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Old 2nd June 2013, 03:52 PM   #51
Bigun is offline Bigun  Canada
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I've not heard this explanation before - it doesn't make sense to me - maybe I'm being dumb though. As you say, the die temperature is determined by the power device and it's temperature is stabilized by the emitter resistors. The use of 26mV across the emitter resistor is to minimize distortion, I don't see it being a big factor in the thermal stability.
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Old 2nd June 2013, 05:33 PM   #52
AndrewT is offline AndrewT  Scotland
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Cordell explained the interaction of supply voltage vs device Rth j-c vs emitter/source resistor for output stage Thermal Stability.
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Old 2nd June 2013, 06:56 PM   #53
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Perhaps I don't explain very well....
Some kit construction and design notes I have seen on integrated Darlington audio amplifiers have also recommended low bias levels because the thermal influence of the output transistor on the driver which, apparently, is significant. I have tried to observe this on the 'scope in 'subtract' mode using LF sinewaves, comparing Vre with Vbias as power is increased. The difference between discrete and integrated output stages (BDV64/65, TIP142/147, MJ100XX etc) is quite noticeable as power climbs above 10W or so. This suggests to me that with a standard Vbe multiplier control and bias setting, the bias could still run away or at least increase well beyond set levels as power increases toward max. limits.

The easiest way to ensure this doesn't happen is probably as here, by limiting the bias to a level that remains controlled in all forseeable situations. Otherwise, you're up for a more sophisticated control system. There is still the possibility that the designer wasn't concerned about lowest THD, as you may infer, and just elected to use an easily trimmed value that was cheaper, yet gave acceptable results. I have to remind myself that these BJT devices are really outmoded industrial switcher parts which don't rate with class AB hi-fi, as I see it, anyway.

I think the reason few have stability problems with Darlingtons is the same as with a lot of audio amplifier DIY and even commercial audio products - we use them at low average levels and a lot of sins go unnoticed - until we throw a party, that is.
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Old 2nd June 2013, 07:30 PM   #54
Bigun is offline Bigun  Canada
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I can understand that with the two junctions in series, in the Darlington, that if they are both on the same heatsink the Vbe multiplier has to be well positioned so that it has adequate control over the output. By mounting the Vbe multiplier onto the active device and not onto the larger thermal mass of the heatsink the thermal control loop can be made much more effective. Still, it's good to know that practical experience with them has been that they can be troublesome. My own experience is limited to Sanken Darlingtons - see my thread on TGM6 - which was thermally stable when optimally biassed for Class AB although I never ran it hard to see how stable it was at high heatsink temperatures.

Ian - what's your take on the benefits of the double bootstrap ?
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Old 2nd June 2013, 09:28 PM   #55
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Quote:
Originally Posted by Bigun View Post
.... what's your take on the benefits of the double bootstrap ?
I only wish I knew my own mind on it. I'm tempted to see it as you described - a good approach to full rail swing of the VAS. That must be good for efficiency and economy and we've seen from your sim. that it doesn't hurt a fabulous harmonic profile either. I would think AKSA has had a look at this more than once and probably could give a more useful reply - It's his kind of territory, if he's out and browsing following his new NAKSA creation.

It really is necessary to see a standard single boostrap sim. in Geirin's design. Otherwise, without pottering about with the sim. it's hard for me to assess the potential, since THD is already running high at ~ -60 dB. There may be ways to improve this, since single bootstrap designs, as we've seen here in several popular threads and texts, need only increase THD from say, .002% to .004% in a careful implementation. (That figure refers to a design with the basic topology of Self's blameless model amplifier or the RCA design it derives from).

I can't imagine double bootsrapping would suddenly render it diabolical but it will affect the linearity, having higher impedance terminations at both rail reference ends of the VAS output. Perhaps this is why we don't see it too often.

OTOH, it may now be making just the kind of distortion contribution we were all looking for.
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Old 15th June 2013, 03:42 PM   #56
GEirin is offline GEirin  Argentina
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Hi Bigun.
You could compare a standart single bootstrap with a double bootstrap in the simulator? Because I think it may be interesting to see the differences between them. Because I have not the software. And I do not know how to do it. Thanks.
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