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From: firstname.lastname@example.org (carltons)
Subject: Re: Q. medium-power RF transistor
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Sat, 09 Nov 2002 03:39:49 GMT
Date: Sat, 09 Nov 2002 03:39:49 GMT
In article , "Helmut Sennewald"
> "Winfield Hill" schrieb im Newsbeitrag
> > I'm thinking about making 20 to 30V pulses with a
> > 1ns max risetime into a 50-ohm line...
> > What's a good low-capacitance transistor to use with
> > say Vceo 30V, 500mA capable (h_fe >10 at 500mA) and
> > with f_T of at least say 1GHz at Ic 200mA, Vce 15V?
> Hello Win,
> I would try four cascodes in parallel with BFG135 transistors.
> The product of output capacitance of your transistor(s) and
> the load(50Ohm) has to be below the wanted rise time.
> The layout will be important important.
> Try a star(+) configuration.
> > For example in the old days we'd consider a 2n3866,
> > etc., but this falls short of the mark for the above
> > specs, and is clearly far behind today's curve. Hmm,
> > what's a common transistor to get 5 watts at 900MHz?
> This is a very different application. It is narrow band only.
> The output capacitance of the transistor(s) is compensated with
> the inductive collector load(resonance circuit).
> Best Regards
I think Helmut is definitely on to something good. I picture it myself as
a kind of blocking oscillator design without the oscillation if you get my
direction. Another way of putting it would be a tuned circuit with a
damper. I also don't know why, but the concept of the pulse forming
network keeps coming back to me and I really don't know why. I've never
designed anything like this. My high power RF expertise is in 50 ohm
class C amps at 37 dbm or more not time domain cicuits which are probably
the same thing only with a Laplace transform or some other mathematical
explanation thrown in.
Another warning bell is going off in my head also. Look out for Miller
cap. Last few stages may require low/no voltage gain. Possiblity of
buckets of feedback to provide low output resistance? (I'm just brain
storming right now)
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