An exquisite mobile phone, mix the melodious ring tones, no matter can attract the countless sight that envy to go everywhere. The quality of the sound tone quality of the mobile phone has great influence on that the mobile phone designs succeeding or not, and reducing quality to sound of power amplifier, have very important function. Audio-frequency amplifier Audio power amplifier below Make simple analysis in application in the mobile phone.

First, audio-frequency amplifier classification

The traditional digital speech returns and includes two pieces of main course systematically: 1, The digital speech data reach the variety of the simulation pronunciation signal utilize high precision to count the converter DAC of the mould Realize; 2 use imitates the power amplifier and pays the analog signal to amplify, such as As, Bs and AB amplifier. Since the early period of 1980 times, a lot of researchers are devoted to developing different kinds of digital amplifiers, this kind of amplifier realizes from the digital speech data power amplification is without needing imitating and changing directly, such an amplifier is usually called the digital power amplifier or D amplifier.

1,A amplifier

The main characteristic of the A amplifier is: The operating point Q of the amplifier is presumed and supported near the mid point of the line, the transistor is being led within whole cycle of the input signal openly. Amplifier can single-hose sharing work, too can recommend, work. Because of work in the intersection of characteristic and linear range of curve by amplifier, so transient state distorted replacing distorted and relatively little. The circuit is simple, it is convenient to debug. But efficiency is relatively low, the transistor consumption is large, the theory maximum of the power only has 25%, and there is bigger distorted nonlinearity. Because efficiency is more low to designs and basically reusing now.

2,B amplifier

The main characteristic of the B amplifier is: Some of static behavior of the amplifier is in VCC, 0 The place, when there is no signal to inputting, the output end hardly consumes the power. Within straight half-period of Vi, Q1 leads open Q2 and ends, it is sinusoidal wave in half a week that export well; Same, as Vi for shoulder half wave sinusoidal wave ‘ ,So must use two tubes to recommend the job. It features efficiency is higher 78% ,But because the amplifier has a section of job in the non-linear area, so the shortcoming is ” Hand in the more distorted ” It is relatively big. Namely when the signal is among – 0.6V 0.6V, Q1 Q2 is unable to lead openly and cause. So this kind of amplifier is abandoned by the designer gradually.

3,AB amplifier

The main characteristic of the AB amplifier is: The leading open time of the transistor is greater than half-period slightly, must use two tubes to recommend the job. Can avoid handing in the more distorted. Replace distortedly bigger, can offset harmonic wave once evenly distortedly. Efficient and relatively high, the characteristic of the transistor with less consumption. Act as the signal yes – 0.6V

4,D amplifier

Ds the power of digital audio frequency The amplifier is that one kind will be input into simulation audio frequency signal or PCM digital information and varied into PWM pulse luminance will be modulated Or PDM pulse density is modulated Pulse signal,last high-power switch device audio frequency power amplifiers coherent / broken, also called the amplifier of the switch. Have outstanding advantage with high efficiency. Digital the intersection of audio frequency and the intersection of power and amplifier look too it is that a piece of power of one bit counts the changer of the mould. The amplifier is dealt with the circuit, switch signal forming the circuit, high-power switch circuit half a bridge types and complete bridges type by the input signal Sum is low to lead to the wave filter LC Wait for four parts to make up. Ds amplify the digital amplifier. Make use of conversion switch circuit of the extremely high frequency to amplify the audio frequency signal.

2.Small, can save very large space than the enlarged circuit of simulation.

3.There is no noise of splitting to put through

4.Low and distorted, it is good that the frequency responds to the curve. There are few peripheral components and parts, it is easy to design and debug.

,PDM signal compares with PWM signal, without fixed operating frequency, it modulates audio frequency signal input into PDM signal that a group of pulses are the same but different in frequency in width, EMI question brought in effective improvement PWM. The products are not numerous yet on the market at present.

PWMPulse Width Modulation /China As, Bs and AB amplifier is simulation amplifiers, the D amplifier is the digital amplifier. Bs and AB analogize and hold the amplifier with high efficiency, more distorted than the A amplifier and smaller, it is relatively little to put the transistor consumption the work, dispel the heat, B until transistor lead coherent and in the transfer process of state will end because their switch of characteristics to or produce and replace the distortion because the circuit parameter is chosen improperly amplifier. And Ds enlarge the efficient level of the utensil distortedly, it is good that the frequency responds to the curve. Few advantages of peripheral components and parts. AB amplifier and D amplifier are basic circuit forms of the amplifier of power of the audio frequency at present.

Second, audio frequency enlarge and think highly of wanting the parameter

1.Power the intersection of line and wave inhibit than PSRR Power the intersection of line and wave inhibit than power supply rejection rate The input that it is the audio-frequency amplifier measures the ratio of the output signal that the deviation of the power voltage couples a simulation circuit. PSRR reflects the request of line unexpected turn of to the power of the amplifier of power of the audio frequency, it is the better that PSRR value is the greater, the audio-frequency amplifier exports the tone quality well.

