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Input Impedance Reduction Through Current Inversion and Negative Bootstrapping

IP.com Disclosure Number: IPCOM000086695D
Original Publication Date: 1976-Oct-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Bond, PW: AUTHOR

Abstract

This circuit decreases the input impedance of common base stage Q8 without the use of lateral PNP transistors or feedback techniques that require compensation.

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Input Impedance Reduction Through Current Inversion and Negative Bootstrapping

This circuit decreases the input impedance of common base stage Q8 without the use of lateral PNP transistors or feedback techniques that require compensation.

The input impedance of Q8 can be written as Rin = delta V/delta I, where delat V is the change in Q8 emitter voltage with respect to ground caused by delta I. The change in the base-emitter voltage of Q8 is, from the diode equation:

(Image Omitted)

Under the assumption of high beta

(I(N) = Current through QN),

I(3) = I delta I.

Now Q5, Q3, Q1, Q2, Q4 and Q6 form a current inversion circuit
[1] for which, under the assumption of matching and equal temperature of the transistors, we may write: (2I-I(1)) I(3)I(1) = L (2I-I(1))I/2/, where L is a constant close to 1 and determined by transistor matching.

Consequently, the current ratio through Q1 is the reciprocal of the current ratio through Q3 which produces

(Image Omitted)

It then follows that delta V = 0, and Rin = delta V over delta I = 0. In actuality, Rin does not exactly equal zero but is determined in part by transistor beta. The addition of Q9 and the two resistors, R1 and R2, provide a convenient output for comparison or further amplification. Further, delta 1 can become a rather large fraction of I before noticeable distortion. [1] B. Gilbert, "A New Wideband Amplifier Technique", IEEE Journal Solid State Circuits, Vol. SC-3, No. 4, December 1968.

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