matched transistors

[terry michaels]

Message text written by "Ian Fritz"
>Hi Juergen --

I agree with you about the noise issue. I was thinking mainly about
converters. Is it possible to improve noise by paralleling transistors?
Seems this should work, but I've never tried it.

  Ian
<

Hi Ian:

Paralleling transistors has some possible benefits.  I remember something
about multiple transistors behaving as follows: the collector currents are
additive, but the noise of each transistor is uncorrelated, so although the
noise from two paralleled transistors is greater than one transistor, it is
not doubled, so you gain a little in signal to noise ratio.  Also, I've
thought about using paralleled transistors for a expo convertor, perhaps to
increase the frequency range, but as far as I can see you don't gain
anything.  Rbe generally defines the upper current limit, the Rbe will be
divided by 2, so your output current can be doubled, but the collector
leakage current will be doubled, so if you double the oscillator timing cap
value, the overall oscillator frequency range does not increase, because
the leakage current is doubled on the low end of the frequency range.

Terry Michaels