Re: gEDA-user: Hfe Gain

[Dan McMahill <dan@xxxxxxxxxxxx>]

Marc Price wrote:
> Seen as the Tesla coil mailing list is down i thought i might ask this 
> question here
>
> i tested 2 x 2n3055 transistors and i read that you need to match the 2 
> but what
> i dont know is, do i need to have them exact or just close.
>
> transistor 1 gain Hfe = 123
> transistor 2 gain Hfe = 194
>
> Any ideas
>
> Marc :-P

You'll have to be more specific than that.  It depends on the details of 
your application which none of us know.  Perhaps you're trying to make a 
low offset differential amplifier.  In that case I'd be at least as 
worried about Vbe matching as I would about Hfe matching.  You can, of 
course, have very good Hfe matching and bad Vbe matching ( take 2 
transistors which are identical in every way except 1 has 2x the emitter 
area, now you have 18 mV of offset at room temperature and the offset is 
proportional to temperature).

My guess is if you're using them in a tesla coil app you probably don't 
need "good" matching but then again I don't have the details of your 
circuit.

-Dan