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[SOLVED] Resistors in Wilson current mirror

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northumber82

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I have some questions concerning analog audio amplifiers: what is the purpose of resistors R1 and R2 in the Wilson current mirror? With what formula are they calculated? Can they be omitted? If they are omitted, what happens to the VAS (Voltage Amplifier Stage)?

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They improve Vbe-match and current-match fidelity (if
the resistors have better matching tolerance than the
transistors' Vbe). You need to establish a current across
the resistors which dwarfs the mismatch voltage (like,
if transistor population has a +/-10mV Vbe scatter,
setting a 100mV (Iset*R) value ought to improve the
mismatch 90%.

You lose a little headroom but in older +/-15V analog
(or discrete) this is tolerable.

An audio circuit doesn't care about DC as long as
it's centered and linear, right? If there's feedback
then small mismatch error in current mirrors may not
amount to anything. You'd need mismatch modeled
to say.
 
Current mirror need monolithic transistor pairs to achieve reasonable matching. The emitter degeneration resistors are a possible means to get a kind of current mirror behavior with discrete transistors.
 
They can also be used to achieve PTAT Current by well adjusting Resistor and Transistor Multiplication number..
 
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