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Emitter Follower Logic

IP.com Disclosure Number: IPCOM000094966D
Original Publication Date: 1965-Jul-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Walsh, JL: AUTHOR

Abstract

This emitter-follower logic circuit has a feedback loop which assures identical input and output logic levels.

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Emitter Follower Logic

This emitter-follower logic circuit has a feedback loop which assures identical input and output logic levels.

NPN transistors 10 and 12 have their emitters connected in common to the base of PNP transistor 14 to form an emitter-follower Or. Under normal circumstances, a logical input signal applied to the base of one of transistors 10 or 12 experiences a base to emitter voltage drop Vbe. This drop is compensated by the Vbe rise through transistor 14 so that the logic signal on output conductor 16 is a replica of the input. If, however, the value of the load connected to conductor 16 varies, the current through transistor 14 likewise varies. This creates a change in its Vbe drop and a disparity between the circuit's input and output logic levels.

Transistor 18 compensates for such variations and assures identical input and output logic levels. In this circuit, transistor 18 is conductive at all times. Assume that input A is raised causing the conduction level in transistor 10 to increase. The resulting rise in potential at the emitter of transistor 10 decreases the conduction through transistor 14 with a resultant rise in potential on output conductor 16. Assuming equal Vbe drops in transistors 10 and 14, the output potential is identical with the input potential.

If, however, the value of the load attached to transistor 14 varies causing the current flowing out of conductor 16 to decrease, the current through transistor 14 must increase, w...