DEFINITIONS APPLICATIONS AND TYPES OF POWER AMPLIFIERS
As the name implies, a power amplifier is designed to deliver a large amount of power to a load. To perform this function, a power amplifier must itself be capable of dissipating large amounts of power; that is, it must be designed so that the heat generated when it is operated at high current and voltage levels is released into the surroundings at a rate fast enough to prevent destructive temperature build up. Consequently, power amplifiers typ,~ally contain bulky components having large surface areas to enhance heat transfer t-9 the environment. A power transistor is a discrete device with a large surface area and a metal case, characteristics that make it suitable for incorporation into a power amplifier.
A power amplifier is often the last, or output, stage of an amplifier system. The preceding stages may be designed to provide voltage amplification, to provide buffering to a high-impedance signal source, or to modify signal characteristics in scrne predictable way, functions that are collectively referred to as signal conditioning. The output of the signal-conditioning stages drives the power amplif ,which in turn drives the load. Some amplifiers are constructed with signal-co stages and the output power stage all in one integrated circuit. Others, especially those designed to deliver very large amounts of power. have a hybrid structure, in the sense that the signal-conditioning stages are integrated and the power stage is discrete.
Power amplifiers are widely used in audio components-radio and television receivers, phonographs and tape players, stereo and high-fidelity systems, recording studio equipment, and public address systems. The load in these applications is most often a loudspeaker (vspeaker”), which requires considerable power to CO’l!t • electrical signals to sound waves. Power amplifiers are also used in electrcmc. ,,:; 1; control systems to drive electric motors. Examples include computer disk • nd tape drives, robotic manipulators, autopilots, antenna rotators, pumps and mo orized valves, and manufacturing? ~ process controllers of all kinds
Large-Signal Operation
Because a power amplifier is required to produce large voltage ·and current variations in a load, it is designed so that at least one of its semiconductor components, typically a power transistor, can be operated over substantially the entire range of its output characteristics, from saturation to cutoff. This mode of operation is called large-signal operation. Recall that small-signal operation occurs when the range of current and voltage variation is small enough that there is no appreciable change in device parameters, such as {3 and r., By contrast, the parameters of a large-signal amplifier at one output voltage may be considerably different from those at another output voltage. There are two important consequences of this fact:
1. Signal distortion occurs because of the change in amplifier characteristics with signal level. Harmonic distortion always results from such nonlinear behavior of an amplifier (see Figure 5-16). Compensating techniques, such as negative feedback, must be incorporated into a power amplifier if low-distortion high level outputs are required.
2. Many of the equations we have developed for small-signal analysis of amplifiers are no longer valid. Those equations were based on the assumption that device parameters did not change, contrary to fact in large-signal amplifiers. Rough approximations of large-signal-amplifier behavior can be obtained by using .average parameter values and applying small-signal analysis techniques. However, graphical methods are used more frequently in large-signal-amplifier design
As a final note on terminology, we should mention that the term large-signal operation is also applied to devices used in digital switching circuits. In these applications, the output level switches between “high” and “low” (cutoff and saturation), but remains in those states most of the time. Power dissipation is therefore not a problem. Either the output voltage or the output current is near 0 when a digital device is in an ON or an OFF state, so power, which is the product of voltage and current, is near 0 except during the short time when the device switches from one state to the other. On the other hand, the variations in the output level of a power amplifier occur in the active region, between the two extremes of saturation and cutoff, so a substantial amount of power is dissipated.
TRANSISTOR POWER DISSIPATION
Recall that power is, by definition, the rate at which energy isconsumed or dissipated (1 W = 1 J/s). If the rate at which heat energy is dissipated in a device is less than the rate at which it is generated, the temperature of the device must rise. In.electronic devices, electrical energy is converted to heat energy at a rate given by P = VI . watts, and temperature rises when this heat energy is not removed at a comparable rate. Since semiconductor material is irreversibly damaged when subjected to temperatures beyond a certain limit, temperature is the parameter that ultimately limits the amount of power a semiconductor device can handle.
Transistor manufacturers specify the maximum permissible junction temperature and the maximum permissible power dissipation that a transistor can withstand. As we shall presently see, the maximum permissible power dissipation is specified as a function of ambient temperature (temperature of the surroundings the rate at which heat can be liberated from the device depends on the temperature of the region to which the heat must be transferred. Most of the conversion of electrical energy to heat energy in a bipolar junction transistor occurs at the junctions. Since power is the product of voltage and current, and since collector andemitter currents are approximately equal, the greatest power dissipation occurs at the junction where the voltage is greatest. In normal transistor operation, the collector-base junction is reverse biased and has, on average, a large voltage across it, while the base-emitter junction has a small forward-biasing voltage. Consequently, most of the heat generated in a transistor is produced at the collector-base junction. The total power dissipated at the junctions is
Equation 16-1 gives, fOT” all practical purposes, the total power dissipation of the transistor.
For a fixed value of P«, the graph of equation 16-1 is a hyperbola when plotted on Ic – VCE axes. Larger values of P, correspond to hyperbolas that move outward from the axes, as illustrated in Figure 16-1. Each hyperbola represents all possible comhinations of Vel, and lc that give a product equal to the same value of PIt. The figure;shows sample coordinate values Uc, Vet:) at points on each hyperbola.
Note that the product of the coordinate values is the same for points on the same hyperbola, and that the product equals the power dissipation corresponding to the hyperbola. Figure 16-2 shows a simple common-emitter amplifier and its dc load line plotted on Ie – VCF.axes. Also shown are a set of hyperbolas corresponding to different values of power dissipation. Recall that the load line represents all possible combinations of Ie and Vn: corresponding to a particular value of Re. As the amplifier output changes in response to an input signal, the collector current and voltage undergo variations along the load line and intersect different hyperbolas of power dissipation. It is clear that the power dissipation changes as the amplifier output changes. For safe operation, the load line must lie below and to the left of
the hyperbola corresponding to the maximum permissible power dissipation. When the load line meets this requirement .. there is no possible combination of collector voltage and current that results in a power dissipation exceeding the rated maximum. It ean be shown that the point of maximum power dissipation occurs at the eel/tel’ of the load line, where Va = Vcc/2 and Ie = Vcc12Rc (see Figure 16-2).
Therefore the maximum power dissipation is
To ensure that the load line lies below the hyperbola of maximum dissipation, we therefore require that
where P(max) is the manufacturer’s specified maximum dissipation at a specified ambient temperature. Inequality 16-3 enables us to find the maximum permissible value of Rc for a given Vcc and a given PAmax):
The amplifier shown in Figure 16-2 is to be operated with Vcc = 20 Valid Rc = 1 k
I. What maximum power dissipation rating should the transistor have?
2. If an increase in ambient temperature reduces the maximum rating found in (1) bv a factor of 2, what new value of Rc should be used to ensure safe operation?
Solution
1. From inequality 16-3,
2. When the -dissipation rating is decreased to a.05 W, the minimum permissible value of Rc is, from (16.4),
