"Roy Lewallen" bravely wrote to "All" (18 Mar 05 09:45:02)
--- on the heady topic of " Darlington Transistors At RF?"

Hmmm, what if the darlington is used in the bottom end of a cascode?
Use as the top end a high ft but low gain transistor. This way the
darlington collector voltage is stable and it relies more on the
common base fae for the gain limit, somewhat higher than ft, I think.
Darlingtons are only mostly good as emitter followers and DC switching
drivers anyways.

A*s*i*m*o*v


RL From: Roy Lewallen
RL Xref: aeinews rec.radio.amateur.homebrew:8924

RL A major problem with the speed is that a common three-terminal
RL Darlington doesn't have any pulldown resistor or current sink to the
RL Q1e-Q2b connection. That leaves no good way to remove charge from Q2b
RL quickly when you want Q2 to turn off. If you can find one with an
RL internal Q2b-Q2e resistor of pretty low value, or better yet have
RL access to that junction so you can long-tail it down to a negative
RL supply or even to ground, you'll have a much faster switching device.
RL I still suspect you'll have trouble getting one to go fast enough for
RL efficient class C operation at VHF.

RL Roy Lewallen, W7EL

RL Paul Keinanen wrote:
On 17 Mar 2005 22:45:11 -0800, wrote:


Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.


You have to look at the fT parameter (which specifies the frequency at
which _current_ gain drops to unity), since a typical power darlington
is a very slow device.

Paul OH3LWR

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