"William Sommerwerck" wrote in message
...
"Ian Field" wrote in message
...

Any sort of decent quality radio would have automatic bias common emitter
stages, the base bias is set by a voltage divider - that and the value of
the emitter resistor determine the collector current.

The self compensating nature of the circuit would tend to ensure a
relatively small reduction in collector current, the main problem I can
think of is that AGC is often fed at the bottom of the base bias divider,
so using a transistor with a higher Vbe may have a similar effect to a
large AGC control voltage - very low gain maybe, but totally inoperative;
doubtful.

This isn't quite the way I learned it in school.

The kind of bias you're describing tends to keep collector current
constant
with beta variations. Vbe is taken into account in the design. I don't
think
a circuit intended for 0.2 to 0.3 volts would work with a transistor
needing
0.6 to 0.7 volts.

Collector current does fall, but there is also a corresponding fall in
emitter current wich reduces the emitter resistor volt drop - which goes
some way to compensating the higher Vbe.

So it doesn't result in the transistor being completely cut off - as would
happen if the emitter went straight to ground without an emitter resistor.