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Differential Amplifier with High Input Impedance

IP.com Disclosure Number: IPCOM000093046D
Original Publication Date: 1967-Apr-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Taub, DM: AUTHOR

Abstract

The principle of the circuit, shown in drawing 1, is that common-mode signals experience series and shunt negative feedback, whereas differential-mode signals experience series negative feedback and shunt positive feedback, though not enough of the latter to make it unstable. The following analysis ignores high-frequency effects and the effects of component tolerances and assumes that transistor base current is negligible.

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Differential Amplifier with High Input Impedance

The principle of the circuit, shown in drawing 1, is that common-mode signals experience series and shunt negative feedback, whereas differential- mode signals experience series negative feedback and shunt positive feedback, though not enough of the latter to make it unstable. The following analysis ignores high-frequency effects and the effects of component tolerances and assumes that transistor base current is negligible.

First, consider the response of the circuit to common-mode signals. Since both input terminals are at the same potential, the circuit can be redrawn, as in drawing 2. For an input voltage vin, currents and voltages at various points in the circuit are as shown. Summing the currents at the output terminal

From this equation it follows that v(out) = 0 when R(3) = 2R(1) + R(2). The effect of collector-to-base capacitance in the transistors can be cancelled by providing an equal capacitance between each emitter and ground.

Analyzing the response to differential-mode signals, with R(3) given as above, See orignial page 1638.

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