150 CHAPTER 4. OPTICAL RECEIVERS
Figure 4.12: Equivalent circuit for (a) high-impedance and (b) transimpedance front ends in
optical receivers. The photodiode is modeled as a current source in both cases.
component. A high-impedance front end cannot be used if∆fis considerably less than
the bit rate. An equalizer is sometimes used to increase the bandwidth. The equalizer
acts as a filter that attenuates low-frequency components of the signal more than the
high-frequency components, thereby effectively increasing the front-end bandwidth. If
the receiver sensitivity is not of concern, one can simply decreaseRLto increase the
bandwidth, resulting in a low-impedance front end.
Transimpedance front ends provide a configuration that has high sensitivity to-
gether with a large bandwidth. Its dynamic range is also improved compared with
high-impedance front ends. As seen in Fig. 4.12(b), the load resistor is connected as
a feedback resistor around an inverting amplifier. Even thoughRLis large, thenega-
tive feedbackreduces the effective input impedance by a factor ofG, whereGis the
amplifier gain. The bandwidth is thus enhanced by a factor ofGcompared with high-
impedance front ends. Transimpedance front ends are often used in optical receivers
because of their improved characteristics. A major design issue is related to the stabil-
ity of the feedback loop. More details can be found in Refs. [5]–[9].
4.3.2 Linear Channel.........................
The linear channel in optical receivers consists of a high-gain amplifier (the main am-
plifier) and a low-pass filter. An equalizer is sometimes included just before the am-
plifier to correct for the limited bandwidth of the front end. The amplifier gain is
controlled automatically to limit the average output voltage to a fixed level irrespective
of the incident average optical power at the receiver. The low-pass filter shapes the
voltage pulse. Its purpose is to reduce the noise without introducing muchintersymbol