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

[jhaible at t-online.de]

> 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.