AN1042

AN1042/D High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs Prepared by: Donald E. Pauly ON Semiconductor Special Consultant http://onsemi.com APPLICATION NOTE With the introduction of complementary bipolar power transistors in the late 1960s, switching amplifiers became theoretically practical. At low frequencies, bipolar transistors have switching efficiencies of 99% and will directly drive a low impedance speaker filter. The requirement for switching frequencies above 100 kHz resulted in excessive losses however. Bipolar drive circuitry was also complex because of its large base current requirement. With the advent of complementary (voltage/current ratings) TMOS power MOSFETs, gate drive circuitry has been simplified. These MOS devices are very efficient as switches and they can operate at higher frequencies. A block diagram of the amplifier is shown in Figure 2. An output switch connects either +44 or –44 volts to the input of the low pass filter. This switch operates at a carrier frequency of 120 kHz. Its duty cycle can vary from 5% to 95% which allows the speaker voltage to reach 90% of either the positive or negative supplies. The filter has a response in the audio frequency range that is as flat as possible, with high attenuation of the carrier frequency and its harmonics. A 0.05 ohm current sense resistor (R27) is used in the ground return of the filter and speaker to provide short circuit protection. The negative feedback loop is closed before the fi...