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Thu Dec 23 15:37:56 EST 2010

OTA mismatch: diff pair + current mirror

An OTA needs two matched pairs: the differential input pair and the
active load current mirror pair.  The effect of mismatch is

      DIFF PAIR:       input offset voltage.
      CURRENT MIRROR:  fixed current ratio between 2 legs.

The CMMR comes from the emittor resistor, which is large if a current
source is used, and mismatch doesn't enter the picture here.  See
Early effect[1].

So what is the effect of these the current mirror mismatch?  Is it
also just offset?

The current ratio in the two legs is proportional to exp(V1-V2 / V_T).
In the case where the current mirror has a mismatch current ration a,
and the output current is zero, the mismatch is carried by a _fixed_
voltage offset between the two inputs.

Does this matter at all when we AC-couple the input and provide proper
DC biasing?  What is the real disadvantage of DC offset for audio
applications?

( Context: can we compensate for mismatch without having to perform
  component selection?  Matched pairs are expensive, though there are
  some cheap FET SMT ones: FDG6303N is $7 in 3000 volumes ).

[1] http://en.wikipedia.org/wiki/Early_effect



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