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Integrating Amplifier with AC Common Mode Noise Rejection

IP.com Disclosure Number: IPCOM000091336D
Original Publication Date: 1968-Jan-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Benson, AB: AUTHOR [+4]

Abstract

The circuit has first difference amplifier 10, emitter-follower 12, and second difference amplifier 14. The output from amplifier 14 is fed back through capacitor C18 to one input of amplifier 10 in order to accomplish the integrating function. The other input to amplifier 10 is grounded through resistor R18.

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Integrating Amplifier with AC Common Mode Noise Rejection

The circuit has first difference amplifier 10, emitter-follower 12, and second difference amplifier 14. The output from amplifier 14 is fed back through capacitor C18 to one input of amplifier 10 in order to accomplish the integrating function. The other input to amplifier 10 is grounded through resistor R18.

In normal operation, as transistors T20 and T22 are driven in one direction, transistors T30 and T32 are driven in the other direction so that the voltage at point A remains at signal ground. However, when AC common mode noise is present, the voltage at point A is driven away from signal ground. AC common mode noise comprises high-frequency noise signals applied to the bases of T20 and T30 in phase. The result is that T22 and T32 are driven in the same direction causing the voltage at point A to move away from signal ground.

The voltage at point A can, in fact, be used to monitor common mode noise since, when there is no noise, the voltage at point A is at signal ground. This characteristic is used to provide a feedback control to a current source in amplifier 10 for the purpose of compensating for the common mode noise thus eliminating it. AC fluctuations at point A are fed back to the base of transistor T34 by capacitor C36. T34 is a current source controlling the combined current flow through T20 and T30.

In operation, AC common mode noise causes an increase in current through T20 and T30 in the sa...