Mon May 12 00:16:31 EDT 2014

Current Mismatch

The two 2N3904 I glued together, with R_C = 10kOhm matched and base
connected give:

3.13V (regulated by opamp)

that's an error of 1.31 or 0.27 V_T or 6mV

So, while the error in current can be significant, the offset itself
isn't very large.

The current seems to be still temperature dependent though it's not
clear if this is just due to bad thermal coupling between the two
transistors.  It's clear that the effect is larger touching one of the
two separate.  One decreases the other increases the current.

If a commercial matched pair[1] advertises 2mV offset (8% current
mismatch), that 6mV doesn't seem too bad.  As long as temperature
drift is under control this can all be compensated by frequency
measurements of oscillator/filter circuits.

[1] http://www.nxp.com/products/bipolar_transistors/general_purpose_bipolar_transistors/matched_pair_transistors/