The figure shows a class B output stage with two complementary output transistors Q3 and Q4 and two complementary driver transistors Q1 and Q2. The transistors are connected in Darlington connection for sufficiently high current gain from input to output at the output stage.
R1 is inserted to increase the quiescent current in Q1 and Q2, and increases the speed of Q3 and Q4 when the transistors to turn off (Class B). RL is the load output stage to drive. Vbb1 and Vbb2 creates a voltage between the bases of the two symmetrical halves amplifier so that the quiescent current IQ in the output transistors can be controlled, it applies to VBB = Vbb1 + Vbb2 and Vbb1 = Vbb2. Rest current is the current flowing through the two output transistors collectors when RL is disconnected and Vout = 0V
For transistors: Is-q1 = Is-q2 = 5 * 10^-14 A. and Is-q3 = Is-q4 = 2 * 10^-12 A.
a) Calculate the maximum power dissipation in the output transistors PQ3 + PQ4.
b) Calculate Vbb1 = Vbb2 so that the quiescent current in the output transistors are I_Q-X, and R1 as the quiescent current of the driver transistors (I_R1) will be equal to I_R1-X.
β is ∞ for all transistors.
c) Determine the maximum efficiency of the amplifier when the maximum output voltage is limited by the following (Vneg + 4V <= Vout <= V POS-4V).
The given values are:
Vpos = 24 V.
Vneg = 24 V.
RL = 7 Ω.
R2 = 768 Ω.
R3 = 768 Ω.
I_R1-X = 27 mA.
I_Q-X = 66 mA.
Vout-X = 16 V.