Tom says:
>All of which is becomes meaningless with high power bipolar
>transistors if the amplifier stage does not use an active bias
>system that provides a constant voltage regardless of base current
>changes with drive.
I think constant voltage applies to FET amps. For bipolar transistors, the
base current bias should stay constant with drive. The bias current should
drop with temperature because transistor hfe goes up with temperature, but
generally you can live with that. Base bias voltage should drop by 2mV/deg C
to keep the current constant (for silicon transistors). The fixed voltage
only applies at constant junction temperature. However, Tom's point that it
mustn't move with signal is absolutely correct, and the old idea of a shunt
diode is just inadequate at any power over a few tens (maybe hundreds) of
milliwatts. Although I've 'got away with it', it was in a marine tx where as
long as you met the spec, that was what was needed - and the spec is nothing
like what we'd class as acceptable on the ham bands.
Ideally, I guess you'd modulate the bias current with signal level and
temperature to try and keep the actual transistor hfe constant, but I doubt
you'd win anything, and in practice, keeping the bias rock steady is much
better.
Quite good is to use the thermal compensation diode to control a DC
amplifier that provides the constant base current: RCA had an RF power
device in the early 70's with the diode on chip, and a 3 transistor DC
amplifier externally to provide constant current base drive. It worked fine.
73
Peter G3RZP
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|