Transistor Current Mirror Circuit

- the transistor current mirror circuit used in many applications including integrated circuit technology to mirror the current of one device in another.

Current mirror circuits are widely used, especially within integrated circuit technology.

The mirror circuit generally consists of two transistors, although other devices such as FETs can be used, and some configurations do use more than two devices in the overall circuit to obtain better performance.

The current mirror circuit gains its name because it copies or mirrors the current flowing in one active device in another, keeping the output current constant regardless of loading.

The current being mirrored can be a constant current, or it can be a varying signal dependent upon the requirement and hence the circuit.

Conceptually, an ideal current mirror is simply an ideal inverting current amplifier that reverses the current direction as well or it is a current-controlled current source (CCCS). The current mirror is used to provide bias currents and active loads to circuits.

Current mirror circuit

The basic circuit is shown in the diagram below. It comprises two transistors, one of which has the base and collector connected together. The base connections of both transistors are then linked, as are the emitters.

Diagram showing the circuit of a transistor current mirror
Current Mirror Transistor Circuit

In terms of the operation of the circuit, the base emitter junction of T1 acts like a diode because the collector and base are connected together.

The current into TI is set externally by other components, and as a result there is a given voltage built up across the base emitter junction of T1. As the base emitter voltage on both transistors is the same, the current in one transistor will exactly mirror that of the second, assuming that both transistors are accurately matched. Therefore the current flowing into T1 will be mirrored into T2 and hence into the load R1.

Circuit limitations

The circuit shown above is often quite adequate for most applications. However the circuit has some noticeable limitations under many circumstances:

  • Current matching dependent on transistor matching:   The current mirroring is dependent upon the matching of the transistors. Often the transistors need to be on the same substrate if they are to accurately mirror the current.
  • Current varies with change in output voltage:   This effect occurs because the output impedance is not infinite. This is because there is a slight variation of Vbe with the collector voltage at a given current in T2. Often the current may vary by about 25% the output compliance range.

Current mirror with emitter resistors

One solution to the variation of current over the compliance range is to introduce a small amount of resistance into the emitter of each transistor. Typically these resistors are chosen to have a few tenths of a volt drop across them

Diagram showing a current mirror with resistors added improve performance
Current Mirror Transistor Circuit with emitter resistors

For this circuit, both emitter resistors and transistors need to be matched. This is obviously easy for the resistors where close tolerance resistors are easily available.

Wilson current mirror circuit

Another variation of the basic current mirror circuit is referred to as the Wilson mirror or Wilson current mirror.

Within the circuit, a third transistor is introduced. This transistor, shown as T3 in the diagram keeps the collector of T1 at a voltage equivalent to two diode drops below the rail voltage Vcc.

This overcomes the previous effect. Also the transistor without the short circuit collector base connection becomes the programming terminal.

Circuit diagram of the Wilson version of the current mirror circuit.
Wilson current mirror circuit

By Ian Poole

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