Wilson Current Mirror - Advantages and Limitations

Advantages and Limitations

There are a number of other possible current mirror configurations in addition to the standard two-transistor mirror that a designer may choose to use. These include ones in which the mismatch from base current are reduced with an emitter follower, circuits that use cascoded structures or resistor degeneration to lower the static error and raise output impedance, and gain-boosted current mirrors that use an internal error amplifier to improve the effectiveness of cascoding. The Wilson current mirror has the particular advantages over alternatives that:

• The static error, the input-output current difference, is reduced to very small levels attributable almost entirely to random device mismatches while the output impedance is raised by a factor of simultaneously.
• The circuit uses minimum resources. It does not require additional bias voltages or large area resistors as do cascoded or resistively degenerated mirrors.
• The low impedance of its input and internal nodes makes it possible to bias the circuit for operation at frequencies up to .
• The four-transistor version of the circuit has extended linearity for operation at high currents.

The Wilson current mirror has the limitations that:

• The minimum potentials from input or output to the common rail connection that are needed for proper operation are higher than for the standard two-transistor mirror. This reduces the headroom available to generate the input current and limits the compliance of the output.
• This mirror uses feedback to raise the output impedance in such a way that the output transistor contributes collector current fluctuation noise to the output. All three transistors of the Wilson current mirror add noise to the output.
• When the circuit is biased for high frequency operation with maximum, the negative feedback loop that maximizes the output impedance may cause peaking in the frequency response of the mirror. For stable, low-noise operation it may be necessary to modify the circuit to eliminate this effect.
• In some applications of a current mirror, particularly for biasing and active load applications, it is advantageous to produce multiple current sources from a single input reference current. This is not possible in the Wilson configuration while maintaining an accurate match of the input current to the output currents.