PURPOSE: To enlarge the open gain of the amplifier circuit and to improve the characteristic such as band width or input converting noise by adding a differential amplifier between two amplifying elements in preceding and following steps.
CONSTITUTION: The received signal of an optical signal from a photodetector 1 is inputted to the gate of an FET 2 in the first step, amplified by the FET 2, differential amplifier 4 and FET 7 in the following step and outputted from the source of the FET 7. The AC open gain of a negative feedback amplifier circuit is almost decided by the product of a transconductance gm of the FET 2, drain load 2 and voltage gain β of the differential amplifier 4. Therefore, when the gain β is enlarged, it is not necessary to enlarge the gm of the FET and the load 3 rather than proper values but the AC open gain can be turned to a sufficient level. On the other hand, since the average value of outputs from the differential amplifier 4 is detected and returned to an inverse phase input, the regular phase input voltage of the amplifier 4 can be made almost equal to the average value of output voltages at all times. Therefore, the characteristic such as band width or input converting noise can be improved.
ERA YOSHIKAZU
MARUYAMA MAKOTO
