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Light Scanner Amplifier

IP.com Disclosure Number: IPCOM000050142D
Original Publication Date: 1982-Sep-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

West, DL: AUTHOR

Abstract

The circuit shown in Fig. 1 is an optical input to a square wave fixed reference output converter. Light on the phototransistor 1 drives the operational amplifier 3, the output of which charges capacitor 5 through parallel, oppositely poled diodes 7 and 9. Capacitor 5 is connected as one input of comparator 11. The other input is amplifier 3. After capacitor 5 is charged, the reduction of light to a point at which the output of amplifier 3 is below the drop across diode 7 switches comparator 11. Feedback of the steady state level at capacitor 5 through resistor 13 in the emitter circuit of phototransistor 1 provides a wide dynamic response.

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Light Scanner Amplifier

The circuit shown in Fig. 1 is an optical input to a square wave fixed reference output converter. Light on the phototransistor 1 drives the operational amplifier 3, the output of which charges capacitor 5 through parallel, oppositely poled diodes 7 and 9. Capacitor 5 is connected as one input of comparator 11. The other input is amplifier 3. After capacitor 5 is charged, the reduction of light to a point at which the output of amplifier 3 is below the drop across diode 7 switches comparator 11. Feedback of the steady state level at capacitor 5 through resistor 13 in the emitter circuit of phototransistor 1 provides a wide dynamic response.

Amplifier 3 has a positive level output and feedback resistor 15, connected from the output to the positive terminal of amplifier 3, is selected to provide a high input impedance of more than 10" ohms, thereby holding current from phototransistor to a very low level. As nearly all of this photocurrent flows through resistor 15, the output voltage of amplifier 3 is a direct function of the photocurrent. By choosing resistor 15 at 12 megaohms in the specific circuit shown, a signal strength change of 150:1 can be easily accommodated.

This photocurrent is controlled by the incident light on the base of phototransistor 1 and by the bias voltage on the emitter of phototransistor 1. The amount of current is a function of these factors as interrelated by the characteristics of phototransistor 1. Phototransistor 1 is a typical one, having characteristics as shown by the characteristic curve diagram of Fig. 2. E(I) is the illumination energy on the base of the phototransistor.

This circuit functions in the narrow region between collector to emitter voltages of zero and the knee of the topmost characteristic curve of interest (0 to
1.5 volt region). In this region a large change of collector current results from a small change in bias voltage.

Increased light applied to the phototransistor 1 lowers its impedance. The output of amplifier 3 increases in the positive direction. The voltage on capacitor 5 is the output of amplifier 3 less the forward voltage drop across diode 7, and this is the reference voltage for comparator 11. While the light input is steady or increasing, the voltage at the positive input of...