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Difference Amplifier

IP.com Disclosure Number: IPCOM000096952D
Original Publication Date: 1962-Feb-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 24K

Publishing Venue

IBM

Related People

Ruben, MA: AUTHOR

Abstract

In difference amplifiers employing transistors of the same conductivity type, the bias currents for the transistors add in a common mode resistor. This necessitates use of a large ideal current source, usually obtained at the expense of a third amplifier element or a very large power supply. By employing complementary type transistors, the bias currents cancel out in the common mode resistor, obviating the need for added circuitry.

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Difference Amplifier

In difference amplifiers employing transistors of the same conductivity type, the bias currents for the transistors add in a common mode resistor. This necessitates use of a large ideal current source, usually obtained at the expense of a third amplifier element or a very large power supply. By employing complementary type transistors, the bias currents cancel out in the common mode resistor, obviating the need for added circuitry.

The circuit employs the NPN and PNP transistors T1 and T2. These respond to A and B input signals at their respective bases to produce X and Y output signals at their collectors. The T1 and T2 emitters are coupled together through a common biasing network, including common mode resistor Rx. Rx provides a slight degree of D. C. stability, but is not large enough to appreciably affect the A.
C. coupling from T1 to T2. In the absence of either input signal, both T1 and T2 conduct and the output signals are at some reference potential. As the level of the A signal rises and the level of the B signal remains stationary, T1 conducts more heavily, increasing the potential at the T2 emitter. This increases the voltage drop across the base-emitter junction of T2, causing it to become more conductive producing an increased potential at the Y output. Since T1 is conducting more heavily, the potential at the X output falls.

When the level of the B signal increases and the level of the A signal remains stationary, T2 conducts t...