ELEC

# Example 1316 calculate the efficiency of the stage

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Example 13.16 Calculate the efficiency of the stage depicted in Fig. 13.26. Assume is chosen so as to allow a peak swing of at the output. Also, . Solution Recall from Section 13.3 that must be at least equal to . Thus, the branch consist- ing of , , and consumes a power of , yielding an overall efficiency of: (13.65) (13.66) We should note the approximation made here: with the diode branch present, we can no longer assume that each output transistor is on for only half of the cycle. That is, and consume slightly greater power, leading to a lower .

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 711 (1) Sec. 13.9 Power Amplifier Classes 711 Exercise If and , what is the maximum efficiency that can be achieved in this circuit? 13.9 Power Amplifier Classes The emitter follower and push-pull stages studied in this chapter exhibit distinctly different prop- erties: in the former, the transistor conducts current throughout the entire cycle, and the efficiency is low; in the latter, each transistor is on for about half of the cycle, and the efficiency is high. These observations lead to different “PA classes.” An amplifier in which each transistor is on for the entire cycle is called a “class A” stage [Fig. 13.29(a)]. Exemplified by the emitter follower studied in Section 13.2, class A circuits suffer t (a) I C T T 2 0 t I C 0 t I C 0 Q 1 Q 2 (c) (b) T T 2 T T 2 Figure 13.29 Collector waveforms for (a) class A, (b) class B, and (c) class C operation. from a low efficiency but provide a higher linearity than other classes. A “class B stage” is one in which each transistor conducts for half of the cycle [Fig. 13.29(b)]. The simple push-pull circuit of Fig. 13.3(a) is an example of class B stages. The efficiency in this case reaches , but the distortion is rather high. As a compromise between linearity and efficiency, PAs are often configured as “class AB” stages, wherein each transistor remains on for greater than half a cycle [Fig. 13.29(c)]. The modified push-pull stage of Fig. 13.11(a) serves as an example of class AB amplifiers. The dead zone in this stage in fact allows conduction for slightly less than half cycle for each transistor.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 712 (1) 712 Chap. 13 Output Stages and Power Amplifiers Many other classes of PAs have been invented and used in various applications. Examples include classes C, D, E, and F. The reader is referred to more advanced texts [1]. 13.10 Chapter Summary Power amplifiers deliver high power levels and large signal swings to relatively low load impedances. Both the distortion and efficiency of power amplifiers are critical parameters. While providing a low small-signal output impedance, emitter followers operate poorly under large-signal conditions. A push-pull stage consists of an follower and a follower. Each device conducts for about half of the input cycle, improving the efficiency.

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