Peter Chadwick wrote:
>
>>You take a high power npn transistor
>>and connect a low power zener from collector to base, with
>>the banded end at the collector
>
>But don't forget that the temperature coefficient is now totally different,
>while the 'zener' voltage has increased by the Vbe of the transistor. So a 6
>volt zener(which would have approximately a zero temp. co) would become 6.7
>volt zener with a temp. co. of -2mV/deg C.(assuming a silicon transistor)
A TL431 "adjustable zener" with a cheap PNP darlington transistor such
as a TIP147 will give continuously adjustable bias voltage that is rock-
stable up to at least 2A (which a zener or a transistor+zener definitely
isn't). The schematic is in the TL431 data sheets.
The transistor is inside the feedback loop, so the temperature
coefficient is that of the TL431 alone, which is typically 10x smaller
than the transistor+zener.
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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