[Amps] Bias Zener Diodes In Parallel ?

[Ian White, G3SEK]

Steve Thompson wrote:

>>   If you need an adjustable bias, you might consider a TL431 (good to
>>36V without additional parts) in conjunction with an appropriately
>>heatsinked PNP power transistor (See either the National or TI
>>databooks... or websites?).
>You took the words out of my mouth, Marv. For a tested design using 
>TL431, you can see a circuit on G3SEK's website.

The design in the databooks can oscillate with capacitive loads (which 
are what we have in am amp, due to the RF bypass capacitors). Adding an 
R-C at the base of the PNP power transistor will fix that... but the 
transistor does need to be a darlington type.

The circuit - with a modification for higher voltages than the 36V 
breakdown of the TL431 - is at:
www.ifwtech.co.uk/g3sek/boards/triode/an5-1.0.pdf

-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)