The most significant problem with the standard Darlington connection is
the amplified Miller capacitance of the base-collector junctions. The
high gain of the darlington pair effectively multiplies this capacitance a
lot more resulting in poor frequency response (it won't go to VHF).
One approach is to tie the collector of the first transistor to an
AC-grounded
DC bias source, reducing this effect a lot.

This approach is described by Alan Grebene (founder of Exar) in his very
readable book "Bipolar and MOS Analog Integrated Circuit Design".

-- Tom




wrote in message
oups.com...
Hi All,

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.

C.W.