Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

G between collector and emitter dissipates a high

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several volts (e.g., between collector and emitter) dissipates a high power and, as a result, heats up . High-power transistors must therefore handle high currents and high temperature. Based on the above observations, we can predict the parameters of interest in the design of power stages: (1) “Distortion,” i.e., the nonlinearity resulting from large-signal operation. A high-quality audio amplifier must achieve a very low distortion so as to reproduce music with high fidelity. In previous chapters, we rarely dealt with distortion. (2) “Power efficiency” or simply “efficiency,” denoted by and defined as (13.3) For example, a cellphone power amplifier that consumes 3 W from the battery to deliver 1 W to the antenna provides . In previous chapters, the efficiency of circuits was of little concern because the absolute value of the power consumption was quite small (a few milliwatts). (3) “Voltage rating.” As suggested by Eq. (13.1), higher power levels or load resistance values translate to large voltage swings and (possibly) high supply voltages. Also, the transistors in the output stage must exhibit breakdown voltages well above the output voltage swings. 13.2 Emitter Follower as Power Amplifier With its relatively low output impedance, the emitter follower may be considered a good can- didate for driving “heavy” loads, i.e., low impedances. As shown in Chapter 5, the small-signal gain of the follower is given by (13.4) We may therefore surmise that for, say, , a gain near unity can be obtained if , e.g., , requiring a collector bias current of 32.5 mA. We assume . But, let us analyze the circuit’s behavior in delivering large voltage swings (e.g. 4 V ) to heavy loads. To this end, consider the follower shown in Fig. 13.1(a), where serves as the bias current source. To simplify the analysis, we assume the circuit operates from negative and positive power supplies, allowing to be centered around zero. For V, we have and mA. If rises from 0.8 V to 4.8 V, the emitter voltage follows the base voltage with a relatively constant difference of 0.8 V, producing a 4-V swing at the output [Fig. 13.1(b)]. Now suppose begins from V and gradually goes down [Fig. 13.1(c)]. We expect to go below zero and hence part of to flow from . For example, if V, then V, and carries a current of 12.5 mA. That is, mA. Similarly, if V, then V, mA, and hence mA. In other words, the collector current of continues to fall. And, in some applications, high voltages.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 687 (1) Sec. 13.2 Emitter Follower as Power Amplifier 687 t 0 4.0 V 0.8 V 4.8 V in V V out (b) Q 1 V +5 V I R L = 8 1 out V in V CC V 5 V EE (a) Q 1 V +5 V I 1 CC V 5 V EE R L out V 0.8 V Q 1 V +5 V I 1 CC V 5 V EE R L out V I 1 (c) (d) = 32.5 mA Figure 13.1 (a) Follower driving a heavy load, (b) input and output waveforms, (c) current path for as input becomes more negative, (d) current path as input becomes more positive.
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