GB2176962A - Pinfet receiver - Google Patents

Abstract

A feedback circuit for an integrated PINFET module includes means for differentiating (8, 9) the electrical output of the module, and an amplifier (10) to which the differentiated output is applied, the amplifier output being applied to the photodiode bias to reduce the bias current when the optical input power increases. The dynamic range of the receiver is thereby increased. <IMAGE>

Description

SPECIFICATION Pinfet receiver This invention relates to a method and means for increasing the dynamic range of integrated PINFET receiver modules, as used in optical high bit rate digital systems.

A typical PINFET module incorporates a photodiode and integrated preamplifier using a FET input. One module currently available is that produced by the Plessey Company plc under the designation HRR7000 Series. The modules of that series have a GalnAs pin photodiode assembled on a thick film hybrid preamplifier using a GaAs FET front end device. The photodiode is biased through a high value resistor and has an optical dynamic range of 23dB. However, in practice it is found that in many applications the rated dynamic range of the module is inadequate to cope with the wide range of optical signal powers which many optical systems produce. It is known, therefore, to introduce optical attenuators in the input to the module to enable the module to handle wide dynamic range signals.

It is an object of the present invention to provide circuitry for increasing the dynamic range of a PIN FET module without the need for optical attenuators in the optical input to the module.

According to the present invention there is provided a method of improving the dynamic range of an integrated PINFET module including the steps of differentiating the electrical output signals from the module, amplifying the differentiated signals and feeding the amplified signals back to vary the photodiode bias current such that the bias current is reduced as the power in the input optical signal is increased.

The invention also provides a feedback circuit for an integrated PINFET module including means for differentiating the electrical output of the module, and an amplifier to which the differential output is applied, the amplifier output being applied to the photodiode bias to reduce the bias current when the optical input power increases.

In a preferred embodiment of the invention the feedback circuit includes a switch in the amplifier output, said switch being controlled by an AGC circuit in an output amplifier to which the module output is fed so as to switch off the feedback signal at low signal levels.

An embodiment of the invention will now be described with reference to the accompanying drawing which illustrates the basic elements of a PINFET receiver module with a feedback circuit for improving the dynamic range of the module.

The PINFET module, e.g. Plessey type HRR 7000, is shown in the dotted line rectangle and comprises a GalnAs photodiode 1 which is biased via resistor 2. The photodiode is coupled to a fibre optic tail (not shown) and its electrical output is fed to a GaAs FET front end device 3, followed by a FET-bipolar cascode 4 and an emitter follower output buffer stage 5. The photodiode bias current is controlled by an operational amplifier 6 which maintains a constant drain-source current irrespective of mean photodiode current. The feedback connections between the amplifier inputs and the FET stages in the module are not shown but can be ascertained from the module data sheet. Resistor 7 serves to isolate the feedback signal from the DC bias applied to the photodiode.

To improve the dynamic range of the module the output signal is first differentiated by capacitor 8 and resistor 9. The differentiated output signal is then fed to amplifer 10. The output of amplifier 10 is applied to the bias resistor 2. If the optical input is a digital signal the resultant square wave of current in resistor 2 will, if in the right phase, subtract from the square wave current in the photodiode, hence reducing the signal level in the module amplifier stages. The gain of amplifier 10 should be shaped to counter the drop in response introduced by the differentiator at high frequency. The value of the gain of amplifier 10 determines how much dynamic range improvement can be obtained. The output of amplifier 10 should be able to swing a large signal voltage as the bias resistor 2 has a high value.

To prevent loss of sensitivity at low signal levels a switch 11 is included in the output of amplifier 10. This switches off the feedback signal and is preferably controlled automatically by the AGC signal occurring in a subsequent amplifier (not shown) in the signal processing circuitry.

Note that the operation of the normal bias control amplifer 6 is unaffected by the addition of the feedback circuit amplifier 10.

1. A method of improving the dynamic range of an integrated PINFET module including the steps of differentiating the electrical output signals from the module, amplifying the differentiated signals and feeding the amplified signals back to vary the photodiode bias current such that the bias current is reduced as the power in the input optical signal is increased.

2. A feedback circuit for an integrated PINFET module including means for differentiating the electrical output of the module, an amplifier to which the differential output is applied, the amplifier output being applied to the photodiode bias to reduce the bias current when the optical input power increases, and a switch in the amplifier output controlled by an AGC circuit in an output amplifier to which the module output is fed so as to switch off the feedback signal at low signal levels.

