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Thu Dec 23 15:00:39 EST 2010

Transistor matcher

Matching transistors for differential pairs and current mirrors is not
about matching current gain, but about matching the voltage to current
curve, which corresponds to matching the reverse saturation current of
the base–emitter diode in the Ebers-Moll model.

The Moog guideline for matching[1] is to match V_BE to 1mV for 100uA.
This is a current mismatch of e^(1mV/26mV) wich is 4% relative current
mismatch.

So it seems a higher matching accuracy is possible by measuring the
current difference directly, i.e. to use a one-to-many current mirror
array.  With high current gain transistors and 4% current matching,
the effect of the double base current in the controlling leg is
negligible.

Can probably be microcontrolled.  Would need calibration of the
resistor values used to measure the current.

[1] http://www.fantasyjackpalance.com/fjp/sound/synth/synthdata/16-minimoog/002/905-matching-transistor.gif



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