Solved: A class AB output stage using a two-diode bias network as shown in Fig. 12.14

Chapter 12, Problem 12.28

(choose chapter or problem)

A class AB output stage using a two-diode bias network as shown in Fig. 12.14 utilizes diodes having the same junction area as the output transistors. At a room temperature of about 20C the quiescent current is 1 mA and VBE = 0.6 V. Through a manufacturing error, the thermal coupling between the output transistors and the biasing diode-connected transistors is omitted. After some output activity, the output devices heat up to 70C while the biasing devices remain at 20C. Thus, while the VBE of each device remains unchanged, the quiescent current in the output devices increases. To calculate the new current value, recall that there are two effects: IS increases by about 14%/C and VT =kT/q changes, where T = 273 + temperature in C, and VT =25 mV only at 20C. However, you may assume that N remains almost constant. This assumption is based on the fact that increases with temperature but decreases with current. What is the new value of IQ? If the power supply is 20 V, what additional power is dissipated? If thermal runaway occurs, and the temperature of the output transistors increases by 10C for every watt of additional power dissipation, what additional temperature rise and current increase result?