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Negative feedback in Op-Amps

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brahma

Advanced Member level 4
feedback...

i started studying negative feedback in opamp, there it is given negative feedback increases stability of the amplifier and also given β*a > 1, where a is the openloop gain, then in positive feedback by nyquist criteria β*a <=1, if this condition is not statisfied then the system is unstable. i dont understand this, anybody could make me clear...

Re: feedback...

brahma,

with negative feedback, the gain of the operational amplifier circuit will depend solely on the feedback gain and independent of the open-loop gain of the opamp.

I suggest that you take a look at this online material on operational amplifiers

Negative Feedback
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opamp2.html#c1

Positive Feedback
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/posfe.html#c1

I hope that you find the material helpful.

Best regards,
v_c

feedback...

For Negative feedback you can read
Design with Op Amp and Linears integrated Circuits by Sergio Franco.

For Positive feedback you can see
Microelctronics Circuits by Rashid

Re: feedback...

v_c said:
brahma,

with negative feedback, the gain of the operational amplifier circuit will depend solely on the feedback gain and independent of the open-loop gain of the opamp.

I suggest that you take a look at this online material on operational amplifiers

Negative Feedback
h**p://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opamp2.html#c1

Positive Feedback
h**p://hyperphysics.phy-astr.gsu.edu/hbase/electronic/posfe.html#c1

I hope that you find the material helpful.

Best regards,
v_c

But i saw A1(with feedback) = A /(1 + Aβ),
A - openloop gain.
I presume, you understand that in positive feedback i am talking abt oscillators...

Re: feedback...

If you make Aβ=-1 you will get oscillations.

h**p://hyperphysics.phy-astr.gsu.edu/hbase/electronic/posfe.html#c1

The whole idea is to design a feedback gain β that will make the product Aβ=-1 at the frequency you want the oscillator to operate at. Does this answer your question?

Best regards,
v_c

Re: feedback...

brahma said:
But i saw A1(with feedback) = A /(1 + Aβ),
A - openloop gain...

No,
"A" is a gain of the OpAmp itself ("forward" gain ;-)),
β is the feedback circuit gain ("backward" gain ;-)) and
βA is the open loop gain, see the enclosed picture.
The cross indicates the point where the loop is open.
Note: don't be confused with the sign of β!

Regards,
Eric

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