Audio Amplifiers – Applications

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The primary function of an Audio Amplifier is to receive a smaller audio signal and make it larger, without creating changes or distortions in the signal shape. It is the basic circuit configuration that is essential to amplify the audio signal, which is received from a transducer, low-level signal processing circuit, or recorded sound storage device such as a microphone, mixer, equalizer, or MP3 player and creates a precisely shaped image of the original electrical signal having significantly larger voltage and/or current and power which can be applied to a speaker, actuator, or wireless transmitter, etc.

Basic Design & Operating Principles

Historically audio power amplifiers were circuits in which the power transistors operated in the linear mode wherein the output transistors operated with high applied voltages simultaneously with high currents. These produced very precise power audio signals but they also produced a large amount of heat. Most of these were very inefficient and produced more heat energy than audio energy. They were also large, hot, and expensive.  Modern audio power amplifiers are typically switched-mode power circuits, called class D amplifiers, that are a sub-class of power inverters or DC to AC conversion circuits. Other common types of inverters, DC to AC conversion circuits, are grid-connected inverters for renewable energy sources such as photovoltaic panels which convert DC power from the photovoltaic panels to AC for the electrical power grid. The grid-connected inverters have as an output a single relatively low-frequency signal.

Class D audio power amplifiers operate from a DC power source but are controlled by an AC audio signal which requires the output to operate over a range of frequencies. Many, if not most, modern audio power amplifiers operate over the full audio range from about 20 Hz to about 20KHz. Class D amplifiers are switching amplifiers that operate at switching frequencies an order of magnitude or higher than the highest audio frequencies. At the output of the class D amplifier, there is a filter that largely eliminates the switching frequency but passes the audio frequency signal unattenuated. The small amount of high-frequency signal that remains is called the ripple frequency, which frequency is far enough beyond the audio range and small enough in magnitude to be inaudible to the human ear. In Class D amplifiers the switches are either fully on, in an on state in which the current through the switch may be large but the voltage across the switch is very near zero so that the power dissipated in the switch, which is the product of the current flowing through the switch and the voltage applied across the switch, is very near zero, or the switch operates in an off state in which the current in the switch is zero so that there is no power lost in the switch during the off state. The switches in class D amplifiers only operate in the condition having simultaneous high current and high applied voltage during the very brief time intervals called switching transitions in which the switches are transitioning from the on state to the off state or vice versa. In a class D amplifier, the fraction of the time that the switches have both a significant amount of current and a significant amount of voltage is small, so the efficiency can be on the order of 90% or higher. Switching power losses are proportional to switching frequency so the efficiency drops as the switching frequency is increased. Higher switching frequency also yields higher performance, i.e., higher precision and less ripple. With switching frequency, there is a trade-off between high performance and high efficiency.

New Audio Power Amplifier Technology

Power Electronics Innovations Laboratory has designed class D audio amplifiers for both actuators and sound reinforcement. We are innovation leaders in the area of reduction and/or elimination of switching power losses, having invented more circuits for reducing and eliminating switching power losses than anyone in the power electronics industry. We are now developing a class D amplifier that dramatically reduces switching power losses operating efficiently with ripple frequencies in the megahertz range, i.e., more than two orders of magnitude higher than the highest audio frequencies. We expect that this new circuit will achieve efficiency greater than 99% operating at full power with higher power density than any currently known technology. We now have recent patents that form a part of the new class D technology. Our ongoing work will result in a new invention and patent primarily intended for audio amplifier applications.

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