loop gain and closed loop gain

[engrvip]

would like to know that whether 3db BW and UGB in unity feedback configuration measured for loop gain are same as that for closed loop configuration.

for example in my current f/b amplifier(CFA) in unity feedback configuration i am expectiong 3db BW to be around 100meg but when i do loop stability anaysis and measure only loop gain i get dc gain of 40db and 3 db BW of 120khz.

So what should i believe to be correct in arriving at my BW figure : closed loop gain analysis or loop gain analysis????

 

[spbhu]

For stability consideration, loop gain is always measured, whose phase margin and gain margin will be used to give the specifications of the stability.
Close loop unity-gain frequency is of course different from the 3db bandwidth of the loop gain, if you really want to know their relationship. You may need to derive it.
For a non-inverting unity feedback configuration, the unity gain frequency of your closed loop apporximately equals to the loop gain unity gain frequency.
If they different too much from each other, then you method of meausring loop gain might have some problem...

 

[sengyee88]

Closed loop gain = Aol/(1+βAol), where Aol is open-loop gain
Loop gain = β

They are two different thing.

 

[strennor]

Hi,

Beta is the feedback gain?
Beta * Aol is the loop gain?

 

[sengyee88]

Yup, you are correct.
My mistake.

 

[engrvip]

so is 3db BW and UGB for closed loop transfer function equal to those of loop gain tranf. function????

 

[wanily1983]

u can think that open loop and closed loop have the same UGB, the 3dB BW of the closed loop is related to the closed loop gain.

 

[sengyee88]

so is 3db BW and UGB for closed loop transfer function equal to those of loop gain tranf. function????

UGB of loop gain = 3dB closed loop

 

[zenisle]

open loop gain is decided by feedback network

 

[qiushidaren]

loop gain: beta*A
closed loop gain: A/(1+beta*A)

Added after 2 hours:

Hi,

Beta is the feedback gain?
Beta * Aol is the loop gain?

Yes! β is the feedback factor.

Added after 5 minutes:

so is 3db BW and UGB for closed loop transfer function equal to those of loop gain tranf. function????

UGB of loop gain = 3dB closed loop

I don't agree with you at this point, UGB of loop gain = βAoωo, but 3dB closed loop = (1+βAo)ωo.
If it is not correct, please let me know.

 

[engrvip]

cloesd loop gain 3db BW, Fcl = Fo ( 1 + A.B) which can be approx. to Fcl = Fo. A.B , where A.B is loop gain anf Fo is BW for open loop gain which is equal to loop gain BW.

hence it proves 3dB BW for closed loop gain = UGB fro loop gain.