| 摘要 |
PURPOSE:To obtain a wide frequency characteristic and a low noise characteristic, by optimizing the trade-off of various parameters concerning the load resistance of a photodetector. CONSTITUTION:The cathode of a photodetector PD is biased backward by a voltage VB, and the anode is connected to the base of a transistor TR Tr1 of a grounded emitter amplifying circuit through a capacitor. The grounded base circuit of a TR Tr2 of a buffer amplifying circuit 2 is cascaded to the collector of the TR Tr1 to reduce the input capacity of the TR Tr1. TRs Tr3 and Tr4 constitute a Darlington circuit to reduce the output impedance. A feedback resistance Rf connects the emitter of the TR Tr4 and the base of the TR Tr1 to use circuits 1 and 2 as a negative feedback circuit. In this circuit constitution, when the value of the feedback resistance Rf, the gain-band width product, and the base current of the TR Tr1 are set to 2.0-2.4kOMEGA, >=2G Hz, and <=50muA respectively, a wide frequency characteristic and a low noise characteristic are obtained. |
