AW: Transistor matching

[Magnus Danielson]

Hi Juergen!

> Just a stupid question about Moog's Transistor matching circuit:
> Do we really need these current sources ??
> I had to select a few transistors recently, and I just used a
> bipolar 15V supply and two 150k resistors. Ok, 150k from
> 15V - Vbe is not an ideal current source, but two identical
> transistors should give the same Vbe at the same 150k
> resistors as well, no? Or did I miss something?

The point is that the Vbe is directly connected to the Ic. However, there is
also some bulk resistances which has to be compensated for etc. so you can 
_not_
rely on any external voltages in that sence... the Ic current source is 
therefor
necessary to get a correct Ic current. The Vbe can now be measured more
accuratly.

Also, an current source will have much higher power-supply rejection than an
resistor. If you happends to connect your circuit to a +/- 12V supply instead
you will not get the same current. One of the points is that you should be able
to do measurements at diffrent times and get the same or close results.

The current source isn't expensive, so WHY cut it out?
There is no point in doing that in a single unit that you build once and that
may very well effect your overall precission... see my point?

Besides, the Moog matching circuit isn't the best around, but it is a good
approximation. Fairly simple curcuits can if built with care be quite 
impressive
in their exactness...

Cheers,
Magnus