Importance

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Abstract

A current mirror having an output-to-input current ratio less than unity comprises first and second transistors connected in cascode between an output terminal and a reference terminal, with the base drive of each transistor coupled to the same diode junction. Cascoding plural transistors effectively reduces the base-emitter voltage of the output transistor by the collector-emitter voltage of the other transistor, which normally operates at saturation. The other transistor may be a multi-emitter transistor, with one of the emitters coupled to its base and another of which coupled to the reference terminal. By the addition of a resistor between the emitter-collector connection of the cascoded transistors and the input terminal, the operation of the circuit may be changed from that of a current mirror to a current switch. Rather than being referenced to the rectifying junction of a diode, the base drives of the cascoded transistors may be referenced to other junction devices, such as a bipolar transistor, diode-connected bipolar transistor, Schottky diode, or a control (cathode) gate of a silicon controlled rectifier (SCR). The latter connection has particular utility in controlling the turn-off of a gate turn-off thyristor (GTO SCR), such as may be incorporated in a regulated power supply circuit.

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First Claim