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A puzzle about negative feedback

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youyang

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puzzle in feedback

Assume A>0
Do both systems form a negative feedback?
If yes, then how explain that they have different transfer functions, and why some books require the gain in system2 is –A to ensure the loop is negative feedback and have the same transfer function as system1?
If not, then how explain they have the same function in terms of letting v2 to track v1, as long as A is big enough.
Additional, the problem puzzles me is that if we have a system configuration like system1, then should we care whether the gain, A ,is positive or negative to say this system is negative feedback?
Any reply is appreciated, thx!
 

Re: puzzle in feedback

the first is negative feedback and the second is positive. the criterion is the whether the feedback signal plays the same function with vi or not
 

Re: puzzle in feedback

Hello there,

First is negative feedback as the output (second inp) tracks the input(source follower).second is +ve feedback if A is +ve and the output is in saturation. if A is negative then the feedback is 180 deg. out of phase and so the feedback is negative and it will surely track the inp if mag(A) is large.

hope u got that.
 

Re: puzzle in feedback

I think the first is negative feddback, and the second is positive feedback. To check whether feedback is negative or not, as long as compare the phase of output with input in the feedback stage. The output of the first system is out of phase 180 with input, in contrast the output phase of the second is the same as the input.
 

puzzle in feedback

i think u may try it ,
system1:
at first o/p is zero then Vousummer=V1 >> V2=A*V1 >> Voutsummer=V1-(A*V1) and so on , note i/p is decreasing.

system2:
at first o/p is zero then Vousummer=-V1 >> V2=-A*V1 >> Voutsummer=-V1+(-A*V1) and so on , note i/p is increasing.
 

puzzle in feedback

Hi youyang,

For distinguishing whether the feedback is positive or negative:
Open the loop at some point. In this way you get an input and an output. Calculate the gain from that input to that output, regardless of the "true" input and output of the system. The sign of this gain is the sign of the feedback.
Feedback is negative in system 1 and positive in system 2.
Regards

Z
 

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    youyang

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