it of a high-impedance receiver front FIGURE 5.53 Electrical scheme of a transimpedance receiver front end. The scheme of a high-impedance amplifier following a photodiode is plotted in Figure 5.51, while its equivalent noise circuit is shown in Figures 5.52 and 5.53. In the equivalent circuit, the photodiode is represented as a capacitor in parallel with the photocurrent generator and the dark current generator while the amplifier is divided into three sections: the load resistance, the noise circuit representing the thermal noise contri- bution, and an ideal amplifier providing the gain G and the lowpass characteristics with bandwidth equal to Be This structure is aimed to minimize the noise contribution with a very high input resis- tance. This design implies a small passband and in order to achieve a large electronic bandwidth the amplifier has to be accompanied by an equalizer. From the equivalent circuit it is clear that the noise results from two main sources: the photodiode noise, which can be modeled as a noise current, and the amplifier noise pro- cess, which can be modeled as a current noise and a voltage noise. Thus the overall circuit noise can be modeled as follows: (t)=G[ntp(t)+nc(t)+nco(t)] (5.36) where the noise contributions are defined in Figure 5.51 and Ncu(t) is the noise current gen- erated by the voltage noise at the amplifier input. The power of the photodiode thermal noise can be written as (5.37) where kB is the Boltzmann constant I is the absolute temperature RL represents the photodiode load resistance Telecommunication Networks The power of the process Ncu(t) can be evaluated by evaluating the input admittance of the amplifier. In particular, calling the admittance Y(@), the power spectral density of the voltage process n.(t) with S(@) and the ideal amplifier transfer function H(), we obtain (5.38) Combining Equations 5.36 with 5.37 and 5.38, the overall power of the thermal noise at the receiver is given by (5.39) where the noise factor of the receiver Fa is given by (5.40) From Equa