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LATERAL TRANSISTOR CURRENT SENSING THAT COMPENSATES FOR EARLY VOLTAGE

IP.com Disclosure Number: IPCOM000006777D
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2002-Jan-31
Document File: 1 page(s) / 50K

Publishing Venue

Motorola

Related People

Thomas Petty: AUTHOR [+3]

Abstract

Transistors Ql and Q2 shown in the above sche- matic are the output devices of an operational amplifier that is capable of a large dynamic output voltage swing. The function of the remaining circuitry is to accurately reproduce a fraction (l/N) of the amplifier's output cur- rent (Iout) at the SENSE OUT terminal. The current Is,,,, must be independent of the voltage which appears at the SENSE OUT terminal.

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MOTOROLA INC. Technical Developments Volume 18 March 1993

LATERAL TRANSISTOR CURRENT SENSING THAT COMPENSATES FOR EARLY VOLTAGE

by Thomas Petty, Robert L. Vyne, and Renwin J. Vee 0 vcc

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  Transistors Ql and Q2 shown in the above sche- matic are the output devices of an operational amplifier that is capable of a large dynamic output voltage swing. The function of the remaining circuitry is to accurately reproduce a fraction (l/N) of the amplifier's output cur- rent (Iout) at the SENSE OUT terminal. The current Is,,,, must be independent of the voltage which appears at the SENSE OUT terminal.

  Relative to lateral PNP transistors, NPNs exhibit very high output impedance. This permits sensing the cur- rent in the output NPN (42) directly with the sense NPN transistor QlO while achieving a high degree of accuracy. Unfortunately, attaining an equivalent level of accuracy while sensing current in the PNP output tran-

sistor (Ql), is not as direct and simple. Because both Ql and QS exhibit poor output impedance, differences in their collector-base voltages will result in significant errors in the sense current. This problem is overcome by forc- ing the collector-base voltage of Q8 to be equal to and also track the collector-base voltage of Ql even though the collector voltage of Ql changes as the amplifier out- put voltage swings throughout its dynamic range. Tran- sistors Q3 and Q9 force the collector voltage of QS to equal the collector vo...