ENGR325 Electronics II

Lab #7 Class B Amplifier Example




                      To see the operation of a class-B amplifier.

                      To compare our experimental response with that expected from theory.

                      To gain experience with noise reduction techniques.




1. In problem D8.39 we are to design a class-B amplifier. I suggest that you use the circuit below. We will use the BS170 for the NMOS transistors (Q1 and Q2) and the BS250 for the PMOS transistors (Q3 and Q4). The simulated circuit shown in Figure 1 below is using different PMOS transistors since the BS250 is not available in Multisim. However, the results of the simulation should give you some idea of the dc currents and voltages that you should expect when you build the circuit.

Build the circuit. Measure the dc currents and voltages as indicated on the schematic. Discuss how they compare with the simulation. Input a 1kHz sine wave and measure the gain of the circuit. Also, note the peak positive and negative excursions at the output. What is causing these limits? Report on the results from these investigations.

2. You may well see that the output is not symmetrical around ground because of a dc offset in the output. Consider how you might balance the circuit to eliminate this offset. Try your ideas and report on your results.


Note: If you experience noise on your waveforms, I suggest that you do three things:

                      Keep your wires short.


                      Insert a 100uF and a 0.1uF capacitor from the +10V supply to ground and from the -10V supply to ground.

                      Try running a separate ground wire from the 100 ohm load back to the power supply so that the signal ground and the high current output ground are not being shared.