2.The total harmonic wave is distorted to add the noise T H D N The total harmonic wave is distorted total harmonic distortion After referring to a simulation circuit and dealing with the signal, the total distorted quantity introduced within the range of a particular frequency. Noise noise Refer to the not common signal of the need. Other electric behaviors interfere with on the circuit board that it is that arise from hot or other physical conditions that sometimes . It is easy to find out the total harmonic wave is distorted and the noise is the less, the better from the definition of T H D N.

3.SNR SNR Usually mean the ratio between useful signal and noise in a analog signal.

4.Gain AO Gain and usually point the ratio between amplifier output power and input power to amplifier of power of the audio frequency. It greatly proves the higher the efficiency of the amplifier is to gain.

5.The biggest output power POCM Output power has reflected the load ability of the amplifier of power of a piece of audio frequency, usually the intersection of audio-frequency amplifier and greatest output power that producer will offer products until working the intersection of voltage and certain terms and amount support.

6.Shut off the electric current Shutdown current With outputting the skew voltage Output Offset Voltage .

Shut off the the smaller electric current, prove the amplifier consumption under the condition of awaiting the opportune moment is little. Output the little extension which is favorable to the battery life-span of the skew voltage.

Third, daily audio-frequency amplifier introduction of mobile phone

The audio-frequency amplifier has AB amplifier and Ds in the mobile phone design to introduce at present briefly, the main manufacturer has national semi-conductive company NS of U.S.A. , Texas Instruments TI The intersection of purpose and the intersection of law and semi-conductive company , U.S.A. fit full of trees American company . Their representative products and performance comparison are shown as Table 1 and Table 2. Among them NCP2890 and NCP2809 fit the American company products full of trees for U.S.A., LM4890 and LM4911 are the national semi-conductive company products in U.S.A., TS4890 is the purpose law semi-conductive company products, TPA6203A1 is the products of Texas Instruments, MAX4410 is the American letter company products MAXIM .

Now take products of ONSEMI Company as examples and introduce the application of the amplifier of power of the audio frequency in the mobile phone, circuit diagram.

NCP2890 can offer the greatest output power of 1W for that 8 ohms are supported continuously in the working voltage 5V, can offer the output power of 320 mW for that 4 ohms support in the working voltage 2.6V. NCP2890 can offer the high-quality tone quality, the logic that chip lead shuts off and designs the mode, make in the circuit design the few and one’s own consumption of the device are little around the chip. Have specially designed the circuit of noise of disappearing within NCP2890, will produce the noise that the ears of people can hear in the course of opening and closing after dispelling the power amplifier. While starting the machine, the logic high level is added to the control end of the switch, the direct current pressing value on the bypass electric capacity begins to increase according to the index law, as voltage value reaches the mould voltage value V p / 2 altogether When, begin output power this course about 50ms ; And shut down, control end connect low level, support the intersection of earthing and end, for zero output power connected to, the static electric current of direct current of the circuit is smaller than 100nA at this moment. Though contain and flow and overheated and protect the circuit within NCP2890, while using, will certainly should notice that the power voltage can’t exceed its extreme to supply power, so as not to cause the chip to be damaged.

NCP4894 is the amplifier of power of a section of all differential audio frequency that ONSEMI Company is designed for mobile device specially. NCP4894 can offer the high-quality sound as an outstanding amplifier of power of audio frequency, especially the advantages are particularly outstanding in using in the mobile phone design. The total harmonic wave of NCP4894 is distorted to add the noise THD N Than less than 0.01%. In working the intersection of voltage and 5V, output of it 1W in maximum power when the 8 of load, in the working voltage 2.6V, output power is 250mW.

NCP2890, NCP4894 and NCP4896, can all be used in the mobile phone design, their young encapsulation form can improve the utilization ratio of PCB.

Fourth, some suggestions that the audio-frequency amplifier selects for use in mobile phone design

Adopt the whole difference divide the intersection of audio frequency and the intersection of power and amplifier that input and output, raise power, inhibit than.
It is high in efficiency to try hard to adopt, low power dissipation, inside intensifies the design of Xiao, so can lengthen the service life of battery and chip. When the limited bandwidth is designed, propose using the D amplifier.
The audio-frequency amplifier will certainly should notice while using that the power voltage can’t exceed its extreme to supply power, so as not to cause the chip to be damaged.

Fifth, audio-frequency amplifier products prospect

Consumers pursue the small and exquisite appearance, comfort of low consumption, low price and hearing now. The supplier realizes gradually too, must improve to some extent in these respects, could make consumers like to spend money buying the apparatus of audio frequency. Meanwhile, they make great efforts to reduce the interference of the noise too. But exist while of digital circuit and simulation circuit, make this task more difficult.

Rely on the advantages, such as extremely good power efficiency, smaller heat and lighter power supply power,etc., the D amplifier is lifting the storm in the audio frequency world, as the maturity of technology and its increasingly good sound reached reproduce the result, the D amplifier will dominate the market of audio-frequency amplifier.