It's a bit more than the usual voltage. The MPSA44 / 94 JUST makes it but the datasheet doesn't give hFE @ low Vce and the '94 seems to be rare.
Any other candidates ? Cascoding could be problematical in this application due to volt drop ( I need a current source of about 6mA capable of handling a high voltage but with a low 'drop-out' voltage ).
Other packages could be OK but I may need TO-92 at least for dissipation reasons.
Graham
NPN:-
ST STX-616 Fairchild KSP44TA C1573
PNP:- Zetex ZTX560
Best regards, Spehro Pefhany
Thanks. The current gain of the Zetex part is nicely high and of course saturates well too. It has an npn near direct complement the ZTX458 too.
Graham
Apex PA94 op amp with 450 volt power supplies wired as a current source.
Bob
Does it also work at 4V ? It sound expensive too.
Graham
4volts for output or supplies? From negative to positive swing of the opamp.
Bob
I think you misunderstood the question. I can do discrete design. I just modelled what I wanted with Spehro's advice and My God it's good. Little change in current ( as far as it matters ) from 2V to 400V.
Graham
build it with a negative supply for the low side of your current source then you can go down to zero volts or negative.
Bob
No need in this application and no negative supply. It also has to be cheap.
Graham
Follow-up. Zetex kindly sent me 10 samples of each by courier despite the fact that Farnell stock them too.
All credit to Zetex.
Graham
due to the hugely increased level of spam please make the obvious adjustment to my email address
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