Re: gEDA-user: Hfe Gain

[al davis <ad106@xxxxxxxxxxxxxxxx>]

On Friday 05 May 2006 16:09, 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

As Dan said, it depends on the application.

Let me guess ...  The transistors are type 2n3055.  This tells 
me that it is some kind of power stage.  Since there are two of 
them, with a comment on matching, this tells me that probably 
they are either in parallel or push-pull.

First, let's assume they are in push-pull.  If there is a 
mismatch, it might increase distortion in the output a little.  
Do you care?

Second, let's assume they are in parallel.  If there is a 
mismatch they will share current unequally.  If this is the 
case, a small resistor in series with the emitter will somewhat 
even them out.

Now, a touch of reality ..   The variation you see here is 
common.  Many high quality audio amplifiers (from 1970 or so) 
used pairs of 2n3055's in the output.  Mostly, they just took 
them as they come.  The circuit is designed so it doesn't 
really matter, as long as the beta is high enough.  Often they 
would use it in conjunction with a smaller transistor as a 
darlington to make the effective beta higher.

If a difference like this matters, in a power stage, it isn't a 
very good circuit.