| 摘要 |
PROBLEM TO BE SOLVED: To protect an isolator and an amplifier circuit even when mismatching to an external circuit is generated at a signal output end by providing a power supply circuit which lowers power feeding quantity to an amplifier circuit with a signal from a temperature detection circuit in the terminating part of the isolator. SOLUTION: The temperature of the terminating part 3e of an isolator 3 is detected by a temperature detection circuit 6, and when the temperature exceeds a certain specified value, a signal is given to a power supply feeding circuit 5. The circuit 5 stops power feeding to an amplifier circuit 2 there. For that reason, a signal from a signal input terminal 1 can not be amplified by the circuit 2 and the power of a signal at an output signal decreases. For instance, even when the connection of a signal output terminal 4 and an external circuit is disconnected to become an open state while evaluation test for the amplifier circuit is performed, because the level of a signal which is reflected by the terminal 4 and added to the part 3e is sufficiently low, it is possible to protect the part 3e of the isolator 3 and the circuit 2.
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