GB2122447A

GB2122447A - Optical receiver

Info

Publication number: GB2122447A
Application number: GB8236511A
Authority: GB
Inventors: William Hugh Powell
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-06-17
Filing date: 1982-12-22
Publication date: 1984-01-11
Also published as: DE3320801C2; AU1572983A; AU558899B2; GB2122447B; DE3320801A1

Abstract

An optical receiver comprising an inverting amplifier with a reverse biassed photo-detector diode D connected in a feedback path from the amplifier output to the amplifier input such that the transfer function of the amplifier at the frequency of operation is primarily determined by the self capacitance CD of the diode. <IMAGE>

Description

SPECIFICATION Optical receiver This invention relates to an optical receiver suitable for, e.g. optical fibre communications systems oper ating at high digital rates.

High speed photo-detector diodes are normally reverse biassed to decrease their capacitance, there by increasing their speed of response when con nected to an electronic circuit. Nevertheless a self capacitance CD remains. The photo-detector diode D then behaves as an illumination dependent current source Is and capacitance CD as shown in Fig. 1.

The most sensitive technique for sensing the current Is generated in the diode D isto allowitto accumulate as charge in its own self capacitance CD and then to amplify the voltage to which it has charged by means of an amplifier as shown in Fig. 2.

The output voltage V is given by V= A jGCD wherein A is the ampifier gain. Field effect transistors are commonly used in thisamptifier because their high input impedance preventsthe escape of the accumulated charge. Similarly when the input stage uses a bipolartransistor not only increased dynamic range but also improved noise performance of the amplifier is achieved.It is found in practice that the gain of the amplifier to sinusoidal variations of the current Is depends not only on the self capacitance CD ofthediodeD butalso on the self capacity and gain stability oftheamplifier. Also, the voltage gain ofthe amplifier can lead to overload at its output even though the voltage change acrossthe diode is relatively small.

According to the present invention there is provided an optical receiver comprising an inverting amplifierwith a reverse biassed photo-detector diode connected in a feedback path from the amplifier outputto the amplifier input such thatthe transfer function of the amplifier at the frequency of operation is primarily determined by the self capacitance of the diode.

An embodiment of the invention is now described with reference to Figs. 3 and 4 of the accompanying drawings, in which: Fig. 3 illustrates schematically an optical receiver; Fig. 4 illustrates a circuit transformation ofthe arrangement of Fig. 3; Fig. 5 illustrates a modification ofthe arrangement of Fig. 3, and Fig. 6 illustrates an embodiment ofthe arrangement of Fig. 5.

Inthe arrangement shown in Fig.3thetwo terminals of the photodiode Dare connected between the inputand outputterminals of an inverting amplifier whose gain is A. The photodiode now constitutes a negative feedback path from the amplifieroutputto its input. Fig.4showsan equivalent circuittransformation ofthearrangementshown in Fig. 3. The capacitive feedback path is effectively provided by the self-capacitance CD of the diode. The single current generator Is of Fig. 3 is replaced bytwo series generators 1'5, each equal to Is, whose mid point is connected to the amplifier ground terminal.

This well known circuit transformation has no effect on the ground current, sincethetwo generators supply and remove equal currents. Inspection of Fig.

4 now shows that one current generator l's is located conventionallyatthe input of the amplifier. The other, at the amplifier output, can be disregarded because the current gain ofthe amplifier is normally so large asto renderthe effect ofthe current generator negligible. Fig. 4 is now recognisable as a negative feedback amplifier with CD as the feedback path and l's as the input signal. As is well-known, the effect of negative feedback isto reduce and stabilise amplifier gain. In this casethe output voltage V1 at angular frequency is given by V1 1 . 1's jCD for large values of A. Consequently the output voltage isstabilised againstvariations inthegainAof the amplifier by the capacitance CD.In addition, since the outputvoltage is reduced by the negative feedback, operation at higher input signal levels can be permitted before overload occurs.

Some amplifiers have morethan one possible output point available. For example, these may be a non-inverting as well as an inverting output. In this casethe photo-diode is connected to the inverting output to provide negative feedback, but any ofthe available outputs can be used to deliver the signal to external circuitry. An example ofthis is shown in Fig.

5.

Atypical embodimentoftheinvention is shown in Fig. 6. The amplifier input istotransistorT1. In this arrangement the emitter oftransistorT2 constitutes the inverting outputto which the photodiode is connected to provide negative feedback. The voltage atthispointistherefore V1 = 1 l's, jEXCD as described above. A capacitor C connected between this output and ground causes a current I = jpC V1to flowthrough 12 emitter and collector. This current may be delivered to an external circuit by the collector of which is the non-inverting outputofthe amplifier.The capacitor C also results in a simple frequency independent relationship between I and Is which is I = C . Is.

CLAIMS CD 1. An optical receiver comprising an amplifier with a reverse biassed photo-detector diode connected in a feedback path from an inverting output of the amplifier output to the amplifier input such that the transferfu nction ofthe amplifier at this output at the frequency of operation is primarily determined by the self capacitance of the diode.

