Re: blast from past

On Wed, 2 Jul 2008 10:11:57 -0700, "BobW"
<nimby_NEEDSPAM@xxxxxxxxxxxxxx> wrote:


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:ss6n64dc1tvvs5un5slbt92n9782r8hcti@xxxxxxxxxx

We're looking for a PNP transistor capable of dissipating about a watt
(in some air flow) and that has low output capacitance, where Co
includes the mounting as well as the part itself. This is for use as a
current source in a 1 GHz bandwidth driver circuit.

The 2nd best choice seems to be 2N2907. The best may be 2N1119, but
they look hard to get.

John


More details, please...How many volts do you have available for the current
source? What's the current? What will be the voltage swing at the output of
the current source?

There used to be some good high-freq uniwatt PNPs (MPSU56, MPSU60 iirc) but
I can't find even a data***. The Cobo for most available parts is really
high (>10pF @ 10V), so if your GHz driver is inflicting a large voltage
swing then you're probably SOL. If you had a bunch of volts available you
may be able to add a series R to help decouple the effect of the Cobo.

You're right -- that 2907 still looks like a crumulent choice.

Bob

Your U60 comment is interesting. Central makes a CEN-U60, in a nice
TO-202 package (good cooling!) but has no proper data***. May be the
sameish thing as MPSU60.

I only need 12 volts Vce, 120 mA max, 1 watt max, 5 volts max p-p
swing, but low Cc.

We do have a ferrite bead in series with the collector, for the really
fast stuff, but lots of capacitance causes slow whoopies (like, 50 ns)
on output edges. No headroom for additional R.

Thanks... I'll look into MPS/CEN U60.

John



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