In most situations, the single-stage amplifiers discussed in Chapters 13 and 14 cannot simultane-ously meet all the requirements of an application (e.g., high voltage gain, high input resistance,and low output resistance). Therefore, we must combine single-stage amplifiers in various ways to form multistage amplifiers that achieve higher levels of overall performance.

• Both ac- and dc-coupling (also called direct-coupling) methods are used in multistage amplifiers depending on the application. ac coupling allows the Q-point design of each stage to be done independently of the other stages, and bypass capacitors can be utilized to eliminate bias elements from the as equivalent circuit of the amplifier. However, dc coupling can eliminate circuit elements, including both coupling capacitors and bias resistors, and can represent a more economical approach to design. In addition, direct coupling is required to achieve a low-pass amplifier that provides gain at dc.
• The most important dc-coupled amplifier is the symmetric two-transistor differential ampli-fier. Not only is the differential amplifier a key circuit in the design of operational amplifiers,but it is also a fundamental building block of all analog circuit design. In this chapter, we studied BJT and MOS differential amplifiers in detail. Differential-mode gain, common-mode gain, common-mode rejection ratio (CMRR), and differential- and common-mode input and output resistances of the amplifier are all directly related to transistor parameters and hence Q-point design.
• Either a balanced or a single-ended output is available from the differential amplifier. The balanced output provides a voltage gain that is twice that of the single-ended output, and the CMRR of the balanced output is inherently much higher (that is, infinity for the ideal case). A two-port model can be used to model the small-signal characteristics at the output of the differential pairs.
• One of the most important applications of differential amplifiers is to form the input stage of the operational amplifier. By adding a second gain stage plus an output stage to the differential amplifier, a basic op amp is created. The performance of differential and opera-tional amplifiers can be greatly enhanced by the use of electronic current sources. Op amp designs usually require a number of current sources, and, for economy of design, these multiple sources are often generated from a single-bias voltage.
• An ideal current source provides a constant output current, independent of the voltage across the source; that is, the current source has an infinite output resistance. Although electronic current sources cannot achieve infinite output resistance, very high values are possible,and there are a number of basic current source circuits and techniques for achieving high output resistance.
• For a current source, the product of the source current and output resistance represents a figure of merit, V_CS, that can be used to compare current sources. A single-transistor current source can be built using the bipolar transistor in which V_CS can approach the β_oV_A product of the BJT. For a very good bipolar transistor, this product can reach 10,000 V. Forthe FET case, V_CS can approach a significant fraction of μ_fV_SS, in which V_SS represents the power supply voltage. Values well in excess of 1000 V are achievable with the FETsource.
• The electronic current source can be modeled in SPICE as a dc current source in parallel with a resistor equal to the output resistance of the source. For greatest accuracy, the value of the dc source should be adjusted to account for any dc current existing in the output resistance.
• Other useful two-transistor dc-coupled amplifiers include the Darlington configuration,which achieves very high current gain, and the cascode circuit, which exhibits a very high amplification factor.
• Class-A, Class-B, and Class-AB amplifiers are defined in terms of their conduction an-gles: 360° for Class-A, 180° for Class-B, and between 180° and 360° for Class-AB op-eration. The efficiency of the Class-A amplifier cannot exceed 25 percent for sinusoidal signals, whereas that of the Class-B amplifier has an upper limit of 78.5 percent. However,Class-B amplifiers suffer from cross-over distortion caused by a dead zone in the transfer characteristic.
• The Class-AB amplifier trades a small increase in quiescent power dissipation and a small loss in efficiency for elimination of the cross-over distortion. The efficiency of the Class-ABamplifier can approach that of the Class-B amplifier when the quiescent operating point is properly chosen. The basic op-amp design can be further improved by replacing the Class-A follower output stage with a Class-AB output stage. Class-AB output stages are often used in operational amplifiers and are usually provided with short-circuit protection circuitry.
• Amplifier stages may also employ transformer coupling. The impedance transformation properties of the transformer can be used to simplify the design of circuits that must drive low values of load resistances such as loudspeakers.