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Old 11th June 2013, 07:34 PM   #3471
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Originally Posted by michaelkiwanuka View Post
Thus, the purpose of isolating it from extraneous circuitry is to establish its intrinsic loop gain.
As I said, why care about that? What's important is stability of the loop in its actual real-life operating condition.
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Old 11th June 2013, 07:39 PM   #3472
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Originally Posted by HarryDymond View Post
What's important is stability of the loop in its actual real-life operating condition.

I agree; that is vital. However Bob's urgument which I disputed was that the intrinsic loop gain of the ANF current source was much larger than I had demonstrated. He asserted that the source of the error in my simulation was the presence of the inductor. This is not true as I have demonstrated.
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Old 11th June 2013, 08:36 PM   #3473
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No, it will not suffice to say that, because they use large inductors to isolate a voltage source (the output of a feedback amplifier) from a medium-impedance load (the input to the feedback network). They don't use the large inductor to isolate a current source from a low impedance load.
On the contrary, the principal is to isolate one circuit node from another regardless of their relative impedances. You clearly haven't read Rosenstark exhaustively; I have.

Last edited by michaelkiwanuka; 11th June 2013 at 08:39 PM.
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Old 11th June 2013, 08:38 PM   #3474
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Originally Posted by Bob Cordell View Post
You are finally beginning to get something that resembles the right answer, but you are drawing the wrong conclusion based on a wrong understanding of the VAS circuit in the closed loop amplifier and with the Miller compensation.
Maybe, or maybe not; but certainly a patronizing tone doesn't stimulate any constructive discussions.
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Old 11th June 2013, 08:43 PM   #3475
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Harry,

Read the following. Towards the end of the page you'll find the following sentence:

"Another PSpice method involves inserting a large inductor (Lins) in the amplifier’s feedback loop (Fig. 6). It opens the loop for the ac signal, but keeps it closed for dc."

Use PSpice To Verify Feedback Amplifier Stability | Analog content from Electronic Design

Additionally, see below:

http://www.spectrum-soft.com/news/wi.../loopgain.shtm

Finally, Rosenstark's book:

http://web.njit.edu/~rosensta/books/Feed_Back_Amps.pdf

Last edited by michaelkiwanuka; 11th June 2013 at 09:09 PM.
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Old 11th June 2013, 09:22 PM   #3476
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Originally Posted by michaelkiwanuka View Post
Harry,

Read the following. Towards the end of the page you'll find the following sentence:

"Another PSpice method involves inserting a large inductor (Lins) in the amplifierís feedback loop (Fig. 6). It opens the loop for the ac signal, but keeps it closed for dc."

Use PSpice To Verify Feedback Amplifier Stability | Analog content from Electronic Design

Additionally, see below:

Plotting Loop Gain and Phase Margin - Winter 2001
I don't need to read these (some of them again), as I really do understand perfectly well the principle of inserting a very large inductor into a feedback loop for loop-gain simulation.

By inserting the very large inductor between the current source output and the TIS, you have made an effectively open circuit at the current source output at all frequencies apart from DC (you must agree with that because that's exactly what the sentence you quote above states). How is this showing the "intrinsic" behaviour of the current source?

Why even simulate the current source with all the other amplifier guff if you're going to isolate it? Why not just simulate it by itself? And in order to show off the "intrinsic" behaviour, surely it would be best to present it with the "kindest" load, which for a current source is a short circuit from DC to daylight (the dual of the "kindest" load for a voltage source being an open circuit from DC to daylight).
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Old 11th June 2013, 09:26 PM   #3477
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Originally Posted by michaelkiwanuka View Post
Thank you for posting this link.
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Old 11th June 2013, 09:37 PM   #3478
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Originally Posted by michaelkiwanuka View Post
This was what I thought initially; viz. that because of minor loop feedback the TIS couldn't funtion as a current sink. I was wrong and so are you.

I established this fact by removing the miller compensation capacitor on the amplifier on the left and removing the inductor in both cases. If the minor loop feedback were to make a difference, then the loop gain local to the ANF current source would be radically different in both cases. It isn't. See below.
Mike,

You are still not getting it right. There are TWO loops of negative feedback that both act to create a very finite impedance at the output of the VAS. You are ignoring the global loop.

You would better off at this point to honestly establish for yourself the truth by abandoning all of the complex mistake magnets in what you are doing and first do the simple sanity check with a feedback current source TOTALLY by itself with a reasonable resistive load to the rail or just conneted to the rail. THEN, once you have convinced yourself of that, you can go on and try to figure out why you are magnetically attracting so many mistakes in your present approach.

Cheers,
Bob
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Old 11th June 2013, 10:48 PM   #3479
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Originally Posted by Bob Cordell View Post
There are TWO loops of negative feedback that both act to create a very finite impedance at the output of the VAS. You are ignoring the global loop.
The major feedback loop has no impact, at least to a first order, on the output impedance of the TIS. You can confirm this by disabling the major loop by means of a large capacitor shunting the inverting input of the amplifier to ground or using a large inductor in series with the inverting input of the amplifier.

Further, Bob, I have also demonstrated that the minor loop has no effect on the loop gain of the ANF current source. Why do you find this so difficult to appreciate?

Tomorrow I'll do as you've asked; viz. run a sim of the loop gain of the current source with a resistive load.
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Old 11th June 2013, 11:17 PM   #3480
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The results with an without the miller cap are the same, because it is global feedback that reduces TIS Zout, not miller feedback, in order to prevent signals at the VAS output from causing error in the output.

Here is your simulation modified to actually show the TIS Zout, instead of assuming what it is. It only changes at RF, where it DOES make a difference as your plot in your post shows.
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