Class A power amplifier. 18. An analog-drive circuit uses a power Darlington transistor. The supply vo 10 V; at 300 rpm, the motor is drawing 500 mA with a voltage of 3 V. How power is the transistor dissipating? 19. An analog-drive circuit uses a power Darlington transistor. The supply vo 12 V; at 400 rpm, the motor is drawing 1 A with a voltage of 7 V. How much is the transistor dissipating? 20. Explain the principle of PWM for motor-speed control. 21. What duty cycle would you specify to make a 2000-rpm, 12-V moto 1500 rpm? (Assume 12-V pulses and no PWM losses.) 22. What duty cycle would you specify to make a 1500-rpm, 12-V moto 1000 rpm? (Assume 12-V pulses and no PWM losses.) 23. What are the advantages of PWM over analog drive for motor control? 24. Explain how an SCR is turned on and off,