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The ideal amplifier would deliver 100 percent of the power it draws from the dc power supply to its load. In practice, 100 percent efficiency cannot be achieved (at this time) because every amplifier uses some percentage of the power it draws from the dc power supply. 
  The efficiency of an amplifier is the ratio of ac output power to dc input power, written as a percentage. By formula:

The lower the position of the Q-point on the dc load line, the higher the maximum theoretical efficiency of a given amplifier. Typical Q-point locations for class A, B, AB, and C amplifiers are shown in Figure 11.1 of the text.

AC Load Lines

The ac load line is a graph that represents all possible combinations of  and  for a given amplifier. Under normal circumstances, the ac and dc load lines for a given amplifier are not identical. A typical ac and dc load line combination is shown in Figure 11-1a. Note that the two lines intersect at the circuit Q-point. The endpoints of the ac load line are defined as shown in Figure 11-1b. As shown, the ac saturation and cutoff points can be defined using circuit Q-point values. The derivations of the equations shown in the figure can be found in Section 11.1 of the text.

(a) (b)

 

FIGURE 11-1  Load lines.

 

The compliance (PP) of an amplifier is the limit that the output circuit places on its peak-to-peak output voltage. The compliance for a given amplifier is found using the following equations:

 and 

These equations are developed as illustrated in Figure 11.4 of the text. The compliance of an amplifier is determined by solving both of these equations and using the lower of the two results, as demonstrated in Example 11.1. Note the following:

• When an amplifier has a value of , exceeding the value of PP results in saturation clipping.
• When an amplifier has a value of , exceeding the value of PP results in cutoff clipping. However, the circuit will experience nonlinear distortion before the amplifier peak-to-peak output reaches the value of PP.

RC-Coupled Class A Amplifiers

The efficiency of an RC-coupled class A amplifier is determined by the amount of power the circuit draws from its dc power supply () and the ac load power (). The total dc power drawn from the power supply is found using

where  is the power supply current. The value of  for a voltage-divider biased amplifier is calculated as shown in Example 11.2 of the text. When  is measured with an ac voltmeter, the value of ac load power can be found using

The use of this relationship is demonstrated in Example 11.3. When  is measured using an oscilloscope, the value of ac load power can be found using

The use of this relationship is demonstrated in Example 11.4 of the text. The maximum value of  for an amplifier is found using

The value of  is used to calculate the maximum efficiency of an RC-coupled class A amplifier, as demonstrated in Example 11.6.

Transformer-Coupled Class A Amplifiers

A transformer-coupled class A amplifier is shown in Figure 11-2. The transformer is used to couple the amplifier output signal to the load.

 

FIGURE 11-2 A transformer-coupled class A amplifier.

The dc biasing of the transformer-coupled class A amplifier is similar to that of other amplifiers, outside of the fact that the value of  is designed to be as close as possible to the value of . 
  The ac load line of a transformer-coupled class A amplifier is more difficult to plot than the ac load line for an RC-coupled circuit. This procedure is outlined in Section 11.3 of the text. The following are typical characteristics for the transformer-coupled circuit:

•  is very close to the value of .
• The maximum output voltage is very close to  and therefore, can approach the value of .

The maximum theoretical efficiency of a transformer-coupled class A amplifier is 50 %. In practice, the transformer-coupled amplifier has a value of . The high theoretical value is a result of assuming that  and ignoring transformer (and other) circuit losses. The efficiency of a transformer-coupled circuit is calculated as shown in Example 11.7 of the text.
  The transformer-coupled class A amplifier has the following advantages over the RC coupled circuit:

• Higher efficiency.
• It is relatively simple to match the amplifier and load impedance using a transformer.
• A transformer-coupled circuit can easily be converted to a tuned amplifier; that is, a circuit that provides a specific value of gain over a specified range of operating frequencies.

Class B Amplifiers

The class B amplifier is a two-transistor circuit that is designed to improve on the efficiency characteristics of class A amplifiers. A class B amplifier is shown in Figure 11-3.

 

FIGURE 11-3 Class B amplifier.

The circuit shown in Figure 11-3 is a complementary-symmetry amplifier, or a push-pull emitter follower. The circuit contains one npn transistor () and one pnp transistor (). The circuit containscomplementary transistors; that is, npn and pnp transistors with identical characteristics (for example, a 2N3904 and a 2N3906).
   Another class B amplifier, called a push-pull amplifier, is shown in Figure 11.16 of the text. The circuit in Figure 11-3 is the preferred of the two because the standard push-pull amplifier requires the use of one or more transformers.
   In its quiescent state, both of the transistors in the class B amplifier are biased in cutoff. During the positive alternation of the input cycle,  is biased on, coupling the alternation to the load. During the negative alternation of the input,  is biased on, coupling that alternation to the load. As a result, a complete cycle is produced at the load. This concept is illustrated in Figure 11.17 of the text. The fact that the two transistors are never fully on (at the same time) is the key to the high efficiency of the circuit.
   As shown in Figure 11-3, the complementary symmetry amplifier has a value of . As long as , the matched (complementary) transistors have equal values of . Because of this, the circuit has a vertical dc load line as shown in Figure 11.19 of the text.
   Because both transistors in a class B amplifier are biased in cutoff, the amplifier is susceptible to crossover distortion. This type of distortion occurs at the zero crossings of the input waveform. (See Figure 11.18 of the text.) During the time that the input signal is between approximately  0.7 V, both transistors are off, causing the flat line between the output alternations. This type of distortion can be reduced (or eliminated) by:

