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Signal Conversion Circuit

IP.com Disclosure Number: IPCOM000077195D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Siverling, MM: AUTHOR [+2]

Abstract

This circuit is designed to receive an input signal having a DC component of several volts upon which is superimposed a small signal of a few millivolts. The circuit removes the DC bias component and converts the input signal into an output current. Such a circuit is useful in, for instance, photodiode amplifiers.

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Signal Conversion Circuit

This circuit is designed to receive an input signal having a DC component of several volts upon which is superimposed a small signal of a few millivolts. The circuit removes the DC bias component and converts the input signal into an output current. Such a circuit is useful in, for instance, photodiode amplifiers.

The input signal is applied to input terminal 1. Operational amplifier 2 provides a voltage gain of approximately 200, which is determined by input resistor 3 and feedback resistor 4. The output voltage of amplifier 2 at terminal 5 is converted into a current by resistors 6 and 7. The output current is available through transistor 8 at terminal 9. Amplifier 10 senses the voltage difference between terminal 5 and point 11. This voltage varies in accordance with the output current of amplifier 2. When the output current decreases to a value in the range of two microamps, the output of amplifier 10 at terminal 21 goes positive to drive positive charge onto the capacitor 12 through diode 13 and resistor 14. The positive voltage on capacitor 12 is applied to the second input of amplifier 2. This is the noninverting input, which is effective to drive the output of amplifier 2 positive to prevent the output current through resistors 6 and 7 from decreasing below the desired minimum value.

Capacitor 12 is large so that its voltage drops only slightly during the intervals between the times when charge is added to it. The voltage across...