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光电二极管后接结型场效应晶体管的光电探测电路,可提供低频散粒噪声限性能。采用现有的微波场效应晶体管,散粒噪声限性能的频带宽可扩展到兆赫范围。低噪声光电探测器用光电二极管可探测的最小信号功率通常受放大器噪声,而不是光电二极管噪声的限制。不过,在光电二极管后接一个场效应晶体管放大器,可大大改进信噪比。对于临界频率f_c以下的频率,可获得散粒噪声限性能,所以,光电二极管与场效应晶体管组合就成了光电参量上变频器、光电倍增管及低频雪崩光电二极管的劲敌。在下面的讨论中,假定光电二极管接到共源组态的场效应晶体管的栅极上。参照图1,信噪比很容易计算。此图中,反向偏置光电二极管由结电容C_d与串联电阻R_s表
Photodiodes followed by junction-type field effect transistor photodetection circuit, can provide low-frequency shot noise limit. With existing microwave field effect transistors, the bandwidth of the shot noise limited performance can be extended to the megahertz range. Low-Noise Photodetectors The minimum signal power that a photodiode can detect is usually limited by amplifier noise, not photodiode noise. However, a phototransistor followed by a field-effect transistor amplifier can greatly improve the signal-to-noise ratio. For the frequencies below the critical frequency f_c, the shot noise limit can be obtained. Therefore, the combination of the photodiode and the field effect transistor becomes the rival of the photoelectric parameter upconverter, photomultiplier tube and low-frequency avalanche photodiode. In the following discussion, it is assumed that the photodiode is connected to the gate of a field-effect transistor of a cascode configuration. Referring to Figure 1, the signal-to-noise ratio is easily calculated. In this figure, the reverse-biased photodiode consists of a junction capacitance C_d and a series resistor R_s