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Chapter 6: Power Amplifiers and Output Stages

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Chapter 6: Power Amplifiers and Output Stages Microelectronics: Analysis and Design© January 9, 2003 Sundaram Natarajan CHAPTER 6: POWER AMPLIFIERS AND OUTPUT STAGES 6.0: INTRODUCTION The analysis and design of small-signal amplifiers were discussed so far. These amplifiers operate in the linear range, and the output will normally be undistorted. The operation of the transistors is confined to the active mode. This permitted the use of small-signal models to analyze these circuits. Such small-signal amplifiers have inherently low power output capability and are not expected to drive low impedance loads such as speakers and motor drives. Requirements of Power Amplifiers In this chapter, we consider power amplifiers. Both discrete and IC power amplifiers will be examined. These amplifiers usually constitute the output stages in a multistage amplifier. There are some specific requirements of such amplifiers. Some important ones are: 1. Power amplifiers must have low output impedance so they can drive low impedance loads with no reduction in the voltage gain. 2. Power amplifiers must deliver a large amount of power while dissipating only a low amount of power internally; i.e., they must have a high power efficiency. There are two important reasons for this: (a) While the amplifier is expected to deliver a specific amount of output (load) power, the input power to the amplifier comes essentially from the dc power supply (Section 1.4) Therefore, if the amplifier is inefficient, there will be a large drain on the power supply. (b) The difference between the battery power and the power delivered to the load must be dissipated by the transistors. If the amplifier is inefficient, the power dissipation ratings of the transistors must be higher, and large size heat sinks will be required to prevent any damage to the transistors. High power transistors are expensive, and the size and cost of the amplifier will escalate. These reasons clearly suggest the importance of efficiency. 3. Power amplifiers handle large voltage and current signals since they are the final stages of multi- stage amplifiers. Therefore, the distortion of the output signal is also an important consideration. These amplifiers should deliver power to a specific load with acceptably low levels of distortion. A measure of the distortion in the output signal is the total harmonic distortion (THD). In a good power amplifier, THD should be less than 0.1 %. 4. Since the power amplifiers handle large signals, the small-signal approximations are not generally 479 Microelectronics: Analysis and Design© January 9, 2003 Sundaram Natarajan valid in the analysis of power amplifiers. Instantaneous values of the signals should be used instead. However, as mentioned earlier, we cannot accept large distortion either. Therefore, linearity is also an important consideration in power amplifiers. In this chapter, we first consider the classification of power amplifiers using the signal-swing considerations. Then, power amplifiers under the different classifications starting with the simplest configuration of the emitter follower circuit are discussed. Thermal considerations due to the power dissipation in the transistors are included. Also examined are short-circuit protection and current-limiting features. We conclude the chapter with a discussion on power MOSFETs. 6.1: CLASSIFICATION OF POWER AMPLIFIERS Power amplifiers are classified according to the waveform of the collector current (drain current if FETs are used). They are Class-A, Class-AB, Class-B, and Class-C amplifiers. While the first three types of amplifiers find wide use in the audio power applications, the Class-C operation is usually employed in the RF (radio frequency) power amplifiers. In this chapter, we focus on the first three categories only. In the Class-A operation, the BJT is biased with a dc bias current ICQ so the instantaneous value of the collector current never becomes zero. The waveform of the collector current under the Class-A operation is shown in Fig. 6.1.1(a) using a sinusoidal signal1 as an example. The transistor conducts during the entire cycle, and the collector current never goes to zero. This is like any other small-signal amplifier addressed in the earlier chapters. All small-signal linear amplifiers belong to the Class-A category. Therefore, in a Class-A power amplifier, the distortion will be the least. The value of ICQ is typically chosen to be equal to half the maximum expected value of the collector current. This provides the maximum symmetrical signal-swing and maximum efficiency in this category. In the Class-B operation, the transistor is biased to operate with a zero bias current; i.e., ICQ = 