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Active Pull-Down for Compensated Current Switch Emitter Follower

IP.com Disclosure Number: IPCOM000061744D
Original Publication Date: 1986-Sep-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Related People

Cavaliere, JR: AUTHOR [+3]

Abstract

The fall-time delay of an internally compensated current switch emitter follower is raised significantly when its output emitter follower must drive a heavy load, for example, a large fan-out or a long on-chip line. By adding the active pull-down device described in this article, the drive capability of the CCSEF is improved and the fall-time delay significantly reduced. Fig. 1 shows the circuit of a compensated current switch emitter follower (CCSEF). When the emitter follower must drive a heavy load, the transistor T OUT will "turn off", thereby reducing its drive capability and significantly increasing the fall-time and delay of the output voltage, as shown in Fig. 2. The addition of an active pull- down device, conceptually shown in Fig.

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Active Pull-Down for Compensated Current Switch Emitter Follower

The fall-time delay of an internally compensated current switch emitter follower is raised significantly when its output emitter follower must drive a heavy load, for example, a large fan-out or a long on-chip line. By adding the active pull-down device described in this article, the drive capability of the CCSEF is improved and the fall-time delay significantly reduced. Fig. 1 shows the circuit of a compensated current switch emitter follower (CCSEF). When the emitter follower must drive a heavy load, the transistor T OUT will "turn off", thereby reducing its drive capability and significantly increasing the fall-time and delay of the output voltage, as shown in Fig. 2. The addition of an active pull- down device, conceptually shown in Fig. 3, restores the drive capability of T OUT, markedly reducing the fall-time and delay of the CCSEF without increasing power dissipation. The disclosed scheme is embodied in the circuit of the compensated current switch with active pull-down shown in Fig. 4, where the diode D1 regulates the up-level and the transistor TD1 regulates the down-level. The transistor TD is the active pull-down device which "turns-on" when V IN is higher than ground potential. V OUT should then be low. TD obtains its base drive from resistors RC2, RD1 and RD2 and the transistor TC1 clamps the output voltage.

In this circumstance the transistor T OUT is off. When V IN is lower than groun...