Question about using feedback resistor on FET

[ssayan79]

Hi to all
in one of my project i had to use a feedback resistor to regulate the gain of a fet.
This resistor is placed between drain and gate and it effectively reduces the gain of my device because it introduces a negative feedback. My problem is that i don't know why this connection, i.e. between gate and drain, introduce a negative feedback and not, for example, a positive feedback. Could anyone give me a link or some article where this concept is explained?
Thank you

 

[lladnar23]

Feedback on FET

If the transfer function from gate to drain is inverting (common source amplifier) then the signal being fed back is negative.

 

[ssayan79]

Re: Feedback on FET

Thank you very much...but at this point my question is:
where i could find the concept you said?
Does some book exsist where it is better explained?
Thanks a lot

 

[FvM]

Re: Feedback on FET

Hello,

I think any textbook covering semiconductor circuits should have an explanation. It's basically necessary to understand the operation of an one-stage common source amplifier. The relations aren't specific to FET, they are similar for BJT or even valve amplifier.

Regards,
Frank

 

[massive]

Re: Feedback on FET

Hi ssayan79,
I think I have seen this simple amp in the book CMOS Circuit Design, Layout, and Simulation, by R. Jacob Baker. There it was presented, but could not remember if the explanation of the circuit was detailed. You could check. The book is also available in the forum as far as I remember.
Best regards,

 

[xuedashun]

Re: Feedback on FET

when NO feedback res (Rf)
gain=gm*(Rload//ro)

with Rf
gain''=(gm-1/Rf)*(Rload//ro//Rf)

if Rf--> very large, gain''=gain=gm*(Rload//ro)
if Rf-->0, gain''=1

 

[zhlc3]

Feedback on FET

the type of feedback depends on the fet transfer function and resistor value. a simple parallel resistor will reduce the amplifier gain,just like xuedashun, but can't regulate the gain.