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Push-Pull Driver Enhancement Using Collector-To-Substrate Capacitance

IP.com Disclosure Number: IPCOM000041513D
Original Publication Date: 1984-Feb-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 54K

Publishing Venue

IBM

Related People

Andersen, JE: AUTHOR

Abstract

This article describes a push-pull driver in which a discontinuity in the voltage waveform of the output of the driver is eliminated and a delay improvement is realized. The push-pull driver includes a current switch emitter follower (CSEF) with the pull-up output device tied to the in-phase side of the switch, and the pull-down device tied to the out-of-phase side. Note the longer path from the switch for the pull-down device. During a falling transition on the output, the pull-up device will turn off before the pull-down device turns on to continue to bring the output node low. This condition is a direct result of the difference in path length to the output device. The output node flattens during the time when neither output device is on, producing a discontinuity.

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Push-Pull Driver Enhancement Using Collector-To-Substrate Capacitance

This article describes a push-pull driver in which a discontinuity in the voltage waveform of the output of the driver is eliminated and a delay improvement is realized. The push-pull driver includes a current switch emitter follower (CSEF) with the pull-up output device tied to the in-phase side of the switch, and the pull-down device tied to the out-of-phase side. Note the longer path from the switch for the pull-down device. During a falling transition on the output, the pull-up device will turn off before the pull-down device turns on to continue to bring the output node low. This condition is a direct result of the difference in path length to the output device. The output node flattens during the time when neither output device is on, producing a discontinuity. The solution to the problem is to slow down the path to the pull-up device and speed up the path to the pull-down device. Physically this can be done by increasing the Ccs capacitance of the current source. An earlier article [*] states that the switching activity of the out-of-phase transistor in this configuration is accelerated, enabling its emitter follower to respond similarly. The point is also made that for CSEF circuits, an AC improvement is only seen on the in-phase output. However, this characteristic is being taken advantage of here by slowing the response of the pull-up (which controls the upper portion of the output wa...