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Signal Driver Having Overvoltage Protection

IP.com Disclosure Number: IPCOM000082117D
Original Publication Date: 1974-Oct-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Bodendorf, DJ: AUTHOR [+3]

Abstract

This circuit solves an overvoltage problem, caused by the fact that the avalanche threshold of the transistors is lower than or about equal to the power supply, VH.

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Signal Driver Having Overvoltage Protection

This circuit solves an overvoltage problem, caused by the fact that the avalanche threshold of the transistors is lower than or about equal to the power supply, VH.

It is a modified "totempole" configuration, in which each of the upper and lower devices of a conventional totem pole is replaced here by two devices in series: Q5/Q6 and Q7/Q8. Instead of a single device, the supply voltage now appears across two devices in series. Voltage across each device is thus reduced roughly by a factor of two.

The collectors of Q2 and Q3 are clamped to one VBE above -VH by Q1 and Q4, irrespective of the value of VH. Resistors R6 and R5 can, therefore, be returned to V1 or ground without encountering any over-voltage problems.

When devices Q1 and Q4 are off, their collectors experience greater than VH. However, their bases then lock into very low-source impedances (overdriven Q2/Q3), and their breakdown characteristics are, therefore, governed by BVces rather than by BVceo. Furthermore, in the event of a transitory avalanching during switching, their currents will be limited by the collector resistors to safe values.

The up and down voltage swings of the output line are reduced by approximately one Vce(sat) at each end, by introduction of the series devices Q6/Q7. Schottky diodes have therefore been removed from the output devices to minimize Vce(sat). The overall loss in the output signal is then insignificant.

In spite of saturatio...