A current mirror is a type of electrical circuit design where 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 mirror each other in value. Current mirror circuits are usually designed with bi-polar junction transistors (BJT) such as the NPN transistor, where a positively doped (P-doped) semiconductor base is sandwiched between two negatively doped (N-doped) layers of silicon. These transistors are specifically designed to amplify or switch current flow. In some current mirror design specifications, the NPN transistor can act as a inverting current amplifier, which reverses the current direction, 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 basic component of analog circuit designs and usually more than one current mirror is present within a circuit. They can be used to produce a much lower-level output current than what was input, or, in cases where a Wilson mirror is used, produce an increased 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 is able to balance output current values regardless of input load or resistance levels over a specified range of operation for the circuit.
One of the reasons that bi-polar junction transistors are used for current mirror design is due to the fact that the base-emitter, or PN portion, of the transistor functions reliably like a diode. Diodes regulate both the amount of current that passes as well as the forward voltage drop for that current. In most circuits, the diode current so closely matches that of the output current for transistors in the current mirrors that reducing the resistance that a diode experiences can be used as a precise calculation to determine the increase in voltage drop across the PN emitter junction of the transistors. This means that the collector current for input values on the transistors also has a direct mirror quality for diode currents within the same circuit.
For output current to be constant, however, in a current mirror, the temperature of all of the NPN transistors must also remain at a constant level. This is controlled in circuit design by physically gluing all of the current mirror transistors together or 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 performed by resistors in the circuit. This is because it is easier to fabricate transistors onto the silicon surface of integrated circuits than it is to etch resistor components onto them.