> To: amps@contesting.com
I wrote:
> >That's right Carl, using a series resistance and a shunt diode for
> >bias is just plain dumb.
Carl sez:
> But the manufacturers keep using it even when Motorola and TI at leas
> have published good bias circuits.
They will continue to use it until more people start to care about
signal purity on VHF, and let them know they care.
> I suspect the original "pattern" started out as AM CB for the HF bricks
> and FM for VHF.
> The "engineering" in either case did not have a clue about linear
> operation of those biplolar devices. When SSB started to become popular
> on VHF it was "one size fits all" .
Actually the diode works very well if the stage has very little RF
drive induced current. It is the rectified RF signal that tends to
drive the bias voltage negative that ruins the linearity, if the
quiescent current through the devices shunting the base is ten or
twenty times higher than the RF induced base current, the cheapo
system works great.
Look at the series resistance most people use. It is about 50 ohms.
13.8(source)-.7(bias)=13.1 volts drop. 13/50 is .26 amperes bias
current. If the RF induced base current is over .026 amperes the bias
will usually waddle around too much. If the collector current is 15
amperes peak, and the beta at the operating frequency is 10, the base
current will be 1.5 amperes.
1.5 amperes will force the base bias far negative, and the
device into class C.
IMD will actually become much "less bad" if bias is removed and
the base is grounded through a low dc resistance choke!
73, Tom W8JI
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