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

Roy Lewallen, W7EL

I thought darlington transistors were just two transistors connected
that way inside the same case, ie not particularly different from
connecting two discrete transistors in the same configuration.

Hence, one could play with darlington transistors, but just
use two distinct transistors, so you have that junction.

Of course, there are other reaosn for having an amplifier chain, besides
overall gain. Those intermediate stages often help to filter the signal.

Michael VE2BVW

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