2. A receiver according to claim 1 wherein the amplifier includes a field effecttransistor input stage.

3. A receiver according to claim 1 whereinthe amplifierincludesa bipolartransistorinputstage.

4. An optical receiver substantially as described with reference to Figs. 3 and 4 or Figs. 5 and 6 of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

SPECIFICATION Optical receiver This invention relates to an optical receiver suitable for, e.g. optical fibre communications systems oper ating at high digital rates.

High speed photo-detector diodes are normally reverse biassed to decrease their capacitance, there by increasing their speed of response when con nected to an electronic circuit. Nevertheless a self capacitance CD remains. The photo-detector diode D then behaves as an illumination dependent current source Is and capacitance CD as shown in Fig. 1.

The most sensitive technique for sensing the current Is generated in the diode D isto allowitto accumulate as charge in its own self capacitance CD and then to amplify the voltage to which it has charged by means of an amplifier as shown in Fig.
2.

The output voltage V is given by V= A jGCD wherein A is the ampifier gain. Field effect transistors are commonly used in thisamptifier because their high input impedance preventsthe escape of the accumulated charge. Similarly when the input stage uses a bipolartransistor not only increased dynamic range but also improved noise performance of the amplifier is achieved.It is found in practice that the gain of the amplifier to sinusoidal variations of the current Is depends not only on the self capacitance CD ofthediodeD butalso on the self capacity and gain stability oftheamplifier. Also, the voltage gain ofthe amplifier can lead to overload at its output even though the voltage change acrossthe diode is relatively small.

According to the present invention there is provided an optical receiver comprising an inverting amplifierwith a reverse biassed photo-detector diode connected in a feedback path from the amplifier outputto the amplifier input such thatthe transfer function of the amplifier at the frequency of operation is primarily determined by the self capacitance of the diode.

An embodiment of the invention is now described with reference to Figs.
3 and 4 of the accompanying drawings, in which: Fig. 3 illustrates schematically an optical receiver; Fig. 4 illustrates a circuit transformation ofthe arrangement of Fig. 3; Fig. 5 illustrates a modification ofthe arrangement of Fig. 3, and Fig. 6 illustrates an embodiment ofthe arrangement of Fig. 5.

Inthe arrangement shown in Fig.3thetwo terminals of the photodiode Dare connected between the inputand outputterminals of an inverting amplifier whose gain is A. The photodiode now constitutes a negative feedback path from the amplifieroutputto its input. Fig.4showsan equivalent circuittransformation ofthearrangementshown in Fig. 3. The capacitive feedback path is effectively provided by the self-capacitance CD of the diode. The single current generator Is of Fig. 3 is replaced bytwo series generators 1'5, each equal to Is, whose mid point is connected to the amplifier ground terminal.

This well known circuit transformation has no effect on the ground current, sincethetwo generators supply and remove equal currents. Inspection of Fig.
4. An optical receiver substantially as described with reference to Figs. 3 and 4 or Figs. 5 and 6 of the accompanying drawings.

4 now shows that one current generator l's is located conventionallyatthe input of the amplifier. The other, at the amplifier output, can be disregarded because the current gain ofthe amplifier is normally so large asto renderthe effect ofthe current generator negligible. Fig. 4 is now recognisable as a negative feedback amplifier with CD as the feedback path and l's as the input signal. As is well-known, the effect of negative feedback isto reduce and stabilise amplifier gain. In this casethe output voltage V1 at angular frequency is given by V1 1 . 1's jCD for large values of A. Consequently the output voltage isstabilised againstvariations inthegainAof the amplifier by the capacitance CD.In addition, since the outputvoltage is reduced by the negative feedback, operation at higher input signal levels can be permitted before overload occurs.

Some amplifiers have morethan one possible output point available. For example, these may be a non-inverting as well as an inverting output. In this casethe photo-diode is connected to the inverting output to provide negative feedback, but any ofthe available outputs can be used to deliver the signal to external circuitry. An example ofthis is shown in Fig.

5.

Atypical embodimentoftheinvention is shown in Fig. 6. The amplifier input istotransistorT1. In this arrangement the emitter oftransistorT2 constitutes the inverting outputto which the photodiode is connected to provide negative feedback. The voltage atthispointistherefore V1 = 1 l's, jEXCD as described above. A capacitor C connected between this output and ground causes a current I = jpC V1to flowthrough 12 emitter and collector. This current may be delivered to an external circuit by the collector of which is the non-inverting outputofthe amplifier. The capacitor C also results in a simple frequency independent relationship between I and Is which is I = C . Is.

CLAIMS CD 1. An optical receiver comprising an amplifier with a reverse biassed photo-detector diode connected in a feedback path from an inverting output of the amplifier output to the amplifier input such that the transferfu nction ofthe amplifier at this output at the frequency of operation is primarily determined by the self capacitance of the diode.

2. A receiver according to claim 1 wherein the amplifier includes a field effecttransistor input stage.

3. A receiver according to claim 1 whereinthe amplifierincludesa bipolartransistorinputstage.