Machines or other stylish portable multimedia players with beautiful melodies can attract envious eyes wherever they go, especially when consumers are demanding more and more sound effects. Good sound design means product success. Whether or not, the iPod and iPhone are the best proof of the importance of sound effects. In the sound design process, the amplifier equipment is crucial, whether it is the traditional A class or the current hot D class, the 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 design a variety of different types depending on the type of amplifier selected? If you still have misunderstandings in these issues, please read this topic carefully. Senior design engineers from IDT and senior engineers from National Semiconductor and American Trust will jointly reveal the R&D problems you should master on the audio design road.
First, the audio amplifier classification
The traditional digital voice playback system consists of two main processes: 1. Digital voice data to analog voice signal conversion (using high-precision digital-to-analog converter DAC); 2 analog power amplifier for analog signal amplification, such as Class A, B Class and class AB amplifiers. Since the early 1980s, many researchers have worked on the development of different types of digital amplifiers that perform power amplification directly from digital speech data without the need for analog conversion. Such amplifiers are often referred to as digital power amplifiers or class D amplifiers.
1, class A amplifier
The main feature of the Class A amplifier is that 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 period of the input signal. The amplifier can be operated in a single tube or push-pull operation. Since the amplifier operates within the linear range of the characteristic curve, transient distortion and alternating distortion are small. The circuit is simple and easy to debug. However, the efficiency is low, the power consumption of the transistor 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 not used again.
Figure 1: Class A amplifier
2, class B amplifier
The main feature of the Class B amplifier is that the quiescent 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 turns on Q2 and the output is positive half-cycle sine wave. Similarly, when Vi is a negative half-wave sine wave (as shown in the dotted line), it must be push-pull with two tubes. It is characterized by high efficiency (78%), but because the amplifier has a period of operation in a nonlinear region, its disadvantage is that the "crossover distortion" is large. That is, when the signal is between -0.6V and 0.6V, Q1 and Q2 cannot be turned on. Therefore, such amplifiers are gradually being abandoned by designers.
Figure 2: Class B amplifier
3, class AB amplifier
The main feature of the class AB amplifier is that the on-time of the transistor is slightly longer than the half-cycle, and two-tube push-pull operation must be used. Crossover distortion can be avoided. Alternating distortion is large and can cancel even harmonic distortion. High efficiency and low power consumption of transistors. When the signal is between -0.6V and +0.6V, in order to make Q1 and Q2 turn on, add two bias voltages between V1 of Q1 and Q2, so that the input signal is between + - 0.6V. , Q1, Q2 can also be linearly amplified. This can achieve higher power efficiency and improve the crossover distortion of the Class B push-pull amplifier. In theory, the 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 diagonal line, under typical listening conditions (about 30% of full power) ), the efficiency of the amplifier is about 35%.
Figure 3: Class AB amplifier
4, class D amplifier
Class D (Digital Audio Power) amplifier is a pulse signal that converts input analog audio signal or PCM digital information into PWM (pulse brightness modulation) or PDM (pulse density modulation), and then uses PWM or PDM pulse signal to control large Power switching devices turn on/off audio power amplifiers, also known as switching amplifiers. It has outstanding advantages with high efficiency. The digital audio power amplifier also appears to be a one-bit power digital-to-analog converter. The amplifier consists of four parts: input signal processing circuit, switching signal forming circuit, high-power switching circuit (half-bridge and full-bridge) and low-pass filter (LC). Class D amplification or digital amplifier. A very high frequency transfer switch circuit is used to amplify the audio signal.
1. It has high efficiency and can usually reach more than 85%.
2. Small size, can save a lot of space than the analog amplifier circuit.
3. No crack noise is connected
4. Low distortion, good frequency response curve. Less peripheral components, easy to design and debug.
As shown in FIG. 4, the PDM signal has no fixed operating frequency compared with the PWM signal, and the input audio signal is modulated into a group of PDM signals with the same pulse width but different frequencies, which effectively improves the EMI problem caused by the PWM. There are not many products on the market today.
Figure IV. This simplified functional block diagram shows the structure of a basic half-bridge Class D amplifier.
Figure V. The output signal pulse width is proportional to the input signal amplitude.
Class A, Class B, and Class AB amplifiers in PWM (Pulse Width ModulaTIon) are analog amplifiers, and Class D amplifiers are digital amplifiers. Class B and Class AB push-pull amplifiers are more efficient and less distorted than Class A amplifiers. Power amplifier transistors consume less power and heat dissipation, but Class B amplifiers have poor switching characteristics during transistor turn-on and turn-off transitions. Or alternate distortion due to improper selection of circuit parameters. Class D amplifiers have high efficiency and low distortion, and the frequency response curve is good. There are few advantages of peripheral components. Class AB amplifiers and Class D amplifiers are the basic circuit forms of current audio power amplifiers.
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