A Four-Transistor Improved Mirror
Adding a fourth transistor to the Wilson current mirror as in Fig. 4a equalizes the collector voltages of Q1 and Q2 by lowering the collector voltage of Q1 by an amount equal to VBE4. This has three effects: first, it removes any mismatch between Q1 and Q2 due to the Early effect in Q1. This is the only first order source of mismatch in the three-transistor Wilson current mirror Second, at high currents the current gain, of transistors decreases and the relation of collector current to base-emitter voltage deviates from . The severity of these effects depends on the collector voltage. By forcing a match between the collector voltages of Q1 and Q2, the circuit makes the performance degradation at high current on the input and output branches symmetric. This extends the linear operating range of the circuit substantially. In one reported measurement on a circuit implemented with a transistor array for an application requiring 10 mA output, the addition of the fourth transistor extended the operating current for which the circuit showed less than 1 percent difference between input and output currents by at least a factor of two over the three transistor version.
Finally, equalizing the collector voltages also equalizes the power dissipated in Q1 and Q2 and that tends to reduce mismatch from the effects of temperature on VBE.
Read more about this topic: Wilson Current Mirror
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