Browse Prior Art Database

Noise Rejecting Phototransistor Circuits in Optical Sensing Applications

IP.com Disclosure Number: IPCOM000087283D
Original Publication Date: 1977-Jan-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Beistle, RG: AUTHOR

Abstract

It is desirable to locate the readback or sensing amplifiers remote from the phototransistor PT or other photo device. Many such applications also require a fairly high frequency response within the optical sensing circuits. The advantage of remotely locating the sensing circuit is cost reduction. For example, if the photo sensing device PT is located on a movable carriage (not shown), then the circuitry in many instances would have to be encased in special, protective packages requiring specially designed circuit boards and other connections. Such items are expensive. When the circuit is located remotely, then the circuit can be combined in a standard package at lower cost.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 53% of the total text.

Page 1 of 2

Noise Rejecting Phototransistor Circuits in Optical Sensing Applications

It is desirable to locate the readback or sensing amplifiers remote from the phototransistor PT or other photo device. Many such applications also require a fairly high frequency response within the optical sensing circuits. The advantage of remotely locating the sensing circuit is cost reduction. For example, if the photo sensing device PT is located on a movable carriage (not shown), then the circuitry in many instances would have to be encased in special, protective packages requiring specially designed circuit boards and other connections. Such items are expensive. When the circuit is located remotely, then the circuit can be combined in a standard package at lower cost.

To achieve the remote location of a photo sensing amplifier, one problem to be overcome is the inherent capacitance in the connecting cable. One approach for avoiding the effect of cable capacitance is to reduce or control the voltage change transmitted across the cable. This action reduces the charging and discharging energy required for driving the cable. Accordingly, the input impedance of the sensing amplifier is designed to be a voltage source presenting a load impedance of essentially zero ohms to PT. An additional advantage of this arrangement is that PT exhibits shorter turn-off and turn-on times than would be the case when the load impedance is substantial.

The circuit shows a differential amplifier DA connected as a voltage source, with the circuit parameters chosen as fol...