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A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and sometimes is, a varying signal current.
The inverse of the design problem is finding the current when the resistor values are known. An iterative method is described next. Assume the current source is biased so the collector-base voltage of the output transistor Q 2 is zero. The current through R 1 is the input or reference current given as,
Fig. 3: Small-signal model for impedance calculation. A circuit is a current source only to the extent that its output current is independent of its output voltage. In the circuits of Fig. 1 and Fig. 2, the output voltage of importance is the potential from the collector of Q 3 to ground.
The output part of the simple current mirror is an example of such a current source widely used in integrated circuits. The common base, common gate and common grid configurations can serve as constant current sources as well. A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the I DSS of ...
In semiconductor electronics, Dennard scaling, also known as MOSFET scaling, is a scaling law which states roughly that, as transistors get smaller, their power density stays constant, so that the power use stays in proportion with area; both voltage and current scale (downward) with length.
Most commonly the active load is the output part of a current mirror [1] and is represented in an idealized manner as a current source. Usually, it is only a constant-current resistor that is a part of the whole current source including a constant voltage source as well (the power supply V CC on the figures below).
Fig. 3: The two operating states of a non-isolated Ćuk converter. The diode and the switch are simplified as either a short circuit when they are on or by an open circuit when they are off. When in the off-state, the capacitor C is charged by the input source through the inductor L 1.
A resistor (with the current simply proportional to the voltage) would be better, and a current source (with the current fixed, independent of voltage) better yet. A depletion-mode device with gate tied to the opposite supply rail is a much better load than an enhancement-mode device, acting somewhere between a resistor and a current source.