[sdiy] Transistor help

[René Schmitz]

Hi Fiercefish,

Fiercefish wrote:
> Hi all, I have just bought a batch of 2n3904's for my soundlab
> minisynth project, I have a meter that can measure the hFE of them so
> will this allow me to select matched pairs by choosing those with the
> closest value? Also I apparently need to find some noisy 3904's for
> the noise generator, what sort of hFE reading (if any) would yield
> the most noise eg high or low? Sorry if these seem like dumb
> questions, I do not know if the hFE value is related to these factors
> but it is the only method I have of testing them, unless there is
> another way of course. I'd be greatful for any input or other
> suggestions, thanks.

hFE is not the right way to select matched pairs. You need to supply a 
constant current through the collector, and you would measure the Vbe 
that you need for that current. What I was always using is to put a 
resistor from collector to V+ and from emitter to V- (with an assumed 
+-15V and the resistors being 150k you get about 100uA.) The base is 
grounded, and you read the voltage between base (GND) and emitter. Find 
the ones that have the reading which most close together.

Cheers,
  René


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uzs159 at uni-bonn.de
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