In most situations, a single-transistor amplifier cannot meet all the given specifications. The required voltage gain often exceeds the amplification factor of a single transistor, or the combination of voltage gain, input resistance, and output resistance cannot be met simultaneously. For example, consider the specifications of a good general-purpose operational amplifier. Such an amplifier has an input resistance exceeding 1 MΩ, a voltage gain of 100,000, and an output resistance of less than 100Ω. It should be clear from our investigation of amplifiers in Chapters 13 and 14 that these requirements cannot all be met simultaneously with a single-transistor amplifier. A number of stages must be cascaded in order to create an amplifier that can meet all these requirements.

Chapter 15 begins our study of combining singletransistor amplifier stages to achieve higher levels of overall performance. Several examples of multistage amplifiers are presented, and important two-transistor configurations, including the Darlington and cascode circuits, are introduced. ac-coupled amplifiers eliminate dc interactions between the various stages forming the amplifier, thus simplifying bias circuit design. On the other hand, in our work in Chapters 11 and 12, most of the operational amplifier circuits provided amplification of dc signals. To realize amplifiers of this type, coupling capacitors that block dc signal flow through the amplifier must be eliminated, which leads to the concept of direct-coupled or dc-coupled amplifiers that can satisfy the requirement for dc amplification. In the dc-coupled case, the operating point of one stage is dependent on the Q-point of the other stages, making the dc design somewhat more complex.

The most important dc-coupled amplifier is the symmetric two-transistor differential amplifier. Not only is the differential amplifier a key circuit in the design of operational amplifiers, it is also a fundamental building block in all analog IC design. In this chapter, we present the transistor-level implementation of BJT and FET differential amplifiers and explore how the differential-mode and common-mode gains, common-mode rejection ratio, differential-mode and common-mode input resistances, and output resistance of the amplifier are all related to transistor parameters.

Subsequently, a second gain stage and an output stage are added to the differential amplifier, creating the prototype for a basic operational amplifier. The definitions of class-A, class-B, and class-AB amplifiers are introduced, and the basic op amp design is further improved by adding class-B and class-AB output stages. In audio applications, these output stages often use transformer coupling.

Bias for analog circuits is most often provided by current sources. An ideal current source provides a fixed output current, independent of the voltage across the source; that is, the current source has an infinite output resistance. Electronic current sources cannot achieve infinite output resistance, but very high values are possible, and a number of basic current source circuits and techniques for achieving high output resistance are introduced and compared. Analysis of the various current sources uses the single-stage amplifier results from Chapters 13 and 14.