Re: small 400V pnp sot-23 transistor, xxxTA94, etc.

Joerg wrote...

Hello Winfield,

The Panasonic isn't a sot-23, but the Zetex fmmt558 is an
ideal replacement for the A92. According to the data***
it has less capacitance, a higher fT, and better gain over
a wider current range. What's not to like? Later today
I'll make the change in my 2.2kV amplifier and find out.

If it wouldn't take more than a usec to turn off I'd really
like it as well.

I'm sure both the fmmt558 and the A92 can turn off in well
under a usec, if it's current sinking or steering you're
thinking of. But do you mean the output has to slew 400V
in under 1us, or > 4x10^8V/s? Assuming an external node
capacitance of 5pF plus an estimated 3pF for the transistor
over most of the excursion, the load current would have to
be i >= C dV/dt = 3.2mA. Since that's quite reasonable,
it would seem these transistors may meet your requirement.

If you'd buy 10k of them you could get them for 12c ;-)

Indeed. It's the nature of high-voltage series-cascode
applications to use a lot of transistors per circuit. For
example, in my circuit I used four for each amplifier, but
I should have used five. If I had added the rail-current
monitoring circuit I envisioned, it would have been 15 per
amplifier. If I had then beefed up the HV amplifier to a
full push-pull configuration, to improve speed, it could
have meant 35 of the A92 or A42 transistors per amplifier
(thanks for machine assembly). And if I had then increased
the voltage range to beyond 2.5kV, it would have been...


--
Thanks,
- Win
.

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