RM> Final cure was a teensy little .01uF across the collector-base
 RM> of the TIP driver.

 RM> Normally, yes. But this circuit was too simple, I wasn't going
 RM> to let mere scruples prevent me from defeating it.

 RM> I suspect the "real fault" was poor layout.

  It's never layout. These things take off around 1-10 MHz and layout
problems don't begin till the tens of Mhz. My guess is that a large
proportion of *all* those converters oscillated, depending on the
actual transistors fitted... a design fault.

  The problem is that a transistor is a 2-pole circuit at HF: the base 
resistance and collector-base junction capacitance, plus the emitter 
resistance and the base-emitter junction... and to make it worse, the 
usual trick is to drive one transistor straight out of the other, so 
Miller effect adds yet another time constant. As soon as you have a 
3-pole R-C circuit you have a possible oscillator and none of the 
elements are linear, varying with voltage and load current... and 
load capacitance.

  That 0.01 you slap on is a real bastard. It starts a top cut at
around 1KHz with the idea of reducing the gain to zero at 1Mhz where
the pole exsits, but it also adds another 90-degrees phase shift that
can sometimes make it worse. So you add another 0.01... somewhere, and
then a spoling 0.1uF/10 ohms across the output, and so on...    

Regards,
Bob
___ Blue Wave/QWK v2.12
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