Power Amplifiers

Abstract

An amplifier is a device for receiving a signal at its input and delivering a larger signal at its output. They are used in a wide range of applications including music systems, in driving industrial loads, and elsewhere. In small-signal amplifiers, the active devices are operated such that the voltage and current changes are small and the devices are operated in their approximately linear regions. In such amplifiers, the main factors are amplifier linearity, gain and noise performance.

Problems

• 1. Using the configuration in Fig. 9.39 and a transistor with a current gain of 50, design a low-power amplifier to deliver 60 mW into an 8-ohm load using a 6-volt supply (Fig. 9.39)

  Fig. 9.39

  

  Circuit for Question 1

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• 2. If the 8-ohm speaker in the above amplifier is replaced by a 15-ohm speaker, calculate the output power.

• 3. A power amplifier has an efficiency of 30% and delivers an output power of 15 W to a resistive load. Calculate the power dissipated and the DC input power to the amplifier.

• 4. A large signal amplifier delivers an output power of 25 W and consumes 75 W from the associated DC supply. Calculate its efficiency.

• 5. Using the configuration shown in Fig. 9.40, design a transformer-coupled amplifier delivering 1.5 W into 8 ohms and operating from a 16-volt supply. Use a power Darlington with β = 2000

  Fig. 9.40

  

  Circuit for Question 5

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• 6. Using the configuration shown in Fig. 9.41, design a transformer-coupled amplifier delivering 2 W into 8 ohms and operating from a 24-volt supply. Use a power Darlington with β = 1000

  Fig. 9.41

  

  Circuit for Question 6

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• 7. Explain the phenomenon of crossover distortion and the reason it occurs. Suggest one method of overcoming this problem.

• 8. Design a low-power amplifier using the topology of Fig. 9.12 and a 12-volt supply.

• 9. For the amplifier circuit shown in Fig. 9.42, determine the quiescent current and the base voltage in transistor Tr₃ if the output voltage is 4.5 volts. Develop a design procedure for this configuration

  Fig. 9.42

  

  Circuit diagram for Question 9

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• 10. What would be the effect on amplifier performance of increasing the input and output capacitors? Discuss the use of a variable Zener circuit to adjust the output bias current.

• 11. Design a 500 mW amplifier using the circuit of Fig. 9.43. Introduce bootstrapping to improve the performance of this circuit

  Fig. 9.43

  

  Circuit for Question 11

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• 12. Design a 5 W amplifier using the configuration of Fig. 9.44

  Fig. 9.44

  

  Diagram for Question 12

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• 13. Compare the performances of the amplifiers designed in questions 11 and 12.

• 14. Identify the type of feedback that is used in the amplifier in Fig. 9.44 as well as the number of voltage gain stages.

• 15. Design a 20 W amplifier driving an 8 ohm speaker using the configuration in Fig. 9.16.

• 16. Design a 60 W amplifier driving a 4 ohm speaker using the configuration in Fig. 9.17. (i) Discuss the effect of including emitter resistors in the input stage. (ii) Design and introduce a constant current source in the differential amplifier stage and indicate how this improves amplifier performance. (iii) Design short-circuit protection for the amplifier.

• 17. The power amplifier shown in Fig. 9.45 utilizes an output configuration referred to as quasi-complimentary symmetry since the two power transistors are of the same polarity. Analyse the circuit shown and investigate the operation of the output stage.

• 18. Design a 75 W power MOSFET amplifier using the circuit in Fig. 9.21.

• 19. In a power amplifier design a power transistor is expected to dissipate 75 W. Using a transistor that can dissipate 100 W at a junction temperature of 100^oC determine the thermal resistance of the heatsink required for safe operation of this transistor. Use θ_JC = 0.4^oC/W and θ_CS ≈ 2^oC/W.

Fig. 9.45

Diagram for Question 17

• 20. As a research project, modify the configuration in Fig. 9.45 to include another transistor stage at the input as in Fig. 9.16.

• 21. An audio power amplifier can be represented in a simplified model as an amplifier A₁ with relatively low distortion d₁ driving an output stage A₂ with significant distortion d₂ (Fig. 9.46). Positive feedback is applied around A₁ via β₁ and negative feedback is applied around the system through β₂. Show that when A₁β₁ = 1, the output is given by ${V}_o=\frac{V_i}{\beta_2}+\frac{d_1}{A_1{\beta}_2}$. This means that in the amplifier output signal V_o, the already low distortion d₁ is further reduced by the factor A₁β₂ and the major distortion d₂ completely vanishes!

  Fig. 9.46

  

  Diagram for Question 21