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# Accurate Current Mirror

IP.com Disclosure Number: IPCOM000120006D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 64K

IBM

## Related People

Buckley III, F: AUTHOR [+1]

## Abstract

In the figures, P, O, and N are the plus supply, ground reference, and the negative supply. The number near resistors is their value in K-ohms.

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Accurate Current Mirror

In the figures, P, O, and N are the plus supply, ground
reference, and the negative supply.  The number near resistors is
their value in K-ohms.

The output of an ideal current mirror is a current source whose
value is a constant (usually one) times its input current.  Current
mirrors are commonly used to bias analog circuitry.  In the simple
transconductance amplifier in Fig. 1, current mirror A allows input
offset voltage to be independent of variations in reference voltage
R.  Ideal offset means the current out of Z is zero when the voltage
from A to B is zero.  This will be nominally true if the collector
current in E and J are the same, or if the current mirror output
current (lead 2) is half the sum of the collector currents of E and
J.  Nominally, the alpha's of C, F, H and the resistances of D, G, I
are identical, so the collector current of B is the sum of the
collector currents of E and J, and if the current mirror has a gain
of exactly 1, the offset will be zero.  Note that variation in
reference voltage R (or the tracking Vbe's of C, F, H or the tracking
resistances D, G, I) changes the collector currents of B, E, J, but
not their ratio, so if the current mirror has a gain of one, zero
offset is maintained.

The collector of a lateral PNP is the only element available in
most bipolar processes to build a high quality (high impedance) cur
rent source from the positive supply. But lateral PNP beta can be so
lo...