[sdiy] Transistor help

[James Patchell]

When I was in college...we used to use a curve tracer to match 
transistors...it had two transistor sockets on it, and there was a switch 
to flip between the two parts...basically, you were matching HFE doing it 
this way...seemed to work...

One thing I have noted about modern batches of transistors is they sure 
seem to be all very similar...I generally match about 10 pairs at a 
time...out of 20 transistors (generally 2n3906) I always get 10 pairs...and 
all of the transistors are within about 2mV of each other....30 years 
ago...it wasn't like that...as I recall.

At 10:01 PM 4/26/2005 -0600, Ian Fritz wrote:
>At 09:05 PM 4/26/05, Bob Weigel wrote:
>>But the way suggested is more proper since it takes into account the 
>>actual things which are going to affect the performance of the circuits 
>>in question usually. -Bob
>
>Well, we've been around on that one many times here.  If the mismatch 
>between two units is due to different mesa areas (say) then the 
>temperature dependences will cancel properly and the mismatch will not 
>matter (just an unimportant current ratioing).  So the standard method 
>could give false negatives.
>
>OTOH, if the differences are due to, say, defects, then these would also 
>affect hFE.  I think that is why I have observed very good correlation 
>between Vbe and hFE.  And why I think simply matching hFE makes sense.
>
>   Ian

         -Jim
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