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A current mirror circuit uses the current flow in one section to regulate the current flow in other sections, typically using bipolar junction transistors. It acts as a current regulator and can produce lower or higher output currents. The use of NPN transistors is reliable due to their diode-like function, but temperature must remain constant for the output current to be constant. Current mirrors are common in analog circuit designs and are easier to fabricate than resistors on integrated circuits.
A current mirror is a type of electrical circuit design in which the current flow in one section of the circuit is used to regulate the current flow in other sections, so that the output of two or more regions mirrors each other in value. Current mirror circuits are typically designed with bipolar junction transistors (BJTs) such as the NPN transistor, where a positively doped (P) semiconductor base is sandwiched between two negatively doped (N) silicon layers. These transistors are specifically designed to amplify or switch the flow of current. In some current mirror design specifications, the NPN transistor can act as an inverting current amplifier, which reverses the direction of the current, or it can regulate a varying pulse current through amplification to create output mirror properties.
The use of a transistor current mirror has become a staple component of analog circuit designs, and there is usually more than one current mirror within a circuit. They can be used to produce a much lower level output current than the input or, in cases where a Wilson mirror is used, produce a higher output resistance level by creating positive feedback loops in the circuit. In its basic form, a current mirror circuit acts as a form of current regulator that balances output current values regardless of input load or resistance levels over a specified operating range for the circuit.
One reason bipolar junction transistors are used for current mirror design is because the base emitter, or PN portion, of the transistor functions reliably like a diode. Diodes regulate both the amount of current flowing through and the forward voltage drop for that current. In most circuits, the diode current so closely matches that of the output current for the transistors in the current mirrors that the reduction in resistance a diode experiences can be used as an accurate calculation to determine the increase in voltage drop across the junction of emitter PN transistor. This means that the collector current for the input values on the transistors also has a direct mirror quality for the diode currents within the same circuit.
For the output current to be constant, however, in a current mirror, the temperature of all NPN transistors must also remain at a constant level. This is controlled in the circuit design by either physically gluing all of the current mirror transistors together or by placing them in close proximity on an integrated circuit (IC) chip so that they share a common temperature. Despite this design limitation, a current mirror amplifier or sinking configuration is common to many circuits as a form of regulator that could also be accomplished by resistors in the circuit. This is because it is easier to fabricate transistors on the silicon surface of integrated circuits than it is to etch resistor components on them.
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