0. With sinusoidal inputs, the collector current exists during either the positive or negative half cycles only. Therefore, the waveform of the collector current of a npn transistor will be as shown in Fig. 6.1.1(b). With the same input, a pnp transistor will conduct during the negative-half cycles of the input only. Since the input voltage has to overcome the cut-in voltage of the base-emitter junction, the collector current will be distorted near the zero crossings. This situation is exactly similar to what exists in a half-wave rectifier. If only one transistor is used to amplify signals with Class-B operation, the output will be heavily distorted. Fortunately, 1 Sinusoidal signals are used to study and assess the properties of many electrical and electronic circuits. The properties of the power amplifiers are studied using a single frequency sinusoidal signal. Therefore, unless otherwise specified, both input and output signals will be sinusoidal throughout this chapter. 480 Microelectronics: Analysis and Design© January 9, 2003 Sundaram Natarajan iC(t) Im iC(t) ICQ Im 0 0 π 2π 3π 4π ωt π 2π 3π 4π ωt (a) (b) iC(t) iC(t) Im ICQ 0 0 π 2π 3π 4π ωt π 2π 3π 4π ωt (c) (d) Fig. 6.1.1: Collector current waveforms of the npn transistors operating in (a) class-A, (b) class-B, (c) class-AB, and (d) class-C power amplifiers. both halves of the input signal can be amplified using the complementary symmetry (push-pull) arrangement. With this arrangement, the power amplifier will be highly efficient because there is no dc power dissipation under the quiescent conditions, i.e., with no input signal. To avoid the distortion during the zero crossings, each transistor in the push-pull arrangement can be biased with a small dc bias current so that the conduction angle is more than 180E as shown in Fig. 6.1.1(c). This is called the Class-AB operation because this operation is intermediate between the Class-A and Class-B operations. Of course, the efficiency will reduce in comparison to the Class-B operation because of the quiescent dc power dissipation but the distortion during the zero-crossings can be avoided. However, in all types of power amplifiers, whether it is Class-A, -AB, -B, the distortion due to the transistor saturation cannot be avoided. Therefore, the maximum signal-swing should be limited to the transistor saturation levels, and we assume this to be the case throughout this chapter. 481 Microelectronics: Analysis and Design© January 9, 2003 Sundaram Natarajan In the Class-C operation, the transistor is biased to conduct for less than half a cycle as shown in Fig. 6.1.1(d). This type of operation is used in RF power amplifiers (in radio and TV transmitters), where efficiency is required to be very high. The output waveform is almost pulsating. The output is fed to an LC tank circuit, which essentially operates as a tuned amplifier (see Fig. 1.9.3) and selects the fundamental component for further amplification. Although the Class-C amplifier is very efficient, it is only used in some special applications where high power is delivered to the load. The analysis of the Class-C amplifier is tedious and beyond the scope of this book. 6.2: CLASS-A POWER AMPLIFIERS The simplest power amplifier is an emitter(or the source) follower. A Class-A power amplifier using a constant-current source biasing, along with its transfer characteristic, has been reproduced here in Fig. 6.2.1 (see Fig. 3.6.9). Emitter- and source-followers have been analyzed in the earlier chapters for their small-signal behavior. Now we consider their large-signal operation and power efficiency. This circuit may be used in a discrete design also. To keep the analysis simple, it is assumed that α . 1 for the BJTs in this and future sections. vO +VCC Q 1 V -V +vI CC CES1 + v BE1 i - E1 i +v R L O IREF v v I BE1 I Q BIAS RL Q3 2 -IBIASRL -(VEE -VCES2 ) -VEE Fig. 6.2.1: (a) A class-A power amplifier with a constant-current source biasing, and (b) its transfer characteristics. Power Dissipation and Efficiency A power amplifier is designed to deliver a specified maximum value of the average amount of power to a given load. It is convenient to assume the output signal to be a sinusoid and develop all the equations in terms of its amplitude. Thus, let 482 Microelectronics: Analysis and Design© January 9, 2003 Sundaram Natarajan vO (t) ' Vm sin ωt , (6.2.1) where Vm # VCC. Then, we find that vCE1 ' VCC & vO ' VCC & Vm sinωt,andiC1 . iE1 ' IBIAS % (Vm /RL )sinωt . (6.2.2) The input current from the signal source is very small in comparison to the collector currents of the transistors. Therefore, in power calculations, the power input from the signal source is ignored, and the input power to the circuit is assumed to be the power supplied by the power supplies only (see (1.4.1)).
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