Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-06-26
2002-07-30
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140LS
Reexamination Certificate
active
06426495
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is related to a logarithm conversion circuit and the like, which detect light reception input intensity from a current of a photodiode of a light receiving circuit employed in an optical communication system, and monitor the detected light reception input intensity, and more specifically, is related to a temperature compensating circuit thereof.
In general, as is well known in the art, in order to perform a logarithm conversion, both a current under measurement and a reference current are supplied to two sets of bipolar elements, and then, a voltage difference between bases and emitters is derived. However, since a bipolar element contains a coefficient which is directly proportional to an absolute temperature as an inherent characteristic, a gain which is inversely proportional to the absolute temperature must be applied by a temperature compensating circuit in order to obtain a logarithm of a ratio of a current to be measured to a reference current irrespective of a temperature.
Conventionally, a temperature compensating circuit contained in a logarithm conversion circuit is arranged as shown in
FIG. 2
, as represented in a publication
1
(“Practical Operational Amplifier Circuit” written by Hideo Tsunoda, pages 40 to 41, Tokyo Electric University publishing section, Jul. 20, 1983), and a publication
2
(“operational Amplifier” edited/translated by Y. Kato, pages 298 to 305, Macgrow Hill K. K., Jun. 30, 1983). Reference numeral
11
shows a temperature sensing resistor, reference numeral
12
indicates a resistor, reference numeral
13
represents an operational amplifier, reference numeral
14
denotes an input terminal, and reference numeral
15
represents an output terminal. This operation is the positive-phase amplifier well known in the art. A voltage gain defined from the input terminal
14
of the positive-phase amplifier to the output terminal
15
thereof may be determined by both the temperature sensing resistor
11
and the resistor
12
of the feedback circuit which is connected to the inverting input terminal of the operational amplifier
13
. That is, this voltage gain is equal to (R
11
+R
12
)/R
11
(note that symbol “R
11
” shows resistance value of temperature sensing resistor
11
, and symbol “R
12
” indicates resistance value of resistor
12
). Since the temperature sensing resistor
11
owns such a temperature coefficient of approximately 0.39%/° C., such a gain is obtained which is inversely proportional to the absolute temperature. As the temperature sensing resistor
11
, a metal such as platinum is utilized as a resistance member.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a low-cost light receiving monitor circuit operable in high precision without a temperature dependent characteristic, while employing a logarithm conversion circuit containing a temperature compensating circuit, is also to provide a light receiver and an optical communication system.
Very recently, in an optical communication system, since light reception input intensity is monitored, abnormal conditions of a light transmitter, a transmission path, and the like, which constitute light transmission sides, are detected. Then, warning information of the abnormal conditions, and also interruption of the optical communication system are required.
However, when a current derived from a photodiode is merely monitored, for instance, in accordance with the recommendations of the ITU-T (International Telecommunication Union-Telecommunication Standardization Section), the light reception input strength becomes −28 to −8 dBm in STM-4, and becomes −28 to −9 dBm in SMT-16. It is required to represent such a light reception input strength range which is approximately 100 times higher than the light reception input strength. As a result, a logarithm conversion circuit for converting the light reception input strength into a strength in a logarithm scale is necessarily required.
Furthermore, there are many cases that an avalanche photodiode (will be abbreviated as an “APD” hereinafter) is employed, while optical communication systems are constructed in long transmission paths. In general, an APD is used in such a manner that a current multiplication ratio specific to this APD is reduced in a high level range of a light reception input level. As a result, in order to monitor a current of the avalanche photodiode, it is desirable to provide a function for correcting a change in the current multiplication ratio other than a temperature compensation made in a logarithm conversion circuit.
Also, since the high-cost platinum resistors and the like are used in the conventional temperature compensating circuit, there is a drawback that the low-cost light receiving monitor circuit could not be realized.
The present invention may remove the above-described drawbacks, and may provide a low-cost light receiving monitor circuit operable in high precision without having a temperature dependent characteristic, while employing a logarithm conversion circuit containing a temperature compensating circuit, and also may provide a light receiver and an optical communication system.
A temperature compensating circuit, according to an aspect of the present invention, is featured by comprising: a first circuit network between an inverting input terminal of an operational amplifier and an output terminal of the operational amplifier; and a second circuit network between the inverting input terminal of the operational amplifier and a reference potential; wherein: at least one of the first circuit network and the second circuit network is made of an arrangement containing a plurality of series-connected thermistor/resistor pairs in which the thermistors are connected parallel to the resistors; and the temperature compensating circuit compensates a temperature-dependent signal which is inputted into a positive phase input terminal of the operational amplifier, and outputs the temperature-compensated signal.
Also, a temperature compensating circuit, according to a modification of the present invention, is featured by comprising: a first circuit network between an inverting input terminal of an operational amplifier and an output terminal of the operational amplifier; and a second circuit network between the inverting input terminal of the operational amplifier and a signal input terminal thereof; wherein: a positive phase input terminal of the operational amplifier is connected to the ground; at least one of the first circuit network and the second circuit network is made of an arrangement containing a plurality of series-connected thermistor/resistor pairs in which the thermistors are connected parallel to the resistors; and the temperature compensating circuit compensates a temperature-dependent signal which is inputted into the signal input terminal of the operational amplifier, and outputs the temperature-compensated signal.
Also, the temperature compensating circuit is featured by comprising a logarithm converting circuit, wherein: an output terminal of the logarithm converting circuit is connected to the input of the above-explained temperature compensating circuit.
Also, a temperature compensating logarithm circuit, according to another aspect of the present invention, is featured by comprising: a third circuit network between the output terminal of the above-explained temperature compensating logarithm converting circuit and a positive phase input terminal of a second operational amplifier; and a fourth circuit network between the output terminal of the second operational amplifier and the positive phase input terminal thereof; in which at least one of the third circuit network and the fourth circuit network is made of an arrangement containing a plurality of series-connected thermistor/resistor pairs in which the thermistors are connected parallel to the resistors; and in the case that a gain defined from the inverting input terminal of the second temperature compensating circuit up to the output of the se
Fujii Tadaaki
Hatano Tadashi
Hayami Akihiro
Ikeuchi Hidehiro
Kikuchi Tomonao
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Kiknadze Irakli
Kim Robert H.
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