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Constant Current Source Generator

IP.com Disclosure Number: IPCOM000088047D
Original Publication Date: 1977-Apr-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Blumberg, RJ: AUTHOR [+3]

Abstract

This is a constant current source circuit providing tightly controlled down level for current switch emitter-follower circuits.

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Constant Current Source Generator

This is a constant current source circuit providing tightly controlled down level for current switch emitter-follower circuits.

As illustrated in the drawing, the voltage at node VT is set by a divider network illustrated by resistors RD1 and RD2. (An external supply could be used to supply the potential at node VT.) The voltage at node VE is one base-to- emitter drop (VBE) below the potential at node VT. The voltage across the resistor RE is determined by the difference in potential between node VE and supply potential VEE.

Since transistors T2 and T3 form a differencial amplifier, half the current demanded by resistor RE will come through T2 and the other half through T3. At node V1, half the current demanded by resistor RE will come through T2 and the other half through T3. At node V1, half the current demanded by RE will flow through resistor R1. Resistor R1 is selected to have twice the resistance of resistor RE, such that half the current through twice the resistance will provide the same voltage drop. The potential at VX will be one VBE below the potential at node V1 through transistor T1. The potential of node VX affects the conductivity of transistor T5, which in turn affects the potential at node V3. The resistors R2, R3 and R4 each have a resistance value equal to that of R1, and twice that of resistor RE.

The constant current source, transistor T5, sets a current level such that the differential amplifier can compare the voltage at node V3 with the incoming voltage at node VT. The voltage at V3 is set by the current level to match the incoming voltage VT. If the voltage at V3 is initially different from VT...