Audio power amplifier classification and application in mobile phone design

A beautiful phone, coupled with sweet tone, no matter where the eye can lead to many envy. Phone voice quality is good or bad on the success of mobile phone design has a significant impact, while the reduction of power amplifier on the sound quality, has a pivotal role. Here on the audio amplifier (Audio power amplifier) in the phone application to do some simple analysis.

1, audio amplifier Categories

Traditional digital voice playback system consists of two main processes: 1, digital voice data to analog voice signal transformation (using high-precision analog converter DAC) to achieve; 2 analog power amplifier using an analog signal amplification, such as A, B Class and Class AB amplifiers. From the early 1980s, many researchers have focused on the development of different types of digital amplifier, this Fangda Qi 直接 from the digital voice data to achieve power amplification without the need for analog conversion, this amplifier is usually referred to as Class D digital power amplifier or amplifier.

1, A Class Amplifier

A class amplifier main features: the amplifier operating point Q is set near the midpoint of the load line, the transistor in the input signal conduction in both the entire period. Single-tube amplifier can work, you can also push-pull work. As the amplifier operates in the linear range of characteristic curves, so small distortion of transient distortion and alternate. Circuit is simple, convenient debugging. However, low efficiency, large power transistors, power is only 25% of the theoretical maximum, and have more non-linear distortion. As the relatively low efficiency of the design is basically now not re-use.

Audio power amplifier classification and application in mobile phone design

2, B Class Amplifier

Class B amplifier main features: a static point amplifier (VCC, 0) office, when no signal input, output almost no power consumption. In the positive half cycle of Vi, Q1 turns on Q2 closed half cycle sine wave output correct; Similarly, when Vi is negative half-wave sine wave (dotted lines shown in Figure), it must work with two push-pull. It features high efficiency (78%), but because there is a work in the non-linear amplifier area, so the disadvantage is that "cross talk" more. That when the signal is between-0.6V ~ 0.6V, Q1 Q2 can not turn caused. This kind of amplifier designers have gradually been abandoned.

3, AB class amplifiers

Class AB amplifier main features: the transistor on-time slightly larger than a half cycle, to work with two push-pull. To avoid crossover distortion. Alternating large distortion can be offset by the even harmonic distortion. Efficient high-power transistors smaller features. When the signal at-0.6V

Audio power amplifier classification and application in mobile phone design

4, D class amplifiers

D (digital audio power) amplifier is an input analog audio signal or PCM digital information converted into PWM (pulse intensity modulation) or PDM (pulse density modulation) of the pulse signal, then the pulse PWM or PDM signal to control a large power switch devices on / off audio power amplifiers, also known as switching amplifiers. The apparent advantages of high efficiency. Digital audio power amplifier is also looking into a bit of power for a digital-analog converter. Amplifier by the input signal processing circuit, the switch signal forming circuit, high power switching circuit (half bridge and full bridge ) and low-pass filter (LC) and other four components. D class amplifier amplification or digital. System using high frequency switch circuit to amplify audio signals.

1. With high efficiency, usually above 85%.

2. Small, you can save more than analog amplifier circuit a lot of space.

3. Without cracking noise connected

4. Low distortion, good frequency response curve. Fewer external components, ease of design and debugging.

As shown in Figure 4, PDM signal compared with the PWM 信號, there is no fixed operating frequency, 其 the input audio signal modulation into a group of the same pulse Kuandu Danshipinshuai different PDM signal, an improvement of the PWM Dailai of EMI 問題. Products currently on the market is not a lot.

Audio power amplifier classification and application in mobile phone design

PWM (Pulse Width Modulation) / in the A Class, B Class and the Class AB amplifier is an analog amplifier, D class amplifier digital amplifier. Class B and Class AB push-pull amplifier than the Class A amplifier, high efficiency, low distortion, power amplifier transistors smaller, heat dissipation is good, but the Class B amplifier transistor and the cut-off state of the conversion process due to their poor switching characteristics or improper choice of circuit parameters produce alternating distortion. The Class D amplifier with high and low efficiency of distortion, good frequency response curve. Few advantages of external components. Class D amplifier and Class AB amplifier audio power amplifier is the basic circuit form.