3. A feedback circuit for a PINFET receiver module substantially as described with reference to the drawing.

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

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Pinfet receiver This invention relates to a method and means for increasing the dynamic range of integrated PINFET receiver modules, as used in optical high bit rate digital systems. A typical PINFET module incorporates a photodiode and integrated preamplifier using a FET input. One module currently available is that produced by the Plessey Company plc under the designation HRR7000 Series. The modules of that series have a GalnAs pin photodiode assembled on a thick film hybrid preamplifier using a GaAs FET front end device. The photodiode is biased through a high value resistor and has an optical dynamic range of 23dB. However, in practice it is found that in many applications the rated dynamic range of the module is inadequate to cope with the wide range of optical signal powers which many optical systems produce. It is known, therefore, to introduce optical attenuators in the input to the module to enable the module to handle wide dynamic range signals. It is an object of the present invention to provide circuitry for increasing the dynamic range of a PIN FET module without the need for optical attenuators in the optical input to the module. According to the present invention there is provided a method of improving the dynamic range of an integrated PINFET module including the steps of differentiating the electrical output signals from the module, amplifying the differentiated signals and feeding the amplified signals back to vary the photodiode bias current such that the bias current is reduced as the power in the input optical signal is increased. The invention also provides a feedback circuit for an integrated PINFET module including means for differentiating the electrical output of the module, and an amplifier to which the differential output is applied, the amplifier output being applied to the photodiode bias to reduce the bias current when the optical input power increases. In a preferred embodiment of the invention the feedback circuit includes a switch in the amplifier output, said switch being controlled by an AGC circuit in an output amplifier to which the module output is fed so as to switch off the feedback signal at low signal levels. An embodiment of the invention will now be described with reference to the accompanying drawing which illustrates the basic elements of a PINFET receiver module with a feedback circuit for improving the dynamic range of the module. The PINFET module, e.g. Plessey type HRR 7000, is shown in the dotted line rectangle and comprises a GalnAs photodiode 1 which is biased via resistor 2. The photodiode is coupled to a fibre optic tail (not shown) and its electrical output is fed to a GaAs FET front end device 3, followed by a FET-bipolar cascode 4 and an emitter follower output buffer stage 5. The photodiode bias current is controlled by an operational amplifier 6 which maintains a constant drain-source current irrespective of mean photodiode current. The feedback connections between the amplifier inputs and the FET stages in the module are not shown but can be ascertained from the module data sheet. Resistor 7 serves to isolate the feedback signal from the DC bias applied to the photodiode. To improve the dynamic range of the module the output signal is first differentiated by capacitor 8 and resistor 9. The differentiated output signal is then fed to amplifer 10. The output of amplifier 10 is applied to the bias resistor 2. If the optical input is a digital signal the resultant square wave of current in resistor 2 will, if in the right phase, subtract from the square wave current in the photodiode, hence reducing the signal level in the module amplifier stages. The gain of amplifier 10 should be shaped to counter the drop in response introduced by the differentiator at high frequency. The value of the gain of amplifier 10 determines how much dynamic range improvement can be obtained. The output of amplifier 10 should be able to swing a large signal voltage as the bias resistor 2 has a high value. To prevent loss of sensitivity at low signal levels a switch 11 is included in the output of amplifier 10. This switches off the feedback signal and is preferably controlled automatically by the AGC signal occurring in a subsequent amplifier (not shown) in the signal processing circuitry. Note that the operation of the normal bias control amplifer 6 is unaffected by the addition of the feedback circuit amplifier 10. CLAIMS

1. A method of improving the dynamic range of an integrated PINFET module including the steps of differentiating the electrical output signals from the module, amplifying the differentiated signals and feeding the amplified signals back to vary the photodiode bias current such that the bias current is reduced as the power in the input optical signal is increased.

2. A feedback circuit for an integrated PINFET module including means for differentiating the electrical output of the module, an amplifier to which the differential output is applied, the amplifier output being applied to the photodiode bias to reduce the bias current when the optical input power increases, and a switch in the amplifier output controlled by an AGC circuit in an output amplifier to which the module output is fed so as to switch off the feedback signal at low signal levels.

3. A feedback circuit for a PINFET receiver module substantially as described with reference to the drawing.