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Old 22nd April 2013, 10:59 PM   #751
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Quote:
Originally Posted by dadod View Post
JLH used 5pF in series with 470k resistor in his 80W MOSFET amp, but he used RC network too, C3, R3.
Damir
Quote:
Originally Posted by steveh49 View Post
Does it still work the same as MIC? With R10 > R14, the series impedance of the MIC compensation components is never <= the global feedback resistance.
I've done some sims of the global loop for resistors in series with the MIC capacitor - schematics and results attached. The two circuits are a basic three-stage amplifier with discrete components and an amplifier built with ideal opamps. In both cases, the ULGF without MIC is ~10MHz. I included the opamps as I've seen Walt Jung use MIC with them a number of times.

The first set of results shows the global loop gain without MIC (upper traces); and the global loop gain with the ULGF aimed at 500kHz by the 17pF MIC capacitor and the 19k feedback resistor (lower traces). Green lines are the discrete circuit and blue are for the opamps. They show MIC working as predicted.

The second set of results has the MIC resistor set to three times the feedback resistor similar to the JLH circuit that dadod posted. It appears that MIC doesn't adequately compensate the global loop on its own in this case, as the loop gain plateaus at ~9dB before falling to unity at the original 10MHz.

My gut feel is that making the MIC resistor 1/3 of the feedback resistor would be good but needs more simulation which will have to wait.
Attached Images
File Type: png MIC_schematics.PNG (69.8 KB, 167 views)
File Type: png Rmic_zero.PNG (60.0 KB, 163 views)
File Type: png Rmic_nonzero.PNG (57.8 KB, 153 views)

Last edited by steveh49; 22nd April 2013 at 11:04 PM.
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Old 23rd April 2013, 02:17 AM   #752
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Originally Posted by steveh49 View Post
I've just checked the few JLH articles I have. The MIC components were:
- 1982: 5pF + 47k; cf 33k feedback resistor
- 1989: 10pF; cf 56k feedback resistor
- 1993: 5pF + 120k; cf 39k feedback resistor

Perhaps the 220pF was only very early on in his writings, or you have misremembered?
The 220p and discussion was in his 1972 HiFi News 75W amplifier article. I may have the instructions for the Powertran kit which has a full reprint but it might be difficult for this beach bum to find.

He has the circuit in his "Valve and Transistor Audio Amplifiers" book.



Its fig 8.12 on pg 154 if you can persuade google books to show it.

Mea maxima culpa for my misleading 1975 reference.
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Old 23rd April 2013, 02:51 AM   #753
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Originally Posted by Waly View Post
.. the large signal slew rate (what is actually called "slew rate" in most data sheets) has little to nothing to do with the small signal slew rate.
Not so.

The 'large signal slew' has to be large enough to support the 'small signal slew' if the amplifier is to remain linear. The 'small signal slew' or rise time is directly related to the -3dB bandwidth (and also whether its 1st order roll-offs bla bla)

eg a 100W @ 8R amp with 300V/us would need to be bandlimited to 100kHz x 300/50.26 = 596.9kHz. (from my 100W amp example)

Otherwise it would slew rate limit on a fast rail to rail square wave. If you have less bandwidth, the amp would overload before it slew limited. That's assuming you don't really want rail to rail sine waves at 2MHz.

You may like to sim. this with some 'perfect' amps with the same bandwidths and rails.
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Bob, I like to achieve supa dupa numbers like 1pp zillion THD and zillion V/us slew too. But I would only do that if it could be done without extra bits. (I'm old fashioned in that respect and have been known to call up young engineers when their new design has more bits than what they replace.

But really, as a speaker man, I'm far more interested in how nice an amp is to my evil speaker loads under ALL conditions.
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Old 23rd April 2013, 03:37 AM   #754
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Originally Posted by kgrlee View Post
The 220p and discussion was in his 1972 HiFi News 75W amplifier article... He has the circuit in his "Valve and Transistor Audio Amplifiers" book. Its fig 8.12 on pg 154 if you can persuade google books to show it.
It's shown as 220uF in that book, but I have another version of the schematic that confirms 220pF was the value (I thought maybe 22p was intended but misprinted as 220).

The feedback resistor is 22k in that design giving ~30kHz ULGF for the global loop. JLH says the design was good for 0.01% THD at 75W/8R (frequency not stated). Is that realistic for a class AB quasi-complementary output stage with bugger all feedback around it? Perhaps I'm missing something in the details...

In relation to your slew rate/band limiting musings, JLH's 1982 amplifier has a 4k7/1nF input lowpass filter for 34kHz.
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Old 23rd April 2013, 10:35 AM   #755
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Originally Posted by wahab View Post
Other than increasing slew rate MIC has no practical value
linearity wise , a moderate TMC yield lower distorsion even
at the upper side of the audio spectrum.
Hi wahab,

In most cases, if I had to choose between MIC and TMC for an amplifier, I would choose TMC, since it gives more bang for the buck where it is needed most - reduction of distortion in the output stage. Output stage crossover distortion, for example, is much more insidious than the typically softer mostly third-order distortion from an LTP input stage (as long as the amplifier is not in slew rate limiting).

