stability feedback connection physics

yef smith

Joined Aug 2, 2020
689
Hello,the component shown bellow is supposed to spabilise the gain over frequency range.
Is there some physical explanation i can use to see the this effect in mathematical way?
Thanks.

Papabravo

Joined Feb 24, 2006
20,990
Feedback allows you to trade gain for bandwidth. Every amplifier has a gain bandwidth product.
The gain usually starts out as some large number which you can reduce to some level in exchange for increased bandwidth.
This happens in mechanics every time you have a product of two things. One of my favorite examples is the horsepower. The definition is the ability to do 550 ft-lbs of work in 1 second. I know a fair number of horses that can lift 550 lbs, by 1 foot in 1 second, but I've never seen one that could lift 1 lb, by 550 ft. in one second.

Wolframore

Joined Jan 21, 2019
2,609
The series RLC component(s) has a resonant frequency of approximately 800 MHz, it also has a low Q, the negative feedback reduces gain to your amplifier keeping the signal within a reasonable range which could otherwise go into cutoff.

yef smith

Joined Aug 2, 2020
689
Hello Wolframore,How do you see that we hav negative feedback?
if we do the barcuasen condition for stability formula,how do we see the Q and resonast frequency effect on the stability?
Thanks.
The series RLC component(s) has a resonant frequency of approximately 800 MHz, it also has a low Q, the negative feedback reduces gain to your amplifier keeping the signal within a reasonable range which could otherwise go into cutoff.

yef smith

Joined Aug 2, 2020
689
Hello , the general rule for stability is the barkausen stability criteria as shown bellow.
Here we have a negative feedback ,the only thing i know that when AB=-1 we are going to have oscilations
given the expression bellow Why exactly at resonance frequency of the loop RLC we will get a more stable signal then at other frequencies?

how do i know the beta value in the real circuit?
Thanks.

Wolframore

Joined Jan 21, 2019
2,609
First your circuit is not complete and just a block diagram, the JFET is not labeled so we have to make some assumptions, a voltage at the gate should allow the JFET to conduct. The biasing is missing but generally that would cause the output signal to go low, since this is being fed back through your RLC components it’s called negative feedback. Is that enough or do we need to go into the math?

your gain is frequency dependent and complex

Is this for homework?

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