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Ideal independent current sources are challenging to fabricate within integrated circuits. Instead, current mirrors using FETs are commonly employed. These circuits consist of two or more FETs where the drain of one FET is tied to the rest of the circuit. The operating conditions of the current mirror determine the dependent current source. This overview explains the conventional design of transistor current mirrors, highlighting the operational limits and conditions for effective functioning, including saturation mode and design specifications.
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FET Current Mirrors ECE 2204
Current Sources • Ideal independent current sources are difficult to make and are almost impossible to fabricate on an integrated circuit. • Instead, current mirrors are fabricated. • These are circuits that contain two or more FETs, where the drain of one of the FETs is connected to the rest of the circuit. • This FET is operating in the saturation/pinch-off mode. Thus, it can be thought of as a dependent current source. • The value of this dependent current source is determined by the operating conditions of the other FETs in the current mirror, not by the operating condition of the rest of the circuit (at least over a certain range of currents).
Common Design in Current Mirrors • At least two transistors • Two transistors are wired in parallel • shared VGS • One transistor has the drain and gate tied together • VDS > VGS – VTN; it is in saturation
Bias Condition of M2 • M2 is assumed to also be in saturation. • This depends in part on the rest of the circuit above M2 • If the circuit above M2 requires too much current, then M2 will be forced into the triode/nonsaturation region. At which point, the current mirror circuit is not functioning properly – which means someone didn’t design their part of the circuit to the correct specification.