Second, the important parameters of the audio amplifier

1. Power supply ripple rejection ratio (PSRR) PSRR (power supply rejection rate) is the measurement of the audio amplifier input bias supply voltage circuit coupled to an analog output signal ratio. Audio Power Amplifier PSRR reflects the ripple on the power requirements, PSRR value the bigger the better, the audio amplifier output quality, the better. Audio power amplifier classification and in the performance of mobile phone design more ">

2. The total harmonic distortion plus noise (THD + N) Total harmonic distortion (total harmonic distortion) is an analog signal processing circuit, in a specific frequency range, the total distortion introduced by volume. Noise (noise) is usually not required signal. Sometimes, because due to thermal or other physical conditions in the circuit board produced by other electrical behavior (interference). THD + N from the definition of total harmonic distortion and is easy to see the noise as small as possible.

3. Signal to noise ratio (SNR) generally refers to an analog signal between the desired signal and noise ratio.

4. Gain (AO) for the gain of the audio power amplifier output power amplifier and usually refers to the ratio between input power. Explain the greater efficiency of the amplifier gain higher.

5. Maximum output power (POCM) output power reflects the ability to load an audio power amplifier, audio amplifier manufacturers will usually provide a certain product in the operating voltage under rated load conditions and maximum output power. Audio power amplifier classification and application in mobile phone design

6. Shutdown Current (Shutdown current) and output offset voltage (Output Offset Voltage).

Shutdown current is smaller, the amplifier in the standby power consumption under the conditions. Output offset voltage of a small benefit to extend battery life.

Third, mobile audio amplifiers commonly used description

Briefly present the design of mobile audio amplifiers are Class AB Class D amplifiers are also major producers are the United States National Semiconductor (NS), Texas Instruments (TI), STMicroelectronics (ST), the United States Semiconductor Corporation (ONSEMI ). Their typical products and their performance comparison as shown in Table 1 and Table 2. NCP2890 and NCP2809 which the United States Semiconductor Corporation products, LM4890 and LM4911 for the National Semiconductor Products, TS4890 to STMicroelectronics products, TPA6203A1 for the Texas Instruments products, MAX4410 is Maxim Products (MAXIM). Audio power amplifier classification and application in mobile phone design

The following example to introduce our products ONSEMI audio power amplifier in the phone application, circuit diagram shown in Figure 5,6.

NCP2890 voltage 5V in 8-ohm load when the able to continue to provide 1W maximum output power, operating voltage 2.6V at 4 ohm load when able to provide 320 mW of output power. Figure 5 NCP2890 provides high quality sound chip with the logic of its own design mode off, the circuit design of a small chip around the device and its power consumption. The NCP2890 is designed within the noise cancellation circuitry, eliminating the power amplifier is turned on and off the course will have the human ear can hear the noise. Boot time, add the switch control logic high side, bypass capacitors on the DC voltage value began to increase exponentially when the voltage value to the common mode voltage (Vp / 2), the starting output power (this process takes 50ms ); the shutdown, control termination low, the load is connected to ground terminal, the output power is zero, then the circuit of the DC quiescent current less than 100nA. Although the NCP2890 includes over-current and overheat the internal protection circuits, but when used, must pay attention to the power supply voltage can not exceed its limit, so as to avoid chip damage.

Audio power amplifier classification and application in mobile phone design

NCP4894 is ONSEMI company designed specifically for mobile devices, a fully differential audio power amplifier. NCP4894 as a good audio amplifier to provide high-quality sound, especially in mobile phone design with the advantage of particularly prominent. NCP4894 total harmonic distortion plus noise (THD + N) ratio is less than 0.01%. Working voltage 5V, 8-ohm load when its maximum power output of 1W, the operating voltage 2.6V, the output power of 250mW.

NCP2890 and NCP4894 and NCP4896, are available in the mobile phone design, the small packages they can improve the utilization of PCB.

Fourth, the design of audio amplifiers use mobile phones Suggestions

The fully differential input and output audio power amplifier, to improve power supply rejection ratio.

Maximize the use of high efficiency, low power consumption, internal temperature of small design, it can prolong the life of the battery and chip. The limited bandwidth of the design, it is recommended to use Class D amplifier.

Audio amplifier in use must pay attention to the power supply voltage can not exceed its limit, so as to avoid chip damage.

5, audio amplifier products outlook

Consumers are now seeking a compact size, low power, low prices and listening comfort. Suppliers are increasingly recognized the need for improvements in these areas in order to make consumers willing to spend money to buy the audio device. They also strive to reduce noise. However, digital circuits and analog circuits exist, making this task more difficult.

With such excellent power efficiency, smaller and lighter power supply heat advantages, D Class amplifiers are the audio world by storm, as the technology matures and more and better voice to reproduce their results, Class D audio amplifier amplifier will dominate the market.

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