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Maximum Signal Selector

IP.com Disclosure Number: IPCOM000086335D
Original Publication Date: 1976-Aug-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 52K

Publishing Venue

IBM

Related People

Siverling, MM: AUTHOR

Abstract

Circuit 10 produces an output voltage 11 which follows the maximum voltage present at any of a number of inputs 12a-12e. Such a circuit is useful in, for example, a threshold generator for parallel-channel optical scanners, where the output would represent the blackest video in a group of adjacent scanner channels.

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Maximum Signal Selector

Circuit 10 produces an output voltage 11 which follows the maximum voltage present at any of a number of inputs 12a-12e. Such a circuit is useful in, for example, a threshold generator for parallel-channel optical scanners, where the output would represent the blackest video in a group of adjacent scanner channels.

It is common practice to achieve the above function by an "analog OR" circuit, in which transistors receiving less than the highest input are degated from an emitter circuit common to all of them. The problem with this approach is that the device offset voltage decreases as the device attempts to degate, resulting in a sloppy and ill-defined disconnection from the load.

In a silicon transistor, e.g., the base-emitter voltage Vbe decreases from about 0.7 volt at the rate of 18mV for every octave of current. It is possible to monitor the output and feed it back, but such approaches have the disadvantage of loading the driven circuit. Constant-current operation to eliminate the variation is not possible, since the device cannot then degate at all.

Input transistors Q1-Q5 are connected as emitter-followers having a common emitter tied to a fixed current source Q6, Q7, R11. That is, the voltage on line 13 is clamped to one Vbe drop below the most positive one of the input voltages 12a-12e.

Diode-connected transistors Q8, Q9 shift the DC level of voltage 13 positive by 2Vbe. Equal-valued resistors R12, R13 shift this level downward by Vbe/2, and transistor Q10 provides an...