Count the classification, important parameters and related introduction of audio amplifiers (I)

A portable multimedia player or other fashionable one with beautiful melodies will attract admiring eyes wherever you go, especially in today's world where consumers have higher and higher requirements for sound effects. Good sound effect design means the success of the product. The popular iPod and iPhone are the best proof of the importance of sound effects. In the process of sound effect design, amplifier equipment is crucial. Whether it is the traditional Class A or the currently popular Class D, different choices mean different characteristics of the product. But do you know how to choose the right amplifier for your design? Do you know how to make a variety of designs according to the different types of amplifiers you choose? If you still have misunderstandings about these issues, please read this special topic carefully. Senior design engineers from IDT and senior engineers from National Semiconductor, Maxim and other manufacturers will jointly reveal the R&D problems you should master on the road of audio design.

　　1. Audio Amplifier Classification

　　Traditional digital voice playback systems include two main processes: 1. The conversion of digital voice data to analog voice signals (using a high-precision digital-to-analog converter DAC); 2. The analog signal is amplified using an analog power amplifier, such as Class A, Class B, and Class AB amplifiers. Since the early 1980s, many researchers have been working on developing different types of digital amplifiers that directly amplify power from digital voice data without analog conversion. Such amplifiers are usually called digital power amplifiers or Class D amplifiers.

　　1. Class A amplifier

　　The main features of the Class A amplifier are: the operating point Q of the amplifier is set near the midpoint of the load line, and the transistor is turned on during the entire cycle of the input signal. The amplifier can work with a single tube or push-pull. Since the amplifier works within the linear range of the characteristic curve, the transient distortion and alternating distortion are small. The circuit is simple and easy to debug. However, the efficiency is low, the transistor power consumption is large, the theoretical maximum power is only 25%, and there is a large nonlinear distortion. Due to the low efficiency, the design is basically no longer used.

　　Figure 1: Class A amplifier

　　2. Class B Amplifier

　　The main characteristics of the Class B amplifier are: the static point of the amplifier is at (VCC, 0), and when there is no signal input, the output consumes almost no power. In the positive half cycle of Vi, Q1 is turned on and Q2 is turned off, and the output end is a positive half-cycle sine wave; similarly, when Vi is a negative half-wave sine wave (as shown in the dotted line part of the figure), two tubes must be used for push-pull operation. Its characteristic is high efficiency (78%), but because the amplifier has a section working in the nonlinear region, its disadvantage is large "crossover distortion". That is, when the signal is between -0.6V and 0.6V, Q1 Q2 cannot be turned on. Therefore, this type of amplifier is gradually abandoned by designers.

　　Figure 2: Class B amplifier

　　3. Class AB amplifier

　　The main characteristics of the class AB amplifier are: the conduction time of the transistor is slightly longer than the half cycle, and two tubes must be used for push-pull operation. Crossover distortion can be avoided. The alternating distortion is large, which can offset the even harmonic distortion. It has the characteristics of high efficiency and low transistor power consumption. When the signal is between -0.6V<+0.6V, in order to turn on Q1 and Q2, two bias voltages are added between the VBE of Q1 and Q2, so that when the input signal is between +-0.6V, Q1 and Q2 can also be linearly amplified. In this way, higher power efficiency can be obtained and the crossover distortion of the class B push-pull amplifier can be well improved. Theoretically, a maximum power of 78.5% can also be achieved, but in fact, the maximum power of about 70% may be affected by the output stage topology and the output stage slope. Under typical listening conditions (about 30% of full power), the efficiency of the power amplifier is about 35%.

　　Figure 3: Class AB amplifier

　　4. Class D Amplifier

　　Class D (digital audio power) amplifier is an audio power amplifier that converts input analog audio signals or PCM digital information into PWM (pulse brightness modulation) or PDM (pulse density modulation) pulse signals, and then uses PWM or PDM pulse signals to control the high-power switching device to turn on/off. It is also called a switching amplifier. It has the outstanding advantage of high efficiency. The digital audio power amplifier also looks like a one-bit power digital-to-analog converter. The amplifier consists of four parts: input signal processing circuit, switch signal forming circuit, high-power switch circuit (half-bridge and full-bridge) and low-pass filter (LC). Class D amplification or digital amplifier. It uses a very high-frequency conversion switch circuit to amplify the audio signal.

　　1. It has a very high efficiency, usually reaching more than 85%.

　　2. It is small in size and can save a lot of space compared to analog amplifier circuits.

　　3. No cracking noise connection

　　4. Low distortion, good frequency response curve. Few peripheral components, easy to design and debug.

　　As shown in Figure 4, compared with PWM signals, PDM signals do not have a fixed operating frequency. They modulate the input audio signal into a set of PDM signals with the same pulse width but different frequencies, effectively improving the EMI problem caused by PWM. Currently, there are not many products on the market.

　　Figure 4. This simplified functional block diagram shows a basic half-bridge Class D amplifier structure.

　　Figure 5. The output signal pulse width is proportional to the input signal amplitude.

　　In PWM (Pulse Width Modulation), Class A, Class B, and Class AB amplifiers are analog amplifiers, and Class D amplifiers are digital amplifiers. Class B and Class AB push-pull amplifiers have higher efficiency and lower distortion than Class A amplifiers, lower power consumption of power amplifier transistors, and better heat dissipation. However, Class B amplifiers will produce alternating distortion during the transition between the on and off states of the transistors due to poor switching characteristics or improper selection of circuit parameters. Class D amplifiers have the advantages of high efficiency, low distortion, and good frequency response curves. They have fewer peripheral components. Class AB amplifiers and Class D amplifiers are the basic circuit forms of current audio power amplifiers.