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Current mirror circuits generally consist two main transistor, although other devices such as FETs can be used. Some current mirror circuits may use more than two transistors to enable the level of performance to be improved.
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 of the transistor current mirror is shown in the diagram below. It comprises two transistors, one of which has the base and collector connected and the other does not. The base connections of both transistors are then linked, as are the emitters which are also taken to ground.
In terms of the operation of the circuit, the base emitter junction of TR1 acts like a diode because the collector and base are connected together.
The current into TRI is set externally by other components, and as a result there is a given voltage built up across the base emitter junction of TR1. 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 TR1 will be mirrored into TR2 and hence into the load R1.
The two transistor current mirror circuit shown above is often quite adequate for most applications. However it has some noticeable limitations under many circumstances:
- 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 TR2. Often the current may vary by about 25% the output compliance range.
- 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.
To overcome some of these issues more advanced current mirror circuits cna be developed and used.
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.
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 TR3 in the diagram keeps the collector of TR1 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.
Current mirror circuits are very useful, especially within integrated circuits. The components can easily be incorporated into the design for little cost. As such they enable balanced currents to be supplied to circuits like differential pairs and the like and this ensures that their operation is improved further. Current mirrors are not widely used outside IC technology in view of the additional number of components required, but nevertheless the principles are the same in both discrete form and when used within ICs.
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