AW: current mirrors (was: AW: [sdiy] discrete SSM2018?)

[Czech Martin]

Well, I thought that you feed a current mirror with a current.
In the typical operation I assume identical current gain,
but one transistor has to feed the two bases.
Of course the exponential law is involved, but it is 
different if you think about a voltage driven differential amplifier.

One one hand I think in terms of current, on the other in terms
of voltage to current conversion. I just wanted to say that
the exponential characteristic of a transistor is something
different then the collector effectivity.
Or in other terms: how much of the emmitter current will
make it to the collector? This depends on base width an impurity
as well as collector doping.
The exponential law will mainly depend on emitter-base diode,
collector is not so important here.

So I think it is possible that offset will have no or weak
correlation with beta. Unfortunetly i can not proove this,
since I anly have access to CMOS data.

Anybody here with bipolar statistics?

m.c.

> -----Ursprüngliche Nachricht-----
> Von: jhaible at t-online.de [mailto:jhaible at t-online.de]
> Gesendet: Mittwoch, 7. November 2001 16:23
> An: Martin.Czech at Micronas.com
> Cc: synth-diy at dropmix.xs4all.nl
> Betreff: current mirrors (was: AW: [sdiy] discrete SSM2018?)
> 
> 
> 
> > Well, the current transfer ratio of a mirror depends basically
> > on current gain. 
> 
> Is this true ? The way I understand a current mirror (at least
> the simple 2-transistor version) is this:
> *Vbe* of both transistors is the same, so if they are identical,
> and if the Early effect is neglegted, they have the same collector 
> current. (The remaining error depends on beta, as one side of the 
> mirror feeds both base currents.) But basically, the mirror function
> depends on the collector current as a function of the base-emitter
> *voltage*, as this voltage is forced identical on both transistors.
> The overall current mirror is formed by a exp(log()) function.
> If one transistor needs a different vbe for a certain collector
> current (the same thing that appears as offset voltage in a 
> differential amplifier), this voltage offset (addition) is transformed
> to a current ratio (multiplication). That's how I always understood
> it.
> And the 3-transistor mirrors will minimize the beta influence and
> also the Early effect, but the principle would otherwise be identical.
> So this large signal Ic(vbe) funtion and its matching for two 
> transistors 
> would be the main effect for both, the offset of a differential pair,
> or a transfer rate unequal 1 for current mirrors.
> I'm aware that this must be a somewhat simplified view - but where is
> the error ?
> 
> JH.
>