Chapters 13 to 16 discussed analysis and design of the midband characteristics of amplifiers. Low-frequency limitations due to coupling and bypass capacitors were ignored, and the internal capacitances of electronic devices, which limit the response at high frequencies, were also neglected. This chapter completes the discussion of basic amplifier design with the introduction of methods used to tailor the frequency response of analog circuits at both low and high frequencies. As part of this discussion, the internal device capacitances of bipolar and field-effect transistors are discussed, and frequency-dependent small-signal models of the transistors are introduced. The unity-gain bandwidth product of the devices is expressed in terms of the small-signal parameters.

In order to complete our basic circuit-building block toolkit, expressions for the frequency responses of the single-stage inverting, noninverting, and follower configurations are each developed in detail.We show that the bandwidth of high-gain inverting and noninverting stages can be quite limited, whereas that of followers is normally very wide. Use of the cascode configuration is shown to significantly improve the frequency response of inverting amplifiers. Narrow-band (high-Q) band-pass amplifiers based on tuned circuits are also discussed.

Transfer functions for multistage amplifiers may have large numbers of poles and zeros, and direct circuit analysis, although theoretically possible, can be complex and unwieldy. Therefore, approximation techniques -- the short-circuit and open-circuit time-constant methods -- have been developed to estimate the upper- and lower-cutoff frequencies ω_H and ω_L .

The Miller effect is introduced, and the relatively low bandwidth associated with inverting amplifiers is shown to be caused by Miller multiplication of the collector-base or gate-drain capacitance of the transistor in the amplifier. Internally compensated single-pole operational amplifiers use Miller multiplication to provide frequency compensation, and the resulting unity gain frequencies can be directly related to amplifier slew rate.

Mixers and modulators are a class of circuits that are capable of translating the frequencies associate with a signal's spectrum. In order to realize the mixing and modulating functions, some form of nonlinear signal multiplication is required. In the closing section of this chapter, we explore how the Gilbert multiplier, introduced in Chapter 16, can be used to achieve both mixing and modulation.