tion 5.39 it results that the minimum noise is achieved by increasing RL as much as possible. The limit to the increase of RL is given by the bandwidth of the front end which is approximately proportional to 1/RL. Even if an equalizer is used to improve the band- width, the relationship between noise and bandwidth constrain the design of low-noise high-impedance front ends. This limitation can be overcome by means of the so-called transimpedance front end, whose scheme is shown in Figures 5.52 and 5.53 while the equivalent circuit is sketched in Figure 5.54. The transimpedance of the front end is defined as the ratio between the amplifier input current (that is the photocurrent) and the output voltage and can be written as (5.41) If the circuit after the front end read the voltage at the front end output as a useful signal, the bandwidth of the front end can be written as (5.42) FIGURE 5.54 Equivalent circuit of a transimpedance receiver front Technology for Telecommunications: Integrated Optics and Microelectronics while the noise at the amplifier output can be evaluated with a procedure similar to that carried out for the high-impedance front end, obtaining (5.43) The principle on which the transimpedance amplifier is based is clear from Equations 5.42 and 5.43: the noise is limited by the high feedback resistance while the bandwidth can be wide if the open loop gain is sufficiently high. 5.8.2 Distortion Compensation Filters During propagation through an optical fiber the signal undergoes both linear and nonlin- ear distortion and the performance of an optical transmission system largely depends on how these distortions are managed. The main deterministic linear distortion is chromatic dispersion and dispersion com- pensation is mandatory in high-performance transmission systems. If dispersion is optically compensated, through a distortion compensation filter (DCF), for example, it is possible to compensate many wavelength channels together; moreover, since optical compensation is applied before the receiver, 