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How can I obtain amplifier circuit loop gain?
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Cerebus:

--- Quote from: Wimberleytech on January 19, 2020, 03:49:21 am ---
--- Quote from: Cerebus on January 19, 2020, 01:47:52 am ---
--- Quote from: bson on January 19, 2020, 12:02:33 am ---Vf/Vin, where Vf is the feedback voltage.

--- End quote ---

What question do you think that is an answer to?

--- End quote ---

You mean.: "To what question do you think that is the answer?"

Anyhooo, it is actually the answer to the title of the original post.

--- End quote ---

A "grammar nazi" remark, on a forum where half the people can't spell and one third of the posts don't even even vaguely meet the rules of grammar? You're going to be busy...

Moreover, I don't think it's the answer to anything. There's no Vf defined anywhere here, and if we take the naïve assumption that Vf is the feedback voltage at the negative input then it's obviously wrong. It'd help if the poster clarified themselves.
Wimberleytech:

--- Quote from: Cerebus on January 19, 2020, 11:57:51 am ---
--- Quote from: Wimberleytech on January 19, 2020, 03:49:21 am ---
--- Quote from: Cerebus on January 19, 2020, 01:47:52 am ---
--- Quote from: bson on January 19, 2020, 12:02:33 am ---Vf/Vin, where Vf is the feedback voltage.

--- End quote ---

What question do you think that is an answer to?

--- End quote ---

You mean.: "To what question do you think that is the answer?"

Anyhooo, it is actually the answer to the title of the original post.

--- End quote ---

A "grammar nazi" remark, on a forum where half the people can't spell and one third of the posts don't even even vaguely meet the rules of grammar? You're going to be busy...

Moreover, I don't think it's the answer to anything. There's no Vf defined anywhere here, and if we take the naïve assumption that Vf is the feedback voltage at the negative input then it's obviously wrong. It'd help if the poster clarified themselves.

--- End quote ---
Lighten up Cerebus...just having a little fun.
Frankly, I thought you were a bit pejorative in your response, so I was just playing along.

I am an old Texan...you should be better than I.

Anyway, back to the technical stuff.  Why do you say Vf/Vin  does not represent the loop gain?

Cerebus:

--- Quote from: Wimberleytech on January 19, 2020, 03:44:41 pm ---Anyway, back to the technical stuff.  Why do you say Vf/Vin  does not represent the loop gain?

--- End quote ---

What is Vf? No, seriously; it's just been introduced out of thin air.

If Vf is, as a I said a naïve assumption would be, the voltage at the negative input, then Vf/Vin can't be the loop gain.

Let's take the classic feedback equation

Vout = Vin A / (1 + Aβ)

Where Vout and  Vin are obvious, A is the open loop gain and β is the feedback fraction.

The loop gain (by definition) is Aβ, if Vf is as assumed above then Vf = βVout. So (substituting)

Vf / Vin = βVout / Vin ~= 1  and is not = Aβ.
 
The bloody SMF editor stole the keystrokes that should have produced a β  and instead decided that I wanted to save a 1/4 written draft message - bastard!
Lucky-Luka:
@Cerebus
That's what I was thinking.
That's why I think that post doesn't add value to the thread.
I have asked a precise question and I have presented 2 solutions.
I can accept other solutions but it would be nice to detail them a little bit...
Cheers

p.s. English is not my first language so be patient :)
Lesolee:


The questions were:
(1) Are they equivalent?
Yes, from an theoretical point of view.
(2) Are they correct?
Yes, with the same caveat.

In some sense the ground generator version is less "accurate" since it removes the load of the feedback network, but 22K compared to the 10R load is pretty negligible.
In some sense the floating generator version is more complicated since the plot is a ratio rather than a direct reading, but again that is evidently not an issue.
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