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Old 6th January 2011, 02:23 PM   #1361
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Quote:
Originally Posted by jcx View Post
I really hope to avoid math but the wye-delta transformation of the TPC, TMC network does seem promising for some insight when interpreting the mixed feedback loop of VAS, OP in TMC - as long as we can deal with FNDR elements in feedback loops

I haven’t been able to install MathCad 11 (the only version to use, the last with the Maple Symbolic engine) on my Vista machine so I may be slow to fire up the old offline computer with the lousy monitor… that would require moving from the only comfy chair in the house
They use MuPAD now, I have 11 tied to an old machine with dead DVD and CD (lost all the docs and receipt of course). Is MuPAD that bad?
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Old 6th January 2011, 03:46 PM   #1362
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4times the LTP tail current requires 4times the value of compensation capacitance !
Is this in the book?
I intended to re-read many chapters. Looks like compensation chapters must be the first.
Hi Andrew,

Yes, that is correct if you keep the relative degeneration factor the same; that means that the 470 ohm resistors would go down to 118 and overall gm of the LTP would go up by a factor of 4.

Cheers,
Bob
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Old 6th January 2011, 03:52 PM   #1363
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Quote:
Originally Posted by AndrewT View Post
yes, the different LTP current would require a different emitter resistor value for the same degeneration.

Bob quotes this 10x degeneration quite a lot.

What are the pros and cons of different amounts of degeneration in each of the various stages.
I assume it does not have to be 10x in the LTP, nor in the VAS, nor in the driver, nor in the output devices.
Where suits less and where suits more and what is lost when one adopts a higher or lower degeneration?
Hi Andrew,

We usually don't think in terms of degeneration in drivers and output stages. I like 10:1 because it linearizes the stages very significantly and usually has little impact on the noise of the amplifier. Some are concerned about the effect of increased amplifier noise with such amount of degeneration in the LTP, but overall amp simulations often show that this does not seriously degrade the noise in such an application. Power amplifier input-referred noise less than about 10 nV/rt Hz is more than adequate.

Cheers,
Bob
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Old 6th January 2011, 04:39 PM   #1364
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Default TMC vs TPC

I've done some more simulations on CMC, TPC and TMC and have attached a zipped folder of three simulations (ready-to-run) of the three techniques showing how they behave when in modest clipping of a 20 kHz sinewave. All of these simulations are of the example amplifier I have used earlier, except that I lightened the HF load to 0.05uF and 2 ohms, doubled all of the base stopper resistors to 50 ohms and 5 ohms for the drivers and output transistors, respectively, and loaded the driver base stoppers with 100 pF to detract somewhat from the phase margin. Once again, the TPC and TMC designs were adjusted to give the same amount of THD-20 reduction as compared to CMC. THD-20 was on the order of 0.002%.

It is easy to see that both TPC and TMC do not recover from clipping as well as CMC. However, TPC is much worse than TMC, getting into an oscillation that does not cease until the polarity of the signal changes (interestingly, the negative polarity is more stable, probably due to differences in the output transistors.

BTW, the TPC and TMC versions have about the same amount of distortion reduction, and the total feedback around the output stage in each has about the same phase and gain margins. Also, small-signal squarewave response shows about the same amount of ringing.

Overall, TMC and TPC give up some stability in reducing distortion as compared to CMC, so they have to be evaluated carefully both in small signal and large signal. They are both very worthwhile techniques, but like any tool, must be properly applied. It appears that large-signal behavior of TPC is significantly worse than that of TMC when margins are close. Overall, I still am inclined to prefer TMC over TPC.

Cheers,
Bob


Cheers,
Bob
Attached Files
File Type: zip CMC TPC TMC Clip.zip (11.3 KB, 66 views)
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Old 6th January 2011, 04:47 PM   #1365
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Hi Bob,

This might be very interesting, but I can't read *plt files. Where I can download a reader (for free, of course)?

Cheers,
E.
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Old 6th January 2011, 05:26 PM   #1366
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Linear Technology - Design Simulation and Device Models (for free, of course!)


re math software - I'm used to the "undocumented" Maple symbolic features hidden in MathCad 11, v12 broke a lot of my worksheets and I never gave 13 a fair trial - the user forum heavyweights certainly seemed to always ask people to repost in version 11 for several years after the later versions

I am intrigued by the Sage math open source project http://www.sagemath.org/ but it looks like you just have to "explore" - I really like comprehensive documentation, working examples of what I'm trying to do - and especially a good indication of what the sw can't do or is so hard as to be impractical - so I don't waste my time

Last edited by jcx; 6th January 2011 at 05:43 PM.
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Old 6th January 2011, 07:24 PM   #1367
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Quote:
Originally Posted by Edmond Stuart View Post
Hi Bob,

This might be very interesting, but I can't read *plt files. Where I can download a reader (for free, of course)?

Cheers,
E.
John Miles' 7470: HP-GL/2 Plotter Emulator should do.

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Old 6th January 2011, 07:54 PM   #1368
jcx is offline jcx  United States
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I stuck in 2 uH series output load isolation inductor and the clipping recovery ringing is pretty much gone with both TPC (well just 1 cycle), TMC (measured at the amp output)

I suppose Mr Curl will object that the 320 KHz low pass is "clearly audible", that would be -0.017 dB, 2nS group delay delta at 20 KHz re 1 KHz - them Golden Ears are pretty good!

it also hides the TPC "bump"

Last edited by jcx; 6th January 2011 at 08:06 PM.
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Old 7th January 2011, 10:01 AM   #1369
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Bob, one quick question for you (great book BTW)!
You wrote about quad emitter follower in NFB amps chapter. Did you just simulated the idea and what about oscillation problem when working in open loop. BTW I never seen a quad EF in real life amplifiers so I guess it`s hard to stabilize it.?
Thanks Borko.
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Old 7th January 2011, 11:28 AM   #1370
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Default clipping

Quote:
Originally Posted by jcx View Post
Thanks, jcx!

It seems that these *.plt files are not regular plot files, rather a kind of proprietary meta-files of LT. Here are the plots for those who don't have LT-spice.
From left to right: bridged TPC, bridged TMC and Miller compensation.

BTW, I also tried different values for the TPC compensation:
C=17p, C2=85p, R=2k2 respectively C=85p, C2=17p, R=2k2, but to no avail.

Cheers,
E.
Attached Images
File Type: png BTC-Clip.png (4.3 KB, 310 views)
File Type: png BTMC-Clip.png (4.0 KB, 306 views)
File Type: png Miller-Clip.png (4.0 KB, 248 views)
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