In this guide on designing class-D audio amplifier it is explained the basic operation of a class-D audio amplifier, the nature of the input signals, the op-amp comparator and its selection, the power switches and their selection and the filter.
Class-D audio amplifier
A class-D audio amplifier works by using pulse width modulation (PWM) to switch the power transistors on and off at a high frequency. This high-frequency switching creates an average output that is proportional to the input signal. The input signal is first converted into a PWM signal by a class-D amplifier driver IC or using high speed comparator op-amp. This PWM signal is then used to control the power transistors, which are typically N-channel MOSFETs, to switch on and off at a high frequency. The output of the transistors is filtered by an LC filter to remove the high-frequency switching noise, leaving only the amplified audio signal.
Circuit Diagram of Class-D audio amplifier
The following shows a simple Class-D audio amplifier.
The basic operation of class-D amplifier is as follows. A class-D power amplifier works by modulating the input audio signal with a high frequency carrier signal which is often a triangle wave. The mixing is done with the help of operational amplifier that outputs audio PWM(Pulse Width Modulation) signal. Here the op-amp is used as differential comparator. The audio information is contained in the PWM signal and only required power is encoded in the audio PWM. This PWM signal is then fed into complementary MOSFET switches in push pull configuration. During the positive half cycle of the PWM signal only the
The switching of the transistors creates a "dead time" which is essential to prevent shoot-through current that can damage the transistors.
The high-frequency switching allows for a high efficiency power amplification, typically around 90% which results in less heat dissipation and less power consumption. Class-D audio amplifiers are widely used in portable audio devices, car audio systems, and other applications where efficiency and size are important.
PWM signal of Class-D audio amplifier
In a class-D audio amplifier, the PWM signal is produced by a class-D amplifier driver IC. The driver IC takes the input audio signal and converts it into a high-frequency PWM signal. The PWM signal is typically a square wave with a frequency in the range of several hundred kilohertz to several megahertz. The duty cycle of the PWM signal is proportional to the amplitude of the input audio signal, with a duty cycle of 50% representing a zero amplitude signal, and a duty cycle of 100% or 0% representing a maximum amplitude signal.
The driver IC typically uses a comparator circuit to compare the input audio signal with a reference voltage, typically a triangular waveform. The comparator output is the PWM signal, which is then used to drive the power transistors. The driver IC also includes a dead-time generator that ensures that the power transistors are never turned on and off simultaneously to prevent shoot-through current.
The PWM signal is also filtered by an LC filter to remove the high-frequency switching noise, leaving only the amplified audio signal. This signal is then fed to the speakers.
Class-D Audio Amplifier Power
Class-D amplifier can be classified as low power and high power class-D power amplifier. Class-D low power amplifier are those which outputs power from few mW to 5W. These low power class-D amplifier are used in low power and portable circuits like mobile phones, computer and laptop audio amplification, ipod and similar devices. Such circuits are often powered by single DC power supply voltage and used mainly in battery operated devices. Such low power class-D amplifier uses H-bridge at the output. The high power class-D amplifier are those which outputs power from 80W to 1500W and are used mainly for car audio system, home theater systems with big subwoofers for professional music performance area. The main supply is used to power such amplifier and heat sinks are required to keep the power switching transistor from overheating.
Comparator
The minimum slew rate of an op-amp comparator is dependent on the signal frequency and the voltage of the input signal. The general rule of thumb is that the minimum slew rate should be at least 10 times the signal frequency.
In this case, if the signal frequency is 300kHz, then the minimum slew rate of the op-amp comparator should be at least 3MHz. This is to ensure that the comparator can accurately switch between the high and low states of the PWM signal in response to the input audio signal.
It's important to note that the minimum slew rate of the op-amp comparator is not only dependent on the frequency of the signal but also on the amplitude of the signal. If the amplitude of the signal is high, the comparator needs a higher slew rate to be able to respond to changes in the signal quickly.
It's also important to verify that the op-amp comparator used has a high enough frequency response to handle the PWM signal frequency, otherwise, it might not be able to provide accurate results.
Output Switching Stage
In class-D amplifier, two or four transistors which could be either BJT or MOSFET are used as power switches and they forms the output switching stage. When two transistors like in the above circuit diagram are used then it is called single ended output switching stage and if four transistors are used then it is called differential ended output switching stage. The differential ended output switching stage topology delivers twice as much power to the load(speaker) as the single ended output switching stage. The signal input to the output switching stage is high frequency modulated PWM signal and the output is series of high frequency pulses.
Although bipolar junction transistor(BJT) can be used it is advantages to use MOSFET(Metal Oxide Semiconductor Field Effect Transistor). The advantage of MOSFET over BJT in Class-D amplifier is that MOSFET are completely cutoff, no drain current flows when it is switched off and have low drain to source voltage when switched on, thereby producing comparatively less power dissipation than class-A amplifier, class-B amplifier or class-AB amplifier. This is reason for high power efficiency of class-D amplifier. Therefore choosing a transistor is crucial in the design of class D amplifier.
The transistor of choice for good class-D amplifier are MOSFET with low Rds(on) values because with such transistor the drain current is high while the drain to source voltage is low when the transistor is in ON state. In such case, high power is delivered to the load.
Examples of MOSFETs for Class-D power amplifiers are as follows.
- IRF540N, IRF9540, IRF3205, IRF4905, IRFP240, IRFP9240, IRFP250, IRFP4468, IRFP450, IRFP3607
Low Pass Filter
A low pass filter(LPF) is inserted between the output switching stage and the loudspeaker. It's function is to lower electromagnetic interference(EMI) and high frequency harmonics from the high frequency pulses coming into the filter. The filter cutoff frequency is typical 40KHz which is twice the highest audio frequency which is usually 20KHz(or 22KHz). Usually second order Butterworth low pass filter is used to minimize the cost and save board space. Toroidal inductor with core current rating higher than the expected current flow in the transistor switches should be selected.
To calculate and design 2nd order LPF design you can use the Second Order LC Low Pass Filter Calculator and similarly to calculate and design a toroidal inductor you can use the Toroid Inductor Online Calculator.
