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

[Jim Patchell]

    This is the way I understand current mirrors to work....

    -Jim

jhaible at t-online.de wrote:

> > 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.