iver can be large. The receiver transfer function is given by Equation 53.56 where A ampl is the low frequency voltage gain of the Equation 53.61, with the bandwidth given by Equation 53.62. amplifier. cd Ca Figure 53.25 High input impedance optical receiver Figure 53.26 Transimpedance optical receiver (53.56) (53.61) The receiver bandwidth is established by the first time constant at the input and it is given by Equation 53.57 where CT is the total input capacitance. (53.62) (53.57) 4-2ARLCT Depending on the technology, bandwidths up to 5GHz are The bandwidth is usually insufficient and some equalisation is possible. Above that transimpedance receivers are not used because required. The simplest approach is to cancel the pole by a zero using the use of feedback may lead to stability problems. For larger a RC network, as in Figure 53.25. The transfer function of this bandwidths the receivers are designed with input impedances corre- network is given by Equation 53.58 with the frequencies f1 and f2 sponding to that of the active devices, which are relative high but given by Equations 53.59 and 53.60. nowhere near the megaohm range used by truly integrating re- ceivers. Equalisation is still necessary but the first pole is in the (53.58) megahertz rather than the kilohertz range. The main reason for using an integrating receiver is the capability of these receivers to deliver very low noise operation because the thermal noise is very small. If an APD is available then the reason for using them is marginal. Their main disadvantage is that the very low bandwidth before equalisation requires the use of coding with (53.59) small digital sum variation, (DSV). As the bit rate increases coding becomes progressively difficult. The use of integrating receivers is expected to be further reduced with the introduction of signal formats with nearly uncontrollable DSV. Transimpedance receivers (53.60) do not impose limits on the signal DSV and in spite of the fact that the noise performance with a p-i-n detector is inferior they