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Old 9th May 2012, 09:04 AM   #201
Bonsai is offline Bonsai  Taiwan
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Edmond, interesting comments about the bipolar vs JFET input! Very difficult now to get matched JFETS, so I guess the fact that they are unobtainium just drives up their street cred . . . but I doubt there is anything magical about them.

That said, for single ended designs they are ok, but a degenerated bip is just as good if not better other than the noise from the degen resistors, but thi sis not an issue on a power amplifer IMO.

Anyway, I really enjoy reading your stuff - very nice - please keep it coming.
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Old 9th May 2012, 12:13 PM   #202
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Hi Andrew,

Thank you for your encouraging words.
>please keep it coming.
I did just that. See fig.17: SuperTIS + Bod Cordell's HEC output stage.

Cheers,
E.

PS >but a degenerated bip is just as good if not better other than the noise from the degen resistors,
They are just 10 Ohms. So, not much to worry about the added noise.
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Last edited by Edmond Stuart; 9th May 2012 at 12:25 PM.
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Old 9th May 2012, 03:31 PM   #203
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Default More on RFI

The previous RFI sims made me suspicious. Therefore I've repeated the sims with a smaller HF signal: 100mV instead of 1V.

Code:
Front-end:               SuperTIS-BJT   SuperTIS-JFET   HEC-BJT    HEC-JFET
Demodulated AM signal:      5.4uV          1.26mV       0.35mV      0.82mV 
THD20k without HF carrier:  0.028ppm       0.071ppm     0.19ppm     0.76ppm*
THD20k with HF carrier:     0.030ppm       0.072ppm     0.24ppm     26.3ppm
THD20k increase:            0.002ppm       0.001ppm     0.05ppm     25.5ppm
Now the differences between AM detection levels and compression distortion are far less extreme.
See also my website.

Cheers,
E.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 9th May 2012 at 03:36 PM.
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Old 10th May 2012, 10:18 AM   #204
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Default high Z & phase margin

Quote:
Originally Posted by jcx View Post
[..]
- I expect best performance would had with error corrected/local feedback output stage giving high input Z, gain close to 1
Hi JCX,

Your absolutely right. As a matter of fact, it needs a very high input Z. Driving for example Bob's HEC-OPS directly (i.e using Bob's Q20 & Q21 as pre-driver ), yields less favorable results. But... using a separate pre-driver, configured as diamond buffer with bootstrapped collectors, works nice: THD20k = 3.1ppm (without TMC). See fig.17.

Quote:
will be interesting to see stability with all of the nested feedback around the output Q.
[..]
Indeed, that's another point of concern. The amp of fig.12 for example, has a phase margin of only 50 degrees (seen at the output). Seen at the input it's much better: 94 degrees, but it is the lowest value that counts. So I definitely have to improve this figure, at least to 60 degrees.

Cheers,
E.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 10th May 2012 at 10:21 AM.
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Old 10th May 2012, 12:13 PM   #205
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Quote:
Originally Posted by Edmond Stuart View Post
Hi JCX,

Your absolutely right. As a matter of fact, it needs a very high input Z.

Cheers,
E.
About how high? Presumably it's asymptotic, so plot of THD20 versus load Z would be nice, but otherwise just an approximate number for a reasonable load would help .

Best Wishes
David

BTW re. your earlier post - Scotland is british.
(Britain is Wales, Scotland and that bit in the middle)
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Old 10th May 2012, 12:46 PM   #206
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Quote:
Originally Posted by Edmond Stuart View Post
Indeed, it's problematic to do it with a conventional circuit, as too many stages are enclosed by the Miller loop. It will result in too much gain and too much phase shift. Perhaps one can tame such an arrangement by means of an additional massive lead-lag compensation, but most likely that is counter productive.
It's not obvious to me why it would be much worse with the conventional circuit. The IPS is identical in both. From the pre-driver onwards it is also (for comparison) identical. The difference is a Baxandall pair compared to, say, the usual VAS pair with the emitter follower before the CE.
If simulation results are very different I'd appreciate an explanation of why.

And if there is too much gain then perhaps it could be used beneficially via nested Miller compensation rather than create problems with massive shunt compensation. That's my main query. The differences in alternative shunt compensation placements is educational but for a fall-back option.

Best wishes
David.

I have a new collection of minor typos if you want them. Would you prefer a PM?
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Old 10th May 2012, 01:02 PM   #207
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Default Zi

Quote:
Originally Posted by Dave Zan View Post
About how high? Presumably it's asymptotic, so plot of THD20 versus load Z would be nice, but otherwise just an approximate number for a reasonable load would help .

Best Wishes
David
Hi David,

Zi = 770kOhms @ 20kHz was apparently too low, because THD20k = 21ppm
With a diamond pre-driver Zi = 96MOhms @ 20kHz and THD20k = 3.1ppm, which is okay.

Quote:
BTW re. your earlier post - Scotland is british.
(Britain is Wales, Scotland and that bit in the middle)
That is, after Scotland became part of the Kingdom of Great Britain in 1706, right?
But my great^n granny lived in 16th century.

Cheers,
E.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 10th May 2012, 01:15 PM   #208
R Dijk is offline R Dijk  Norway
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Any thoughts abt adding an inner current mirror to the long tail pair?
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File Type: jpg This front end double mir.jpg (28.9 KB, 166 views)
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Old 10th May 2012, 01:15 PM   #209
AndrewT is offline AndrewT  Scotland
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Great Britain, UK, British Isles, etc. all have their own quite precise but different definitions.

The green & blue Euler diagram depicts it well. It's about half way down.
British Isles - Wikipedia, the free encyclopedia
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Old 10th May 2012, 01:35 PM   #210
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Quote:
Originally Posted by Dave Zan View Post
It's not obvious to me why it would be much worse with the conventional circuit. The IPS is identical in both. From the pre-driver onwards it is also (for comparison) identical. The difference is a Baxandall pair compared to, say, the usual VAS pair with the emitter follower before the CE.
If simulation results are very different I'd appreciate an explanation of why.

And if there is too much gain then perhaps it could be used beneficially via nested Miller compensation rather than create problems with massive shunt compensation. That's my main query. The differences in alternative shunt compensation placements is educational but for a fall-back option.

Best wishes
David.
Hi David,

I'm afraid we are at cross purposes, aren't we? I thought you meant by 'a conventional circuit' something like the 'blameless' IPS and VAS, including a 'beta enhancement' tranny. It's this circuit that is incompatible with input inclusive compensation (at least, without additional measures to tame it), because the compensation encloses too many stages with gain.

>The difference is a Baxandall pair compared to, say, the usual VAS pair with the emitter follower before the CE.
The current gain of the Baxandall pair is just 1x, while the current gain of usual VAS pair with EF is about 10000x. That's a huge difference.

Anyhow, please correct me if I misunderstand you.

Quote:
I have a new collection of minor typos if you want them. Would you prefer a PM?
Yes, please!

Cheers,
E.
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goed verliezen dan dooft het licht…(H.M. van Randwijk)
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