Bipolar current mirror - Circuit diagram
The circuit diagram example "Bipolar current mirror" was redesigned from the Wikipedia file: Current mirror.png.
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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. Conceptually, an ideal current mirror is simply an ideal inverting current amplifier that reverses the current direction as well or it is a current-controlled current source (CCCS). The current mirror is used to provide bias currents and active loads to circuits. ...
Basic BJT current mirror.
If a voltage is applied to the BJT base-emitter junction as an input quantity and the collector current is taken as an output quantity, the transistor will act as an exponential voltage-to-current converter. By applying a negative feedback (simply joining the base and collector) the transistor can be "reversed" and it will begin acting as the opposite logarithmic current-to-voltage converter; now it will adjust the "output" base-emitter voltage so as to pass the applied "input" collector current.
The simplest bipolar current mirror ... implements this idea. It consists of two cascaded transistor stages acting accordingly as a reversed and direct voltage-to-current converters." [Current mirror. Wikipedia]
The circuit diagram example "Bipolar current mirror" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Electrical Engineering solution from the Engineering area of ConceptDraw Solution Park.