To avoid this situation, an active load can be used, for example, a current mirror.

There are three main specifications that characterize a current mirror.

All three transistors of the Wilson current mirror add noise to the output.

The input voltage of the Wilson current mirror is .

This impedance is twice the value of for the standard two-transistor current mirror.

The Wilson current mirror has the particular advantages over alternatives that:

The current mirror is used to provide bias currents and active loads to circuits.

The simplest bipolar current mirror (shown in Figure 1) implements this idea.

One way to increase R is to replace this resistor by an active load, for example, a current mirror.

But if the two transistors are widely separated, the precision of the current mirror is compromised.

I also use a laptop to keep a current mirror of the PC that contains most of my data.

There are three principal metrics of how well a current mirror will perform as part of a larger circuit.

In large-signal hand analysis, a current mirror is usually and simply approximated by an ideal current source.

Series negative feedback is also used in the two-transistor current mirror with emitter degeneration.

Figure 3 shows a bipolar current mirror with emitter resistors to increase its output resistance.

An approximate analysis due to Gilbert shows how the Wilson current mirror works and why its static error should be very low.

First, the emitter-collector voltages of the inner current mirror formed by Q1 and Q2 are not the same.

The result is that new designs tend to use some variant of a wide swing cascode current mirror configuration.

The current mirror inverts Q3 collector current and tries to pass it through Q4.

Figure 3 shows a small signal model of a Wilson current mirror drawn with a test voltage source, , attached to the output.

Thus a current mirror consists of two cascaded equal converters (the first - reversed and the second - direct).

The Wilson current mirror has an output impedance that is higher by the factor , on the order of 50X.

The collector resistors can be replaced by a current mirror, whose output part acts as an active load (Fig. 3).

The output part of the simple current mirror is an example of such a current source widely used in integrated circuits.

The input stage is a composed differential amplifier with a complex biasing circuit and a current mirror active load.