Audio Power Amplifier Classification and Application in Mobile Phone Design

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A beautiful mobile phone with a pleasant ringtone can attract countless envious eyes wherever you go. The sound quality of mobile phone sound has a great impact on the success of mobile phone design, and the power amplifier plays a decisive role in the quality of the sound reproduction. Let's do a brief analysis of the application of an audio amplifier in a mobile phone.

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.

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.

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 at -0.6V

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 of high efficiency. The digital audio power amplifier also appears to be a one-bit power digital-to-analog converter. The amplifier consists of input signal processing circuit, switching signal forming circuit, and high-power switching circuit (half-bridge and full-bridge). And a low-pass filter (LC) and other four parts. 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.


PWM (Pulse Width Modulation) / Medium 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 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.

Second, the important parameters of the audio amplifier

1. Power Supply Ripple Rejection Ratio (PSRR) The power supply rejection ratio is the ratio of the input of the audio amplifier's input measured supply voltage to the output signal of an analog circuit. The PSRR reflects the ripple requirements of the audio power amplifier for the power supply. The larger the PSRR value, the better the audio amplifier output quality. Performance comparison">

2. Total Harmonic Distortion plus Noise (THD+N) Total harmonic distortion is the total amount of distortion introduced in a particular frequency range after an analog circuit processes the signal. Noise is a signal that is usually not needed. Sometimes it is due to other electrical behavior (interference) on the board due to heat or other physical conditions. It is not difficult to see from the definition of THD+N that the total harmonic distortion and noise are as small as possible.

3. Signal-to-Noise Ratio (SNR) Generally refers to the ratio between the wanted signal and the noise in an analog signal.

4. Gain (AO) For an audio power amplifier, the gain is usually the ratio between the amplifier's output power and the input power. The higher the gain, the higher the efficiency of the amplifier.

5. The maximum output power (POCM) output power reflects the load capacity of an audio power amplifier. Usually, the audio amplifier manufacturer will provide the maximum output power of the product under certain operating voltage conditions and rated load.

6. Shutdown current and Output Offset Voltage.

The smaller the shutdown current, the lower the power consumption of the amplifier under standby conditions. The small output offset voltage is beneficial to the extension of battery life.

Third, mobile phone commonly used audio amplifier introduction

Brief introduction to the current mobile phone design, there are Class AB amplifiers and Class D. The main manufacturers are National Semiconductor Corporation (NS), Texas Instruments, STMicroelectronics, and ONSEMI. ). Their representative products and their performance comparison are shown in Table 1 and Table 2. NCP2890 and NCP2809 are products of American company, LM4890 and LM4911 are products of National Semiconductor, TS4890 is product of STMicroelectronics, TPA6203A1 is the product of Texas Instruments, and MAX4410 is the product of Maxim.

Let's take the ONSEMI product as an example to introduce the application of audio power amplifier in mobile phones. The circuit diagram is shown in Figure 5 and 6.

The NCP2890 is capable of delivering up to 1W of maximum output power to an 8 ohm load at an operating voltage of 5V and 320 mW of output power to a 4 ohm load at an operating voltage of 2.6V. As shown in Figure 5, the NCP2890 can provide high-quality sound quality. The chip's own logic-off design mode makes the device around the chip less in the circuit design and consumes less power. A noise cancellation circuit has been specially designed inside the NCP2890 to eliminate the noise that the human ear can hear when the power amplifier is turned on and off. When turned on, a logic high level is applied to the switch control terminal, and the DC voltage value on the bypass capacitor begins to increase exponentially. When the voltage value reaches the common mode voltage value (Vp/2), the output power starts (this process is about 50 ms). When the power is off, the control terminal is connected to the low level, the load is connected to the ground, and the output power is zero. At this time, the DC quiescent current of the circuit is less than 100nA. Although the NCP2890 contains overcurrent and overheat protection circuits internally, it must be noted that the power supply voltage must not exceed its limit value to avoid chip damage.

The NCP4894 is a fully differential audio power amplifier designed by ONSEMI for mobile devices. The NCP4894 is an excellent audio power amplifier that delivers high-quality sound, especially in mobile phone designs. The total harmonic distortion plus noise (THD + N) ratio of the NCP4894 is less than 0.01%. At a working voltage of 5V and a load of 8 ohms, its output maximum power is 1W. At a working voltage of 2.6V, the output power is 250mW.

The NCP2890 and NCP4894 and NCP4896 can be used in mobile phone designs, and their small package form can improve PCB utilization.

Fourth, some suggestions for the selection of audio amplifiers in mobile phone design

  1. An audio power amplifier with fully differential input and output is used to increase the power supply rejection ratio.

  2. Try to use high efficiency, low power consumption, and low internal temperature rise, which can extend the battery and chip life. Class D amplifiers are recommended for limited bandwidth designs.

  3. When using the audio amplifier, be sure to pay attention to the power supply voltage should not exceed its limit value, so as not to cause chip damage.

V. Audio amplifier product outlook

Consumers are now pursuing a small form factor, low power consumption, low price and comfortable listening. Suppliers are also increasingly recognizing that improvements must be made in these areas in order for consumers to be willing to pay for these audio devices. At the same time, they also strive to reduce noise interference. However, the simultaneous presence of digital and analog circuits makes this task even more difficult.

Class D amplifiers are setting off storms in the audio world with advantages such as excellent power efficiency, lower heat, and lighter power supplies. As technology matures and more and more sound reproduces, D Class amplifiers will dominate the audio amplifier market.

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