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Operational Amplifier with High Common Mode Voltage Capability

IP.com Disclosure Number: IPCOM000080727D
Original Publication Date: 1974-Feb-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Thoma, NG: AUTHOR

Abstract

An operational amplifier that can be used with a high common-mode voltage is described.

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Operational Amplifier with High Common Mode Voltage Capability

An operational amplifier that can be used with a high common-mode voltage is described.

In many applications of operational amplifiers, the single voltage or current to be amplified is superimposed on a voltage exceeding the breakdown voltage of available operational amplifiers. The described circuit allows usage of low- voltage amplifiers with power supplies having hundreds of volts, if common-mode power supplies are utilized.

Amplifiers Al and A2 are powered by a floating power supply PSl having a common output mode 10, a negative output mode 11, and a positive output mode 12. The common terminal 10 of power supply PS1 is tied back to the negative input signal point 13. This permits the common-mode input voltage to be sensed. The differential between the negative input voltage 13 and the positive input voltage 14 is amplified by amplifier A1, and the signal converted to a current by amplifier A2 and transistors T1, T2, T3, or T4.

Resistor 20 between amplifiers A1 and A2 establishes what value a single- current resistor 30 will have. The magnitude of the current can be sensed by differentially measuring the voltage drop across resistor 30 with amplifier A3, which is powered by a ground common power supply PS2. Diodes D1 and D2 protect the transistors T1, T2, T3, and T4 from large reverse polarity collective voltages. The maximum common-mode input signals are fixed by the breakdown voltages of the tra...