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LATERAL TRANSISTOR CURRENT SENSING THAT COMPENSATES FOR EARLY VOLTAGE

IP.com Disclosure Number: IPCOM000006738D
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2002-Jan-28
Document File: 1 page(s) / 50K

Publishing Venue

Motorola

Related People

Tom Petty: AUTHOR [+3]

Abstract

SENSE cur 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 amplier's output cur- rent (Iout) at the SENSE OUT terminal. The current Isense must be independent of the voltage which appears at the SENSE OUT terminal.

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MO-LA INCm Technical Developments Volume 18

March 1993

LATERAL TRANSISTOR CURRENT SENSING THAT COMPENSATES FOR EARLY VOLTAGE

by Tom Petty, Bob Vyne, and Renwin Yee

SENSE cur

  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 amplier's output cur- rent (Iout) at the SENSE OUT terminal. The current Isense must be independent of the voltage which appears at the SENSE OUT terminal.

  Relative to lateral PNP transistors, NPNs exhiiit very high output impedance. This permits sensing the cur- rent in the output NPN (Q2) 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 Q8 exhibit poor output impedance, dierences 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 ampliier out- put voltage swings throughout its dynamic range. Tran- sistors Q3 and Q9 force the collector voltage of 48 to equal the collector voltage of Ql by level shifting the collector voltag...