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Circuit for Maintaining Constant Impedance Ratio With Photocell

IP.com Disclosure Number: IPCOM000091172D
Original Publication Date: 1969-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Coburn, RL: AUTHOR [+2]

Abstract

The impedance of photocell 10 can drift very slowly with time. The circuit dynamically adjusts the effective resistance between node 12 and ground so that a constant impedance ratio is maintained between the impedance of photocell 10 and the impedance between node 12 and ground. Thus, the voltage at node 12 tends to remain constant even though the impedance of photocell 10 drifts.

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Circuit for Maintaining Constant Impedance Ratio With Photocell

The impedance of photocell 10 can drift very slowly with time. The circuit dynamically adjusts the effective resistance between node 12 and ground so that a constant impedance ratio is maintained between the impedance of photocell 10 and the impedance between node 12 and ground. Thus, the voltage at node 12 tends to remain constant even though the impedance of photocell 10 drifts.

Voltage changes at node 12 are buffered by emitter-followers 14 and 16. The output from follower 16 is fed back to the base of transistor 18 which acts as a current source. When the voltage at node 12 decreases, the voltage at output terminal 17 decreases. This reduces the signal applied to the base of transistor
18. The current conducted by transistor 18 decreases and, therefore, the voltage at node 12 increases. Thus, the voltage at node 12 tends to remain constant.

When the photocell detects information, there is a more rapid change in voltage at node 12. This voltage pulse appears at the output terminal 17 and is also fed back to transistor 18. However, because of large capacitor 20, the time constant is charging such capacitor is so long that a pulse of short duration has no effect on transistor 18. Thus, the voltage at node 12 can vary for a short duration, but is dynamically adjusted to be constant over a long period of time. Capacitor 20 has to be large so that there is a long time constant for capacitor 20 and the...