Published online by Cambridge University Press: 05 March 2013
In the previous chapter, we have examined the design of front-end low-noise amplifiers for narrowband wireless receivers. The purpose of these LNAs is to amplify a small input signal level while adding as little noise as possible such that the receiver sensitivity is as high as possible. But what about broadband applications – are low-noise front-ends required in high-speed digital receivers? In this chapter, we will build on the small signal properties of broadband amplifiers introduced in Chapter 5 and on the noise analysis and low-noise design techniques developed in Chapters 3 and 7, respectively, to learn how to analyze and design very broadband, DC-coupled, low-noise amplifiers. Along with high-speed logic, discussed in Chapter 11, and broadband large swing output drivers, covered in Chapter 12, these amplifiers form the back-bone of all fiber-optics, backplane, and other wireline communications systems. We will discuss first the specifications and requirements of low-noise broadband amplifiers in high-speed digital receivers. Next we will explore the design of low-noise transimpedance amplifiers (TIAs) commonly found in optical receivers. Finally, design methodologies for other LNA topologies used in broadband electrical links will also be discussed. In all cases, representative design examples in InP HBT, SiGe BiCMOS and nanoscale bulk and SOI CMOS technologies will be provided.
Low-noise broadband high-speed digital receivers
Consider the simplified receiver of Figure 8.1, where a noisy input amplifier is directly followed by a decision circuit (a D-type flip-flop) that determines if the received bit is a logical 1 or 0. In situations where small input amplitudes are received, noise added by the input amplifier could cause the decision circuit to make an incorrect decision. Ultimately, this leads to a poor bit error rate (BER) for the broadband receiver. This simple example illustrates the importance of low-noise designs even in high-speed digital receivers.
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