I used MIC in my MOSFET amplifier with error correction because I already had dealt with the output stage distortion with the use of error correction. In that situation, the MIC helped achieve high slew rate and very low input stage distortion. I also was quite frankly unaware of TMC at the time.

In the absence of TMC, MIC does have value linearity-wise beyond increasing slew rate. That, of course is the linearity of the input stage. Linearity of the input stage is important insofar as what used to be called "soft TIM". In some cases, the added linearity provided by MIC might also allow a smaller amount of input stage degeneration to be used, if desired.

Cheers,
Bob
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Old 23rd April 2013, 01:04 PM   #756
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by kgrlee View Post
The 'large signal slew' has to be large enough to support the 'small signal slew' if the amplifier is to remain linear. The 'small signal slew' or rise time is directly related to the -3dB bandwidth (and also whether its 1st order roll-offs bla bla)

eg a 100W @ 8R amp with 300V/us would need to be bandlimited to 100kHz x 300/50.26 = 596.9kHz. (from my 100W amp example)

Otherwise it would slew rate limit on a fast rail to rail square wave. If you have less bandwidth, the amp would overload before it slew limited. That's assuming you don't really want rail to rail sine waves at 2MHz.
Not sure I follow. The small signal slew rate may be (and usually is) far below the large-signal value. A common opamp as the TL071 is specified with a (large signal) slew rate of 13V/uS. The rise time is specified at 100nS/20mV step, so the small signal slew rate is not more than 0.16V/uS. If you compare the rise times (or the small signal slew rates) for two different opamps, you'll note that the small signal slew rates are in the same ratio as the gain-bandwidth products.

Perhaps I'm missing something, but your example of limiting the bandwidth for high (large signal) slew rates amplifiers doesn't make any sense. A input low pass filter set at a few 100's of KHz has a completely different role of filtering the ingress RF. If your amp doesn't fully behave with fast input transients, without the input LP filter, then you have a global or local stability issue and the design has to be revisited.
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Old 23rd April 2013, 01:27 PM   #757
jcx is offline jcx  United States
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Default TMC is not comparable to single pole compensation - it is 2-pole

Quote:
In most cases, if I had to choose between MIC and TMC for an amplifier, I would choose TMC
thats a faulty comparison - TMC is a "2-pole" compensation – TMC should be compared with other 2-pole schemes – TMC requires the same level of knowledge, skill to safely apply as other 2-pole compensations

2-pole could be done with “MIC” and then the "extra" loop gain that could be equivalent to some tuning of TMC would be available to the entire amplifier, input and output - just like the conventional Miller position TPC


I am afraid the "single pole" appearance of the global loop with TMC is deceptive - leads people to think TMC is "trivial" - to think incorrectly that TMC stability is some how better than other 2-pole schemes, that it is fine to recommend to less sophisticated builders without the inclination or skill to use other 2-pole schemes

we apparently need to keep repeating the warning that TMC is 2-pole

Last edited by jcx; 23rd April 2013 at 01:38 PM.
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Old 23rd April 2013, 01:47 PM   #758
jcx is offline jcx  United States
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Quote:
Perhaps I'm missing something, but your example of limiting the bandwidth for high (large signal) slew rates amplifiers doesn't make any sense. A input low pass filter set at a few 100's of KHz has a completely different role of filtering the ingress RF. If your amp doesn't fully behave with fast input transients, without the input LP filter, then you have a global or local stability issue and the design has to be revisited.
the TMC/TPC debate brings to the front the issue of amplifier step response interpretation, significance

we are used to single dominant pole compensated amps where overshoot, ringing are 1st order related to stability margins

this is not the case generally - control theory regularly separates system response design from feedback loop frequency/step response with added prefilters - this can allow different and possibly better performance in some feedback controlled characteristics while meeting system frequency response requirements
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Old 23rd April 2013, 02:00 PM   #759
Waly is offline Waly  United Kingdom
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Originally Posted by jcx View Post
we apparently need to keep repeating the warning that TMC is 2-pole
Unbelievable. After hundreds of messages and threads discussing this, it still doesn't get through. TMC appears to be one of the most deceptive ideas I've seen in audio.

What also doesn't go through is that for a minimum phase system (as usual in audio, not much use of all pass filters) the maximum loop gain is a function of ULGF and the loop gain order. That's it, no way around, no free lunches, everything else comes to a stability margin price.
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Old 23rd April 2013, 02:09 PM   #760
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Hi JCX,

What about output stage inclusive compensation (a la Cherry). Is it single pole or two pole?
Provided that the whole thing has been made stable, I'm inclined to call that also two pole compensation.

Cheers,
E.
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