltage sampling) when the source of Q1 is connected directly to the drain of Q2.    



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