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
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
( 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 ).