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The current mirror shown in the figure is a modification of a so-called composite PNP current source and has good DC and transient performance.
English (United States)
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Integrated Circuit Precision PNP Current Mirror
The current mirror shown in the figure is a modification of a so-called
composite PNP current source and has good DC and transient performance.
In operation, input current I(in) drawn from high gain composite structure
formed from lateral PNP transistor T(1) and NPN transistor T(3) is required to
generate an identical output current I(out) from an identical interconnected
composite structure formed from transistors T(2) and T(5). The circuitry is
required to operate over a wide range of input current and to be suitable for
production as an integrated circuit.
A problem associated with the basic arrangement is that the gain of lateral
PNP transistors is low and varies considerably with variation in current. This
variation in gain gives rise to variation of base current of transistors T(1) and T(2)
which, as a result of the additional current flow through diode T(4), produces
mismatch between input and output currents. The problem is solved by the
insertion of an additional resistor R between the base and collector of transistor
T(1) and by scaling the emitter ratio N of transistors T(3) and T(5) with respect to
diode T(4). The value of the additional resistor R and the emitter ratio N are
determined by calculation so as to introduce the required degree of nonlinearity
to the circuit necessary to provide the compensation needed for a high precision