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Old 23rd May 2007, 09:08 AM   #501
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Quote:
Originally posted by PMA
Glen,

maybe I did not get it. I cannot imagine a NFB system without frequency compensation, even if it was an intrinsic one.

Me three.

Jan Didden
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Old 23rd May 2007, 09:22 AM   #502
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Quote:
Originally posted by Bonsai
Rock solid approach Glen - you can also use the Lissajous figures as well to get the phase margin.

Speaking of which, doe s anybody know of a simple phase meter circuit - I'd love something that I could hang a DVM or mc meter onto and just read it off.
Hi Bonsai,

Rock solid?
See my next post.

Cheers,
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Old 23rd May 2007, 09:23 AM   #503
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Quote:
Originally posted by G.Kleinschmidt
OK, for a real amplifier, I disconnect the feedback resistor (Rf) from the output and short it to ground, so the amplifier is running open loop with the impedance as seen by the inverting input to ground unaltered. I then wire-in an external op-amp servo amplifier which stabilises the DC operating point of the amplifier. I then connect an audio oscillator to the input and measure the gain and the phase of the output with respect to the input signal on a dual trace oscilloscope (with the amplifier driving a suitable dummy load of course).
Then all I have to do is to adjust the frequency compensation for the desired phase margin at the gain crossover frequency.

Easy-peasy

Cheers,
Glen
Hi Glen,

First, I suppose you have bypassed the input filter.
Second, strictly speaking, this approach isn't foolproof.
It depends on configuration of the input stage. If the phase and gain properties of the inverting and non-inverting input are identical, no problem. But this isn't always the case. For example, think of the classic JLH amplifier, a CFB op-amp or the Alexander amplifier. In these cases, the feedback signal is injected into the emitter of the input stage, not into a base, and the phase and gain properties of the inverting and non-inverting input are different.
So to be sure of reliable results, I always inject the test signal somewhere into the feedback path (and of course, ground the input).

Cheers, Edmond.
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Old 23rd May 2007, 09:38 AM   #504
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Quote:
Originally posted by estuart


Hi Glen,

First, I suppose you have bypassed the input filter.
Second, strictly speaking, this approach isn't foolproof.
It depends on configuration of the input stage. If the phase and gain properties of the inverting and non-inverting input are identical, no problem. But this isn't always the case. For example, think of the classic JLH amplifier, a CFB op-amp or the Alexander amplifier. In these cases, the feedback signal is injected into the emitter of the input stage, not into a base, and the phase and gain properties of the inverting and non-inverting input are different.
So to be sure of reliable results, I always inject the test signal somewhere into the feedback path (and of course, ground the input).

Cheers, Edmond.

G'day Edmond.

I agree that the test would not be foolproof for all amplifier topology’s, but as a open loop test as I described it, for amplifiers unlike the JHL and the others you mention, with a differential input stage, the technique is pretty much rock solid.
Any phase and gain differences between the inverting and non-inverting inputs are very small and practically negligible.

Cheers,
Glen


edit:

Oh, and no I don't bother bypassing the input filter, I just connect the cro probe directly to the non-inverting input.

 
Old 23rd May 2007, 09:59 AM   #505
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Quote:
Originally posted by G.Kleinschmidt
G'day Edmond.

I agree that the test would not be foolproof for all amplifier topology’s, but as a open loop test as I described it, for amplifiers unlike the JHL and the others you mention, with a differential input stage, the technique is pretty much rock solid.
Any phase and gain differences between the inverting and non-inverting inputs are very small and practically negligible.

Cheers,
Glen
Hi Glen,

I agree on that. Phase differences of input stages we normally use, are very small, less than +/- 1 deg.
But please, explain what 'rock solid' exactly means, as English isn't my native language. I thought something like 'foolproof under all circumstances', thus applicable to all amplifiers.

Cheers,
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Old 23rd May 2007, 10:43 AM   #506
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Quote:
Originally posted by john curl
............................
I realized then that you can't 'help' people if they do not wish it, and that often they would deliberately compromise a project, just to show their spite. I promised myself at the time, that just being 'right' was not enough, and that I should use more diplomacy, just to get people to do what is right and best for the project. I have temporarily forgotten this lesson here on the subject of coils, and I wish everyone the best, as far as they can go with their individual designs. Best to leave it at that.
Always remember: "C'est le ton qui fait la chanson"
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Old 23rd May 2007, 10:51 AM   #507
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'Rock Solid' = good, understandable, robust, doable with the tools to hand.

I don't know where it came from - I just appropriated it.


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Old 23rd May 2007, 10:52 AM   #508
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Quote:
Originally posted by estuart


But please, explain what 'rock solid' exactly means, as English isn't my native language. I thought something like 'foolproof under all circumstances', thus applicable to all amplifiers.

Cheers,
unmovable, steady, reliable
 
Old 23rd May 2007, 11:00 AM   #509
Bonsai is offline Bonsai  Taiwan
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Pity that this thread has got so emotive - from both sides.

I guess one of the problems with this hobby/vocation is designers tend to settle on a toplogy or technique that they get familiar with and it becomes part of their toolbox and when someone else disagrees with it, they defend it.

I think this is the situation we have here. The coil camp and the coil-less camp.

Who is right? My guess is both but for different reasons. JC gets away with it because of how he compensates his amp and some other (old) tricks while the guys that use an output inductor are looking to satisfy some different design aims. Like I said before, how an amp sounds to your ear is a very personal thing and the best example is tube amps versus solid state.

I'm outta here.

Peace

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Old 23rd May 2007, 11:14 AM   #510
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Quote:
Originally posted by estuart

But please, explain what 'rock solid' exactly means, as English isn't my native language.

It means as sturdy and reliable as a drop-pit, brick s***house, mate.

Cheers,
Glen
 

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