Browse Prior Art Database

Low Drift AC Coupled Photodiode Amplifier

IP.com Disclosure Number: IPCOM000076402D
Original Publication Date: 1972-Feb-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 54K

Publishing Venue

IBM

Related People

Keidl, SD: AUTHOR

Abstract

In order to achieve high bandwidth with high-capacitance photodiodes, such diodes usually drive common-base transistor stages. To maintain the necessary low-input impedance, such stages are externally biased with a steady current much larger than the maximum signal current. The of accurately removing this bias current in spite of the nonperiodic nature of the video signal to be amplified.

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Low Drift AC Coupled Photodiode Amplifier

In order to achieve high bandwidth with high-capacitance photodiodes, such diodes usually drive common-base transistor stages. To maintain the necessary low-input impedance, such stages are externally biased with a steady current much larger than the maximum signal current. The of accurately removing this bias current in spite of the nonperiodic nature of the video signal to be amplified.

Video system 10, Fig. 1, includes identical document and reference channels, each having photodiode 11, preamplifier 12, logarithmic converter 13, and correction circuit 14. Both channels drive differential amplifier 15 having DC level correction circuit 16. If diodes 11 receive document and reference video signals, then analog-digital converter 17 produces a signal related to the logarithm of the quotient of the two signals. Units 15-17 of this system are more fully described in
U. S. Patent 3,626,092 to M. G. Wilson.

Fig. 2 shows common-base amplifier 21 receiving diode current Id and a large prebias current Ip from fixed current generator 22. Graph 41, Fig. 3, shows the output voltage Va while scanning a blank white document. Negative going spikes 42 indicate a minimum light level, occurring while the scanner is turned off during retrace. AC coupling capacitor 23 charges to the average value of voltage Va; for a blank document, this voltage is almost equal to the maximum of Va. Since ls, the output current of transadmittance amplifier 24, goes negative under this condition, curre...