Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-01-28
2001-02-13
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100
Reexamination Certificate
active
06188059
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photocurrent monitor circuit for detecting a photocurrent flowing through a photodiode for receiving an optical signal, and an optical receiver equipped with the photocurrent monitor circuit.
2. Related Background Art
Optical receivers have been employed in communications using optical fibers and the like. Such an optical receiver is equipped with an avalanche photodiode (APD) for receiving optical signals. Usually, a bias voltage of over 80 V is applied to the APD.
SUMMARY OF THE INVENTION
The photocurrent cannot be detected correctly, because its detecting circuit influences the photocurrent. It is thus an object of the present invention to provide a photocurrent monitor circuit which can correctly monitor the photocurrent flowing through a photodiode, and an optical receiver equipped therewith.
The photocurrent monitor circuit in accordance with the present invention comprises a photodiode for receiving an optical signal; a first current mirror circuit having two parallel lines with respective currents flowing therethrough in proportion to each other, one of the lines connecting with one end of the photodiode; a second current mirror circuit having one of parallel lines connected to the other line of the first current mirror circuit; and a photocurrent monitor terminal connected to the other of the parallel lines of the second current mirror circuit.
Since this photocurrent monitor circuit can detect a current which is in proportion to the photocurrent flowing through the photodiode at the photocurrent monitor terminal, it can monitor the correct photocurrent without its detecting circuit influencing the photocurrent of the photodiode itself.
Preferably, the first and second current mirror circuits are constituted by bipolar transistors having polarities opposite to each other. Namely, in this configuration, connecting bipolar transistors having opposite polarities in series can make their current flowing directions coincide with each other, thus simplifying the circuit configuration.
Preferably, the photodiode is an avalanche photodiode, whereas one of the bipolar transistors constituting the first current mirror circuit has a collector connected to the photodiode and an emitter connected to a multiplication factor control circuit for supplying a bias potential with a positive temperature coefficient to the emitter.
The multiplication factor of an avalanche photodiode has a temperature dependence and a bias voltage dependence. Here, in the avalanche photodiode, the temperature dependence of its multiplication factor can be compensated for when a potential with a positive temperature coefficient is given as its bias voltage. The collector potential of the transistor in the first current mirror circuit is uniquely determined by the emitter potential, because the base and collector are short-circuited.
As a consequence, if a multiplication factor control circuit is connected to the emitter, and a potential with a positive temperature coefficient is supplied thereto, then the temperature coefficient of its multiplication factor can be compensated.
Preferably, the multiplication factor control circuit comprises a temperature compensation circuit in which a Zener diode having a positive temperature coefficient and a transistor whose base-emitter voltage has a negative temperature coefficient. The temperature compensation circuit and the transistor are connected in parallel such as to yield a positive temperature coefficient on an output. The multiplication factor control circuit has an output connected to the emitter of the first current mirror circuit.
Namely, when a Zener diode and a transistor having temperature coefficients with polarities opposite to each other are connected in parallel, then the temperature coefficient of their output potential can be adjusted according to contributions of the individual devices.
The optical receiver in accordance with the present invention further comprises a transimpedance amplifier connected to the other end of the photodiode.
While the photocurrent from the photodiode is indirectly monitored as mentioned above, the direct photocurrent is converted into a voltage via the transimpedance amplifier.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.
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Nishie Mitsuaki
Nishiyama Naoki
Takahashi Satoshi
Le Que T.
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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