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# Current Limiting Achieved by Variable Impedance

IP.com Disclosure Number: IPCOM000040795D
Original Publication Date: 1987-Jan-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 1 page(s) / 11K

IBM

## Related People

Trythall, LM: AUTHOR

## Abstract

Described is a circuit which uses variable impedance for controlling output current. With the variable impedance method, current limiting is a function of output voltage.

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Current Limiting Achieved by Variable Impedance

Described is a circuit which uses variable impedance for controlling output current. With the variable impedance method, current limiting is a function of output voltage.

The figure shows a circuit in which the output current is a function of the output voltage. Resistors ROUT1 and ROUT2 establish a voltage threshold. This voltage threshold controls the operating region of Q2 with respect to the output voltage. By adjusting the operating region of Q2, the effective emitter impedance of the reference current source, Q1 is varied. The variation of the effective emitter impedance controls the current source that controls the base drive of the output PNP transistor Q4. This control of base drive of Q4 enables output current limiting. Thus, the output current limiting is directly a function of output voltage. Furthermore, output current limiting is achieved by a logarithmic gain feedback loop. The feedback loop includes resistors RDIF, R1, R2, ROUT1, ROUT2, diodes D1 - D3 and transistors Q2-Q5 .

The logarithmic gain feedback loop is dominated by the equation of the base- emitter diode of Q5. This exponential "knee" of the output current limiting curve, not shown, provides significant changes in output current for small changes in output voltage.

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