• Biasing the transistors at soft cutoff as shown in Figure 11.20 of the text
• Using diode bias

The use of diode bias is discussed later in this chapter.
   Typical dc and ac load lines for a complementary symmetry amplifier are shown in Figure 11-4. The position of the ac load line (blue) is determined by the relationships shown in the figure. The load lines shown in the figure are plotted as demonstrated in Example 11.8 of the text.

 

FIGURE 11-4 Class B amplifier load lines.

 Since the complementary-symmetry amplifier is an emitter follower, it has the overall characteristics of any emitter follower. That is, it typically has:

• Relatively high current gain
• Low voltage gain (< 1)
• Relatively high input impedance
• Relatively low output impedance

Also, because of the two-transistor configuration, the amplifier has a maximum theoretical efficiency of 78.5%. (The derivation of this value can be found in Appendix D of the text.) Of course, any practical value is significantly lower, as demonstrated in Example 11.13 of the text.

Class AB Amplifiers (Diode Bias)

The Class B amplifier is susceptible to crossover distortion and thermal runaway. Diode bias is often used to overcome these drawbacks. Diode bias is accomplished by connecting a pair ofcompensating diodes between the transistor bases, as shown in Figure 11-5. When the diodes are properly matched to the transistors, crossover distortion and thermal runaway are prevented.

 

FIGURE 11-5 Diode bias.

 When diode bias is used, the transistors are biased just above cutoff. The values of  are still approximately half the value of . However,  is at some measurable value that is greater than . The quiescent currents through a diode bias circuit are illustrated in Figure 11.29 of the text.
   Since each transistor in a diode bias circuit has a value of , each conducts for slightly more than 180° of the input cycle, as shown in Figure 11.30. As a result, the circuit cannot technically be referred to as a class B amplifier. Instead, it is referred to as a class AB amplifier, indicating that each transistor conducts for more than 180°(and less than 360°) of the input cycle.
   Crossover distortion occurs during the time that both transistors in a class B amplifier are in cutoff. Since one (or both) of the transistors in a class AB amplifier is always conducting, crossover distortion is prevented. For the circuit to eliminate thermal runaway, several conditions must be met:

• The diodes must be very nearly perfectly matched to the base-emitter characteristics of the transistors. That is, the thermal and forward operating characteristics of the two devices must be as closely matched as possible.
• The diodes and the transistors must be in thermal contact to ensure that they are operating at the same temperature. Thermal contact can be accomplished by connecting both components to a common heat sink.

When these conditions are met, any heat-related increase in transistor current is accompanied by an increase in diode conduction. As diode conduction increases, the value of  decreases and the value of  increases. These changes cause the value of  for each transistor to decrease, which causes the values of  and  for each transistor to decrease. As a result, thermal runaway is prevented. 
   Swamping resistors are sometimes added to the emitter circuits in a class AB amplifier, as shown in Figure 11.31 of the text. These resistors are used to overcome any minor differences in the characteristics of the transistors.
   Class AB amplifier troubleshooting is discussed in Section 11.6 of the text. Several common variations on the basic class AB amplifier are shown in Figures 11.40 and 11.41 of the text.

Maximum Power Ratings

The transistor(s) used in a given power amplifier must have a maximum power dissipation rating that is sufficient to meet the demands of the circuit. In a class A amplifier, maximum transistor power dissipation occurs when the circuit has no active input signal. By formula:

For class B and class AB amplifiers:

Examples 11.14 and 11.15 of the text demonstrate the use of these relationships.

Component Cooling

When used in an enclosed system, the power dissipation of the various resistive components can cause a significant rise in temperature. As temperature rises, the power derating factor of a given component can cause its power dissipation rating to fall below the required value. In extreme cases, the maximum junction temperature ratings of some components may be exceeded.
   Many enclosed systems incorporate a cooling fan to decrease the ambient temperature. Another method used to cool components is to mount them on a heat sink. A heat sink is a large metallic object that effectively increases the surface area of a component. Several heat sinks are shown in Figure 11.45 of the text.
   When mounted to a heat sink, the effective surface area of a component is increased, allowing it to dissipate heat much faster. As a result, the component is kept cooler than it would be without the sink. Heat-sink compound is applied between a component and the heat sink to improve the flow of heat to the sink. (It does not provide a mechanical bond between the component and the heat sink.)