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Wide Range Emitter Amplifier

IP.com Disclosure Number: IPCOM000081214D
Original Publication Date: 1974-Apr-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 49K

Publishing Venue

IBM

Related People

Keidl, SD: AUTHOR

Abstract

An amplifier operating over a wide input current range amplifies pulses generated by a photoelectric emitter. The wide-range requirement emanates from the wavering emitter disk as it rotates, and the wide variation in efficiencies of an inexpensive light-emitting diode (LED)-phototransistor combination used in low-cost emitters, causes the emitter to generate valid pulses having widely varying amplitudes. The wide variation in efficiencies, causes the average current level (IAV) from the phototransistor to vary more than an order of magnitude. To cope with this condition, the output of the emitter is fed into a log amplifier, whose output is AC coupled into a comparator with AC hysteresis. The AC coupling eliminates the effects of IAV and allows the use of a fixed reference to the comparator.

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Wide Range Emitter Amplifier

An amplifier operating over a wide input current range amplifies pulses generated by a photoelectric emitter. The wide-range requirement emanates from the wavering emitter disk as it rotates, and the wide variation in efficiencies of an inexpensive light-emitting diode (LED)-phototransistor combination used in low-cost emitters, causes the emitter to generate valid pulses having widely varying amplitudes. The wide variation in efficiencies, causes the average current level (IAV) from the phototransistor to vary more than an order of magnitude. To cope with this condition, the output of the emitter is fed into a log amplifier, whose output is AC coupled into a comparator with AC hysteresis. The AC coupling eliminates the effects of IAV and allows the use of a fixed reference to the comparator.

Phototransistor 10, Fig. 1, provides the output of the photoelectric emitter. Its collector is connected to one input of log amplifier 15 formed by operational amplifier 16 and diode 17. The output voltage of amplifier 16 is proportional to the natural logarithm of the ratio of light to dark phototransistor currents; i.e., Vout(p- p) = N 0.026 Log(e) I light over I dark; 1 </- N </- 2. This output signal is AC coupled by capacitor 20 and resistor 25 to comparator 30.

Comparator 30 compares this output signal with a reference voltage Vref. The comparator output corresponds to the average value points from the waveform appearing at the output of am...