[Amps] Transistor Bias

[Steve Thompson]

Gene Bigham wrote:
> I have been looking at various bias supplies for the 2sc2879
> transistor. 
> I am going to run 4 of them in my amp.  An appealingly simple diagram
> shows a 7808 (8v @ 1 amp) regulator supplying bias for a pair of
> these transistors.  Anyone with any experience with this sort of set
> up?  I could easily put two 7808's in my circuit to feed the center
> tap of the input transformer to each pair of 2sc2879s.  The circuit I
> am looking at has an output resistor on the 7808 of 8.2 ohms and a
> choke in series to the center tap, and also two resistors shunting
> one rated 1 ohm the other labeled TBD (is that to be defined?)  This
> is on a Telstar Electronics schematic for what is identified as a
> Skywave 2879AB.         
> Any comments about this arrangement?
It's a common, but poor, substitute for doing it properly. Have a read of
http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm
and/or look up some Motorola hf amp app. notes which use a circuit with a
723 regulator. I'd recommend a bias circuit for each pair, that way you
don't have to buy matched quads. Slight differences in rf performance
between pairs matter little.

73, Steve