Browse Prior Art Database

Precise Current Mirror

IP.com Disclosure Number: IPCOM000041663D
Original Publication Date: 1984-Feb-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Schmerbeck, TJ: AUTHOR

Abstract

The current mirror circuit shown in the figure is designed for use on semiconductor integrated circuits. The function of a current mirror circuit is to generate a mirror current IM which is an exact copy of a reference current IR. The measure of the accuracy of a current mirror circuit is how closely the mirror current equals the reference current. Resistors R1 and R2 are of identical geometry and are in close proximity so that their resistance values are equal and track very closely. Transistors Q1 and Q2 are of identical geometry and are in close proximity so that their base-emitter voltage characteristics and current gains are equal and track very closely, and likewise for transistors Q3 and Q4.

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Precise Current Mirror

The current mirror circuit shown in the figure is designed for use on semiconductor integrated circuits. The function of a current mirror circuit is to generate a mirror current IM which is an exact copy of a reference current IR. The measure of the accuracy of a current mirror circuit is how closely the mirror current equals the reference current. Resistors R1 and R2 are of identical geometry and are in close proximity so that their resistance values are equal and track very closely. Transistors Q1 and Q2 are of identical geometry and are in close proximity so that their base-emitter voltage characteristics and current gains are equal and track very closely, and likewise for transistors Q3 and Q4. Resistors R1 and R2 are emitter degeneration resistors that reduce the small effect of Q1-Q2 base emitter-voltage mismatch even more. (The circuit accuracy improvement is achieved even if this assumption of device equality is not made, but the mathematics is made much simpler by the assumption.) Since transistors Q1 and Q2 have the same base termination point and their emitter circuits are identical, they will carry identical emitter currents. Also, since their current gains BETA 1, 2 are closely matched, their base currents and collector currents are equal. The reference current IR is made up of Q1 collector current IC and Q3 base current IB3. The mirrored current IM is made up of Q2 collector current IC and Q4 base current IB4. Since Q1 and Q2 have equal collector currents, the only difference between the reference and mirrored currents is the difference between IB3 and IB4. The only differenc...