loop gain vs closed loop gain

[cjron]

Hi All, I have a big doubt while reading many technical papers regarding amplifier gain.Often I come across the word 'LOOP GAIN' denoted by
(Aβ). is the term Loop gain mean the gain of the closed loop system(system with feedback) or it is different from that??

thx

 

[grizedale]

yes its bad choice of terminology from the world of control engineering.

Loop gain as i portray it just means the gain going round the loop, but with the loop not yet closed.

so it considers the gain going through all the loops facets, but with it not just yet closed.

you examine the loop gain as prior to your decision to close the loop.

When you finally understand control loop mathematics in full, some huge institution will employ you and pay you a fortune, and you will be part of that tiny group of people who really understand it.

Meanwhile, you will be with the rest of us, such as my old company in the south of uk, where we could not find a control engineer to help us with controlling a multi-smps system....because no matter how much money they offered...nobody wanted the job who could do the job.

 

[barry]

As Grizedale explained, Loop gain is gain all the way around the loop (through the feedback network) without closing the loop. Just to add more confusion, it's not the same as 'open loop gain' which is just A.

Open loop gain=A
Loop gain=Aβ
Closed loop gain=Aβ/(1+Aβ)

And that's why I don't work in controls.

 

[FvM]

I assume, that the expressions for loop gain and closed loop gain are referring to a feedback amplifier structure corresponding to the below diagram. The closed loop should be corrected to
A/(1+Aβ), approaching 1/β for sufficient high A.

File:Block Diagram for Feedback.svg - Wikipedia, the free encyclopedia

 

[cjron]

thq u all for explaining me but i can't understnd the part that loop gain is not the actual open loop gain, and not closed loop gain either. but it is a gain around the loop before the loop is closed?

 

[FvM]

In my view, the "gain without closing the loop" point is somewhat hair-splitting. I think, the loop gain is still there, when you close the loop, but you don't see it directly. The expressions given by barry (except for a typo with closed loop gain) are however very clear about the problem, and it's possibly better to think about their substance rather than about words.

Open loop gain A is clearly the gain of the amplifier itself, without external circuitry. It's a number written in the datasheet.
Loop gain Aβ, the gain around the loop, has been visually described by barry, don't know what I should add.
Closed loop gain finally is the signal gain of the feedback amplifier, Vout/Vin.

 

[barry]

In my view, the "gain without closing the loop" point is somewhat hair-splitting.

I never quite understood the point of 'loop gain'. It obviously means something to people smarter than me. In my designs (which, to be honest, don't usually involve control theory) I have never needed to know loop gain.

 

[mtwieg]

I've never seen the term "loop gain" used as anything other than shorthand for "open loop gain."

 

[FvM]

I've never seen the term "loop gain" used as anything other than shorthand for "open loop gain."

You'll find all three terms distinguished with the said meaning in text books, e.g. Gray, Hurst, Lewis and Meyer, Analysis and Design of Analog Integrated Circuits.

---------- Post added at 16:58 ---------- Previous post was at 16:48 ----------

I never quite understood the point of 'loop gain'.

I think, it's important to determine the perfomance of OP circuits. It's the available feedback gain, if you have e.g. a loop gain of 10 at a certain frequency, it means, that non-ideal OP properties are improved by a factor of 10, non-linearity reduced, output impedance reduced. Finally, complex loop gain (gain and phase) characteristic is the criterion for stability of the feedback circuit.