[AMPS] Bias question

[Ian White, G3SEK]

Peter Chadwick wrote:
>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. 

Tom is right- bipolar power transistors do need a constant-voltage base
bias supply (leaving aside the point about temperature compensation).

When RF drive is applied, it creates both AC and DC current in the base-
emitter junction. The AC component flows back through the RF ground
return, but the DC component flows back though the base bias supply. (If
I haven't made myself clear, there's a diagram on my web site.) 

>However, Tom's point that it
>mustn't move with signal is absolutely correct,

To prevent the varying DC current from changing the bias point, the
supply needs to be a constant-voltage design.

> and the old idea of a shunt
>diode is just inadequate at any power over a few tens (maybe hundreds) of
>milliwatts.

Hear, hear... you can often hear it all over the band! There are many
better bias circuits, and a simple two-transistor design by
Mullard/Philips is described on my web site.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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