Hi Peter,
> 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.
Ya'll said,
> 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.
No, the bias current can NOT remain steady because the bipolar
requires a change in base current to produce a change in collect
current.
If the bias current remains steady with varying drive, the amplifier
will have gain compression.
If the VOLTAGE supplied to the bias system remains fixed at a
level that produces the correct Ic for the device junction
temperature, the amplifier will not gain compress when the exciter
power rectified in the EB junction tries to add negative voltage back
into the bias supply.
If the current is regulated, the bias will go negative as drive is
increased.
That's why high power transistors can not be correctly biased
without an active bias system that maintains constant voltage with
a very low bias dynamic source impedance .
FET's, because they have no gate current (hopefully), require only
a regulated voltage source of almost any impedance.
73, Tom W8JI
w8ji@contesting.com
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