AW: Re: AW: Transistor matching

[Magnus Danielson]

> Who needs an exact current of 100.00 uA ??
> The only important thing is that you measure the Vbe of *all*
> transistors at the *same* current.

Rigth. I never said an specific current... that is your interpretation.
I only said that an current source will give a more stable current considering
a number of factors.

> The voltage drop over the 150k will determine the current.
> It will be approx. 14.4 V, and it will vary 2 millivolts, if you match
> transistors for 2 mV.
> If you had two identical transistors, there would be no error at all
> with the 2-resistor-circuit. If you have matched two transistors that
> are 2mV apart, say 600mV and 598mV, the two currents would
> be 14.400V/150kOhm = 96uA and 14.402V/150kOhm = 96.01uA.
> Should not have a great effect on Vbe.

Juergen, we just look at this from two diffent perspectives... and there is no
point in us building up arguments against each other...
You asked an question and I gave my view...

> > The current source isn't expensive, so WHY cut it out?
> 
> I build it once, on veroboard. Searching it somewhere in the
> closet or basement takes me longer than soldering two
> resistors to a transistor socket. AND the two-resistor
> method works bipolar, i.e. one socket for both NPN
> and PNP: Base is tied to gnd, Collector and Emitter are
> connected to adjacent rails with 150k resistors.
> So you measure at a *nearly* constant current, and at
> a collector voltage of 0.6V. If you plug in a PNP, just
> turn it upside down, the circuit is symmetrical!

OK, if you feel happy about doing that, then do that. What I wanted to say was
that I would probably spend the effort of doing a little more serious device if
building one at all.

Also, recall that the transistors should be close in temperature, so while you
let them stabilze you can dig up that curcuitboard...

You asked if you where missing something and I thougth you where... maybe not
on a large scale, but I still wanted to point out a few limits of the curcuit.
If you feel like disregard some of those, that's your decision.

Currently I am quite tired of the flame-happines that exist on some other lists
so I won't go into deep argument without a good reason. I will go back to my
semiconductor book that I am supposed to do deep studies in anyhow...
If you don't feel my views on the topic comply with yours just leave out, OK?

So, how is that VCA project going?

Sligth Cheers,
Magnus