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Amplifier with High Common Mode Input Impedance

IP.com Disclosure Number: IPCOM000093810D
Original Publication Date: 1966-Feb-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Bakke, EP: AUTHOR [+4]

Abstract

High common mode input impedance is generated in a potentiometric feedback differential amplifier. This is effected by driving the first differential stage transistors 1 and 2 through emitter-followers 3 and 4 which are supplied by high impedance current sources 5 and 6. The common mode signal applied to the emitters of 1, 3 2 and 4 is fed back to the collectors of 3 and 4 through resistors 9 and 10. The latter have resistances considerably less than the impedances of 5 and 6.

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Amplifier with High Common Mode Input Impedance

High common mode input impedance is generated in a potentiometric feedback differential amplifier. This is effected by driving the first differential stage transistors 1 and 2 through emitter-followers 3 and 4 which are supplied by high impedance current sources 5 and 6. The common mode signal applied to the emitters of 1, 3 2 and 4 is fed back to the collectors of 3 and 4 through resistors 9 and 10. The latter have resistances considerably less than the impedances of 5 and 6.

Since the common mode signal is applied to the collectors as well as the emitters of 3 and 4 common mode inputs to their bases do not reduce the reverse bias potential at the base-collector junctions of 3 and 4. This generates an extremely high common mode input impedance.

Resistors 9 and 10 function to keep 3 and 4 out of saturation and can also be a string of forward biased diodes or a zener diode. Resistors 11 and 12 increase the operating current in 3 and 4 to reduce noise generation.

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