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Basic current mirror
Introduction and Theory

The conventional biasing techniques for MOS amplifiers prove inadequate for high-performance microelectronic circuits. For example, the bias current CS stage is a function of the supply voltage—a serious issue because in practice, this voltage experiences some variation. The rechargeable battery in a cellphone or laptop computer, for example, gradually loses voltage as it is discharged, thereby mandating that the circuits operate properly across a range of supply voltages. Another critical issue in biasing relates to ambient temperature variations. A cellphone must maintain its performance at 20_C in Finland and +50_C in Saudi Arabia. Another critical issue in biasing relates to ambient temperature variations. A cellphone must maintain its performance at 20_C in Finland and +50_C in Saudi Arabia.

Fortunately, an elegant method of creating supply- and temperature-independent voltages and currents exists and appears in almost all microelectronic systems. Called the “bandgap reference circuit” and employing several tens of devices, this scheme is studied in more advanced books. The bandgap circuit by itself does not solve all of our problems! An integrated circuit may incorporate hundreds of current sources, e.g., as the load impedance of CE or CS stages to achieve a high gain. Unfortunately, the complexity of the bandgap prohibits its use for each current source in a large integrated circuit. Let us summarize our thoughts thus far. In order to avoid supply and temperature dependence, a bandgap reference can provide a “golden current” while requiring a few tens of devices. We must therefore seek a method of “copying” the golden current without duplicating the entire bandgap circuitry. Current mirrors serve this purpose.