[Amps] FET bias

[David Cutter]

But the 723 allows an easy way to introduce temperature dependent bias 
voltage.

David
G3UNA


----- Original Message ----- 
From: "Steve Thompson" <g8gsq at eltac.co.uk>
To: <d.cutter at ntlworld.com>
Sent: Wednesday, December 26, 2007 12:04 PM
Subject: Re: [Amps] FET bias


> d.cutter at ntlworld.com wrote:
>> That's useful to know.  It's only on the Motorola application note from 
>> Concept that I've seen this technique.   Thanks for the explanation.
>
> In my opinion, the 723 based regulator that Helge Granbeg used on FET 
> designs is much more complex than needs be. I think it's a hangover from 
> the bipolar bias that was probably already on a pcb used for an early 
> experimental circuit and it ended up sticking.
>
> I normally just use a 10V zener feeding 10k pots with 10k series to the 
> gates and 100k from the gates to ground. Ive found that's fine with all 
> brands of FETs for domestic/amateur use. The diode has another potential 
> downfall - depending on the circuit, the gate could be left open if the 
> remote end of the diode loses power. That can be catastrophic, always have 
> a fixed resistor from gate to source. I learned the hard way - I've lost 
> transistors to 'crackly' pots without it.
>
> 73, Steve