EE 2212

POWER AMPLIFIER RELATED PROBLEMS

FINAL EXAM PREPARATION

S. G. Burns

I strongly recommend that you review text pages 1006 through 1012 as well as our class discussions related to power amplifiers prior to the Final Exam. In addition to some text problems, I have provided an amalgamation of sample problems from old quizzes and final exams that include power amplifier topics. I do like to use real circuits as the basis for problems.

The problems will not be collected but I strongly suggest you bring copies of your solutions to the Final Exam

1. Text 15.102. Also sketch and label the back-to-back load lines.

2. From an old quiz.

Assume a Class B audio amplifier operating between ±48 volts and driving a 4 W speaker. Neglect the effect of crossover distortion.

(a)Provide required maximum ratings as indicated for the transistors using a well-labeled sketch of the back-to-back load lines to illustrate your answers.

I_C(max) = ____________ V_CE(max) = ___________ P_C(max) = __________

(b) Design a protection circuit to limit the transistor to the I_C(max) you computed in part (a) should the amplifier output be short-circuited. Incorporate this protection circuit in your basic Class B circuit diagram.

3. Amplifier Analysis

A partial circuit diagram for a Fujitsu MB 47358 operational amplifier is given below. Assume V⁺ = 15 volts and V^- = -15 volts. For the npn transistors, assume b = 150, V_BE(on) = 0.7 Volts, and V_AN = 100 Volts. For the pnp transistors, assume b = 100, V_BE(on) = 0.55 Volts, and V_AP = 50 Volts.

(a) Briefly describe the circuit function/operation of :

Q₁₀ and Q₁₁

Q₁₃ and Q₁₄ (Diode connected transistors)

Q₇ Circuit topology

Q₅ and Q₆

(b) Assume a sinusoidal input voltage that results in an output voltage v_o(t) = 12 sin (wt) volts. R_L = 1 kW.

Compute values for :

1. Peak and average power to the load resistor.

2. Resultant collector efficiency.

3. Power required from the power supplies for the output stage.

4. A design value for R_SC to limit the short circuited load current to 30 mA .

(c) Estimate the following :

1. Collector currents in Q₅ and Q₆

_2. Collector currents in Q₁ and Q₂

_3. Voltage at the base of Q₅ and Q₆

4. Voltage at the emitter of Q₇

4. Power Amplifiers

I had a problem similar to this on a past final exam. Who know? Something like this could be on the Final in a couple of weeks! Look at Review Text 15.107. Follow the same approach we used for a sinusoidal signal and look at Text 15.108. Think Class D to provide a one-line answer. Note the similarity.

(a) In class, for a Class B (AB) amplifier, we derived the collector efficiency for a sinusoidal signal with a maximum value of 78.5%. Now consider a periodic triangular signal shown in Figure 1. Derive the maximum collector efficiency, h_C, for this waveform. I suggest using symmetry to minimize the algebraic effort.

(b) By inspection, (no calculations are necessary) provide a value for the approximate collector efficiency, h_C, for the Figure 2 waveform. Explain your answer.