Comparison of advantages and disadvantages of Class A, Class B, Class AB and Class D power amplifier ICs

    However, the connection between amplification technology and sound quality cannot be completely based on sound quality. From the perspective of traditional ClassA/AB amplification technology, Class A amplification has the best sound quality. Its principle is purely linear amplification with analog signals, and the sound quality is not distorted.

    However, although Class A amplifier has the best signal authenticity, it consumes quite a lot of power. Generally speaking, the power utilization rate is only 20%~30%. For example, when 100W power is supplied to Class A amplifier, only about 25W is actually output to the speaker‘s sound power, and the rest 75W is consumed. In addition to poor power efficiency, high energy consumption also represents high waste heat, which often needs to be equipped with large heat sink on the amplifier transistor to assist heat dissipation. For handheld devices, high power consumption and high heat are the original sin of design, and developers should avoid it.

    Another class B amplifier design, although it has good power saving, but the signal distortion has a great impact on the sound quality. Therefore, the derivative class AB amplifier with both advantages has achieved an acceptable balance between sound quality and power saving, and has become the most popular amplification method in the field of consumer audio in the past.

    However, at present, in the digital playback device, more and more attention is paid to reducing the overall circuit configuration space, and reducing power consumption to make the product more power-saving to maintain battery life and other development trends, Class D Audio Power Amplifier, proposed in 1958, has become a popular fried chicken.

    The principle of Class D amplification is PWM (Pulse Width Modulation), which is similar to the concept of switching power supply on the motherboard. That is, it uses the density of digital frequency wave pattern to output the analog amplitude. The frequency density means the amplitude is high, while the frequency density means the amplitude is low. Therefore, in operation mode, Class D amplifier is generally referred to as digital power amplifier or digital power amplifier.

    Class D amplifier omits the traditional class AB crystal amplifier. The large transformer and super-large filter capacitor required for making high-power models can be replaced by small capacitors and small transformers similar to the switching power supply mode, so that the electric energy can drive the horn unit quickly and directly. This operation mode provides extremely high power utilization rate, which is 100% in theory, and in practice, the power utilization rate is often 80% 90% level.

    It can be seen from the above driving mode that the Class D amplifier circuit is not easy to generate high heat during operation, so it can eliminate the heat sink that occupies space. In addition, due to the PWM mode, the volume required by the Class D amplifier circuit is much smaller than the traditional power amplifier circuit, and the cost is also lower. In addition to the advantage of low heat consumption, the Class D amplifier circuit is very suitable for plugging in a small space.

    However, from the perspective of sound quality, the disadvantage of Class D amplification is that the amplification formed by the modulation procedure must have some differences with the original signal. However, in the music playback of general consumer products, its quality can be accepted. Even under the current technology, the sound quality of Class D amplification is getting better and better, especially under the condition of low wattage.

    For example, with the IRAUDAMP5 design scheme using the Class D amplifier IRS2092S D, compared with the Class AB amplifier input of the same level, the THD+N of Class D amplifier is actually lower than that of Class AB amplifier in the area below 80W at 1KHz and connected with 4 Ω load, while the THD+N of Class D amplifier has improved to 0.005% near 60W.

    Only in the region of 80W to 140W, the performance of Class AB amplification is slightly higher than that of Class D amplification, but even to 120W, the THD of Class D amplification is still less than 1%. For most digital audio devices, high power is rarely used, and low power is common. At this time, Class D amplifier has the advantage of sound quality.

    The power supply rejection rate (PSRR) of Class D amplification must be as high as possible in dB. In terms of sound quality, it is the total harmonic distortion plus noise ratio (THD+N) requirement, which is expressed in percentage (%). THD+N is as low as possible, generally not more than 10%, and lower than 0.1% and 0.01% in terms of high standard requirements. However, THD+N is recommended to be compared with load impedance RI, output power, PWM modulation frequency (f), etc., so as to accurately judge.

    In addition, Class D amplification has unique electronic characteristics, so attention must be paid to EMI electromagnetic interference when evaluating the sound quality. Because Class D amplification IC will continuously and frequently conduct the conduction and closing of the transistor, it is easy to generate electromagnetic interference, which will certainly have a negative impact on the sound quality. Therefore, the products that can reduce the emission of electromagnetic interference are better. However, if the space and cost permit, the metal shell can be used to strengthen the shielding.

    In addition, many Class D amplification ICs have their own characteristics in terms of power saving, controllability, volume reduction, flexibility and protection. Most of them have little to do with the sound quality. However, if some designs are too excessive, they will have an impact on the sound quality.

    For example, some Class D amplification ICs reduce the quality of LC low-pass filter circuit in the rear section, such as using smaller capacitance, or simply omitting capacitance (Cap-Free), and even some designs have the same external inductance? Omit (LC-Free, Filter-Free) and directly use the leakage sense of the voice coil of the speaker as LC. Of course, this Class D amplifier IC can reach a relatively small volume, because its output pin can be directly connected to the speaker and headset.

    Although this design method can reduce the volume to a minimum, the high-frequency part can be directly output without filtering. It is not easy for speakers to emit frequencies above 20KHz. Even if the speaker emits frequencies above 20KHz that cannot be heard by the human ear, but if it is too small, it may also have a negative impact on the sound quality within the audible range. The providers of Class D amplification IC include ADI, Cirrus Logic, MAXIM, Motorola, NS, Philips, Sanyo, ST, TI, TriPath, etc.