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The Basic Current Mirror

The Basic Mirror Circuit

The basic current mirror.

We got an initial glimpse of current mirrors when looking at the internal circuitry of the 741 op amp. This little circuit is extremely useful in a wide range of applications, especially in integrated circuits. Indeed, it is such a handy little building block that it is well worth taking the time to understand it and its many variations.

The basic current mirror is shown to the right. It consists of nothing more than two bipolar junction transistors with their emitters connected together (and grounded in this example), and with their bases connected together as shown. One transistor, designated to carry the reference current IREF has its collector connected to the common base connection. We have arbitrarily designated this transistor as Q1. The second transistor, Q2, has its collector connected to some external load or circuit. Its collector current is also the output current, IO, of the current mirror.

In this configuration, Q1 automatically sets its base-emitter voltage, VBE1, to the exact value which will cause its collector current, IC1, to flow through the transistor. In so doing, it must at the same time set VBE2 to the same value, since the two transistors are connected together. Therefore IC2, which is also the output current, IO, is the same as IC1.

Thus, by controlling the reference current IREF in a known circuit, we can equally control the current through an external circuit. We can say then that the output current, IO, mirrors the reference current, IREF. This circuit is quite appropriately known as a current mirror.

Setting the Reference Current

The basic current mirror

The easiest way to set the reference current, IREF, is with a single resistor, R, as shown to the left. In this circuit, IREF is set according to Ohm's Law: IREF = (+V - VBE)/R. +V is normally the power supply voltage, which is specified as part of the circuit design parameters. VBE for the transistors is known. Therefore, it is easy to set IREF simply by choosing the value of R in the circuit.

If the two transistors are manufactured side by side at the same time as part of an integrated circuit, they will match very closely in characteristics. If you construct this circuit using individual transistors off the shelf, it will still work, but not as accurately or precisely as if the transistors were matched.

If the transistors are matched, then IO = IC2 = IC1. Of course, there is the matter of the base current for the two transistors, which is supplied through R. However, the current gain (β) of the two transistors is generally more than 100, so the base current is very small and can usually be ignored in practical applications. We'll take a closer look at the details of this issue when we look at Errors in Current Mirrors.

Using Multiple Mirror Transistors

Current mirror with three outputs.

The current mirror circuit is not limited to a single independent output transistor. As shown in the circuit to the right, one reference transistor can support multiple mirror transistors. Each mirror transistor can supply current to a circuit that is independent of all others, and, so long as all transistors are matched, all output currents will be the same, and will still match IC1.

It is also possible to connect the output collectors together and apply them to a single external circuit. In that case, the total output current will be 2IO or 3IO, etc., depending on how many output transistors are interconnected. Or, we could interconnect two transistors on the reference side of the circuit. Then, IO for each mirror transistor will be IC1/2. This is quite practical in integrated circuits, to obtain fixed current ratios in related circuits.

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