EET2222 – Electronic Devices and Circuits Lab

Lab 10 – Class B Power Amplifier

Resources: Floyd Text, Chapter 9, Section 2

Adjust a 100 Ω potentiometer to zero ohms. Set up the circuit below, using the 100 Ω potentiometer for R₂. Use a silicon power diode for D₁. Note the polarity of the 10 µF capacitors; make sure that you connect them properly.

* The 1 Ω resistors are already mounted on the heatsink with Q1 and Q2.

1. Adjust v_in to 1 kHz at about 2 V_p. Monitor the input and output voltages with the oscilloscope. Print or sketch the input and output voltages showing the phase relationship between them (you should see some crossover distortion of the output signal – see reverse for an explanation of crossover distortion). Label the printed graphs or sketches appropriately.

2. With the input signal applied, adjust R₂ to eliminate crossover distortion by looking at the output signal on the scope. Adjust just to the point where the distortion is eliminated, further increases in the value of R₂ will increase the no signal power dissipation of the transistors.

3. Increase the amplitude of v_in, and measure the maximum undistorted peak to peak output voltage you can obtain across R_L (V_LMAX). Print or sketch v_in and the voltage across R_L, showing the phase relationship between them. Label the printed graphs or sketches appropriately.

4. Take measurements to determine the output power to R_L [P_ac = (V_L²_rms)/R_L], the DC power input to the amplifier [P_dc = (V_cc+ * I₁) + (V_cc- * I₂)], the power dissipated by the transistors, and the efficiency of the amplifier for output voltages of V_out = V_LMAX, and V_out = (1/2)V_LMAX. (Hint: see page 442)

Your lab report is due at the end of the lab period. Please use the green engineering paper.