[sdiy] Transistor help

[Pete]

You can get tempco resistors from the site below but they are not
inexpensive. I got a quote for $6.71 each and a 15 piece minimum. I
have not orderd any.

PT146 2000 ohm 1%
            @25 C
            TC 3500 ppm/degree C
15 pc.min.@$6.71 ea.

http://www.precisionresistor.com/


On 4/26/05, René Schmitz <uzs159 at uni-bonn.de> wrote:
> 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
> 
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
> 


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