In Chapter 5 we saw that the front-end plays a major role in determining the noise performance of a receiver. In this chapter, we will explore four principal types of front-end designs that are used in optical receivers. We will review the use of bipolar and field-effect transistors in front-end amplifiers and we will examine representative examples of receiver front-ends using p-i-n photodiodes and APDs.
6.1 Front-End Architectures
An optical receiver's front-end design can usually be grouped into one of four basic configurations: 1) resistor termination with a low-impedance voltage amplifier, 3) high-impedance amplifier, 4) transimpedance amplifier, and 5) noise-matched or resonant amplifier. Any of the configurations can be built using contemporary electronic devices such as operational amplifiers, bipolar junction transistors, field-effect transistors, or high electron mobility transistors. The receiver performance that is achieved will depend on the devices and design techniques used.
The names typically used to describe these four configurations can be somewhat misleading. Fundamentally, the front-end of an optical receiver responds to an optical signal by generating a photocurrent with a photodetector. The photocurrent is then converted to a voltage. Electronic signal processing stages process the recovered voltage to extract the desired information. The dimensions of the transfer function associated with the front-end will consequently be volts per amp or ohms. Thus the transfer functions of virtually all optical receivers are actually transimpedance in nature.
6.1.1 Low Impedance Voltage Amplifier
A simple optical receiver front-end is illustrated in Fig. 6.1. It consists of a photodetector, a load resistor, and a low input-impedance voltage amplifier.
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