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Old 10th June 2013, 03:22 PM   #3441
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Quote:
Originally Posted by keantoken View Post
With an inductor at the collector, open-loop gain will dramatically worsen due to the output transistor's Rin=Rm.
Completely untrue.
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Old 10th June 2013, 05:42 PM   #3442
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Well, base resistance interferes significantly. So what? Using a high impedance load alters the internal loop, making the results wrong.
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Old 10th June 2013, 06:10 PM   #3443
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Guess what. Your "accurate" models completely support my first statement because they are flawed. Cordell didn't neglect to add base resistance.
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Old 10th June 2013, 06:39 PM   #3444
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I have no idea what you're talking about; which "base resistance" are you refering to? I cannot make head or tail at your attempt to simulate loop gain of the ANF current source.

Do you even know how to use the "Tian" lope gain probe in LTSpice? If so, why haven't you used it?

Anyway, I don't owe you an explanation for anything. I have demonstrated to my satisfaction the typical loop gain of the ANF current source. That is sufficient.

Last edited by michaelkiwanuka; 10th June 2013 at 06:42 PM.
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Old 10th June 2013, 07:13 PM   #3445
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I never said you owed me an explanation. But it would be appreciated because we can't both be right at the same time. It is strange that you would stop talking now when you are normally so eager to tell us how wrong we are.

Those were not actually attempts to simulate OLG, and because Rbe and Rm are ratios, in AC analysis it doesn't matter anyways. As long as there is a stimulating source the impedances only need to be derived from local signals.

Here is the loopgain. Nothing changes.

Of course, because feedback is not taken from the output, loopgain tells us nothing but whether the circuit is stable. Notice that when simulated correctly, phase margin dips to 37 degrees.
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Old 10th June 2013, 07:25 PM   #3446
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Default .. from #3430 etc

.. from #3430

ANF_loop_gain.asc
BJT_models.txt


Apart from Bias Spreader, LTP CCS, Zobels & output L , this might even be a 'real life' working amp.

Michael, can we take this as an example of your views on good practice and your target level of THD/stability? leaving out your L1 of course
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Quote:
Originally Posted by michaelkiwanuka View Post
Running the current source in isolation is precisely what I am doing by isolating it from the rest of the circuit with a very large inductor. The use of a large inductor for this purpose is routinely recommended by such luminaries as Rosenstark, Sedra and Smith.
Err.rrh! They recommend using a large inductor to isolate a Voltage source or something with rather less Z than a 'current source'

Quote:
You cannot obtain valid results by simply connecting the current source to a voltage source via a resistor because that resistor would define the output impedance of the current source giving wildly inaccurate results.
But you aren't measuring its output Z. You are measuring Loop Gain. For this you want to isolate the current source from other stuff. To do this, SHORT OUT IT'S OUTPUT.

If you want the Loop Gain of the CCS in situ with the effects of OPS, Ccb etc, remove the inductor.

If you want to measure its output Z, then use your inductor and connect the other end to earth.

If you want to know the relative effect of OPS on VAS & VAS on OPS, use your inductor and measure Z on either side of the inductor. I do this in #2877 http://www.diyaudio.com/forums/solid-state/171159-bob-cordells-power-amplifier-book-288.html#post3475959

Quote:
I have never heard of an inductor being described as a current source by anyone; it is no such thing.

Rather, the inductor is a virtual open circuit at all frequencies of interest allowing the output impedance of the current source to be defined solely by the output impedance of the source BJT.
Err.rrh! Isn't a perfect current source 'a virtual open circuit at all frequencies of interest' while supplying a DC current (or a controlled current for a VCCS etc)?

The description of an Inductor as a current source was used by many basic electronic texts in da old days to describe the evolution of simple amplifiers from a single BJT with collector resistor ... to full Class B with LTP i/p .. sorta the Jurassic prelude to Bob's Fig 3.1

It is also the basis of many Golden Pinnae SE amps bla bla.
___________

But why are we discussing this? I think MikeK's analysis is flawed ... but so what? Is this helping us design better amps or understand them better?

[deleted : 104 pg rant about how I analysed all this in my 1000 pg unpublished book on how to grow Cox's Orange Pippins in Antartica ]
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Old 10th June 2013, 07:29 PM   #3447
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For the same reason that some voltage regulators/amps oscillate with capacitive loads, I can provoke oscillation in this "unconditionally stable" circuit with a 5uH inductor at the output. Not applicable to most circuits, but worth knowing, and even more worth understanding.

But wait, with your loopgain simulation, we have false results and no way of testing stability into any legitimate load. Oops!
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Old 10th June 2013, 07:37 PM   #3448
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Originally Posted by michaelkiwanuka View Post
Using a current source, as you did in your analysis, is unrealistic and probably explains your somewhat exaggerated loop gain result. I used a more realistic resistor.




Running the current source in isolation is precisely what I am doing by isolating it from the rest of the circuit with a very large inductor. The use of a large inductor for this purpose is routinely recommended by such luminaries as Rosenstark, Sedra and Smith.

You cannot obtain valid results by simply connecting the current source to a voltage source via a resistor because that resistor would define the output impedance of the current source giving wildly inaccurate results.

I have never heard of an inductor being described as a current source by anyone; it is no such thing.

Rather, the inductor is a virtual open circuit at all frequencies of interest allowing the output impedance of the current source to be defined solely by the output impedance of the source BJT.
Mike,

Feeding it with a current source gives us the worst case. Using 22k instead makes only a small difference in the end result, assuming your transistor beta is 100.

A huge inductor is a virtually infinite impedance at AC, but it allows DC current to flow, some in some regard it acts like a current source in the AC domain. My use of the term current source was poorly chosen, but you should have gotten the picture anyway.

As I suggested, you should run your own sanity check with the current source by itself. The complexity of the way in which you are trying to simulate a fairly simple issue is undoubtedly tripping you up.

Remember, with all these powerful tools, garbage in, garbage out. These tools can be mistake magnets. I have seen it all too often. What we see here is a perfect example. Whenever possible, good engineers do a ballpark sanity check, ESPECIALLY if someone else questions their result and explains how to do such a sanity check. This is how we all learn.

Cheers,
Bob
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Old 10th June 2013, 08:01 PM   #3449
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Originally Posted by Bob Cordell View Post
A huge inductor is a virtually infinite impedance at AC, but it allows DC current to flow, some in some regard it acts like a current source in the AC domain.
I afraid thinking of an inductor as a current source is simply absurd and need not be further considered.

I am satisfied from the erroneous responses I have seen on the issue that my approach is wholly correct. We'll have to agree to disagree and leave it at that.

Last edited by michaelkiwanuka; 10th June 2013 at 08:07 PM.
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Old 10th June 2013, 10:17 PM   #3450
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Quote:
Originally Posted by michaelkiwanuka View Post
I afraid thinking of an inductor as a current source is simply absurd
Not at all. Inductors are used as effective current sources in switch-mode circuits all the time, especially the ubiquitous clamped inductive load test circuit. What Bob said:

Quote:
Originally Posted by Bob Cordell View Post
A huge inductor is a virtually infinite impedance at AC, but it allows DC current to flow, [so] in some regard it acts like a current source in the AC domain.
Is 100% true.

Once current is set up in a large inductor it behaves a lot like a current source. You know, just how a very large capacitor that has been previously energised (aka "charged") behaves a lot like a voltage source.

Last edited by HarryDymond; 10th June 2013 at 10:23 PM.
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