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Output Circuit for Fast Switching of Highly Capacitive Loads

IP.com Disclosure Number: IPCOM000082617D
Original Publication Date: 1975-Jan-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 37K

Publishing Venue

IBM

Related People

Wernicke, FC: AUTHOR

Abstract

If the output potential of an integrated circuit has to switch over the total voltage range of the power supply, the circuit shown in Fig. 1 may be utilized. Depending on the state of transistor TX1 transistor TX(out) is cutoff (V(out) in uplevel) or TX(out) is drawing current (V(out) in downlevel, VCE(sat)). The transient time of an up-going transition at the output depends primarily on the RL.CL time constant (assuming high-load impedance).

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Output Circuit for Fast Switching of Highly Capacitive Loads

If the output potential of an integrated circuit has to switch over the total voltage range of the power supply, the circuit shown in Fig. 1 may be utilized. Depending on the state of transistor TX1 transistor TX(out) is cutoff (V(out) in uplevel) or TX(out) is drawing current (V(out) in downlevel, VCE(sat)). The transient time of an up-going transition at the output depends primarily on the RL.CL time constant (assuming high-load impedance).

If the load capacitance is very high its impact is dominating, and the speed of a down-going transition at the output depends almost entirely on the time it takes to discharge the capacitance CL. Therefore, to draw a very high current through transistor TX(out), resistor RB must be low enough in resistance to permit sufficient base current. Since RB is connected from the most positive power supply to almost GND (when transistor TX(out) is turned off), a decrease in the value of RB causes a significant increase in power dissipation. What is actually required is a resistor RB whose value is high in the steady state, but small when the capacitive load at the output has to be discharged during down-going transitions.

The circuit depicted in Fig. 2 provides this feature by the provision of the circuitry contained with the enclosed broken line labelled Circuitry replacing RB of Fig. 1.

Referring to the circuit of Fig. 2, the output level is switched from V(cc) to almost GND when transistors TX1 and TX3 are turned off by lowering the potential at node 1. The potential at node 2 and node 3 rises until transistor TX(out) and transistor TX2 turn on. The voltage level at node 2 is clamped one VBE above GND and the level at node 3 is clamped two VBE's (~1.6v) above GND. Since the base of TX2 (node 3) is connected through 4.0K ohms to V(cc), a base current of about 0.85mA will be...