Today several types of basic amplifier topology, AB power amplifier has been widely used in various audio products, including mobile phones, mp3 players and PMP systems. Considering the class AB amplifier to provide signal amplification with high quality performance, and therefore very suitable for small power headphone speaker. However, due to more and newer products for portable multimedia capabilities have higher requirements, stereo audio, 3D surround sound and a large volume of output has gradually become a new generation of portable multimedia products essential to the function. Smaller form factor design, using a thinner battery, less efficient become the Achilles heel of class AB power amplifier. In order to solve the conflict between energy efficiency and high power audio output, Class D amplifier compared to class AB has been greatly improved in terms of efficiency, it has been gradually used in some high-end products. The following will briefly Class D amplifier works, as well as basic testing methods.
Take the Anni Mei Semiconductor NCP2820 as an example, the D power amplifier is completely different from the work mode of the AB class, as shown in Figure 1, the NCP2820 consists of three parts: pre-substantial, sampling and pulse width modulation and H-type power output part .
Pre-amplified by a fully differential analog amplifier configuration, accordingly amplified differential input signal, the gain of the internal fixed resistance Rf divided by the resistance Ri come. Ci and Ri constitute a low-pass filter circuit, an input signal isolation unwanted DC component. Since NCP2820 use of fully differential amplifier, if the input signal is fully differential, Ci of may be omitted.
Sampling and pulse width modulation to form a pulse width modulated square wave of 250kHz signal and preventing large signals after a triangular wave is compared 250kHz. Real time width of each pulse reflects the amplitude of the input signal.
H-output circuit shown in Figure 1 is constituted by two pairs of the MOSFET, the driving pulse signal generated by the preceding stage. Generates a pulse signal having a power output to the load speaker. Now widely applied 8Ω dynamic speaker, its internal inductance coil having an impedance approximately 15mH itself and 8Ω constituting the demodulation circuit can restore the analog signal to a square wave pulse. This is why the low pass filter does not require additional reason to do at the output of the demodulation circuit.
NCP2820 using 1.5mm × 1.5mm flip-chip compact package, and a few external components, simpler circuit design, the circuit board layout is also more convenient. However, It should be emphasized that since the output of the class D amplifier is a pulse waveform approximately 250kHz, so when the wiring, the amplifier close to the speaker as far as possible, to avoid unnecessary noise and interference.
Engineers in the class AB to be tested can be captured directly by the probe to the actual input and output waveforms. However, when testing the class D amplifier, since the output of the analog audio signal is not, but a certain square wave digital pulse frequency, which requires an additional filter to be tested. FIG 2 is a Class D audio amplifier test configuration.
From the point of view engineer, THD + N (total harmonic distortion + noise) and PSRR (power supply rejection ratio) are two relatively important parameters, these parameters directly reflects the performance of the audio amplifier.
Figure 3 shows the circuit configuration of the test harmonic distortion + noise. Analyzer using an audio signal generator may produce a standard test for sinusoidal single-ended or differential signal is input to NCP2820, and the detection output of the NCP2820. The input and the output signal may be analyzed in accordance with the requirements set by the test, and according to the set, the test results show.
If the test output is set when the THD + N performance relative output power of the audio analyzer will adjust the amplitude of the input signal sine wave generator to NCP2820 fixed frequency at a point, the harmonic input and the test output signal at different amplitudes wave value, calculated according to a given load power. This power value is obtained by dividing the baseband power THD + N of the test results, and manifested in graphical form in FIG. 4 is NCP2820 at 4.2V supply voltage, the load 8 ohms, the test results obtained. NCP2820 can be seen that at an output power of 1W, THD + N "1% by mass to ensure the output of the music
If desired test output THD + N performance relative signal when the frequency of the audio analyzer will adjust the signal generator on a fixed amplitude sine wave input to the frequency point from NCP2820 20Hz ~ 20KHz, while the analyzer receiving port in the test different harmonic frequency of the input signal when the output value of each test values compared to the baseband frequency required then manifested in graphical form in FIG. 5 is a characteristic curve NCP2820 obtain THD + N performance relative frequency at 8Ω loads.
THD + N performance determines the output audio quality. Due to the working principle of the Class D amplifier, so the performance will be slightly worse than class AB amplifiers.
Another important performance parameter is the relative power supply rejection ratio PSRR, this parameter reflects the extent of the amplifier power supply voltage fluctuations. It is particularly important in portable wireless communications products.
6 in the basic configuration suppression ratio of the test power supply. And harmonic distortion can be seen that different test input terminal at this time is NCP2820 to ground, differential signals obtained BTL output ideally should be zero. However, due to power fluctuations cause fluctuations produce a corresponding output signal, thereby generating noise can be heard. When measuring the power supply rejection ratio of the relative frequency of power fluctuations, the amplitude of the signal generator generates a fixed frequency standard 20Hz ~ 20KHz sine signal from the DC component to provide the desired NCP2820 operated by driving BUFFER. Analyzer receiving port at a test frequency of 20Hz ~ 20KHz, the effective value of the output signal, and then comparing the input signal RMS, and graphically represented, in dB NCP2820 7 expressed in units of power supply rejection ratio characteristic in FIG. Bode plots.
In summary, the test configuration of an audio amplifier to rely on professional audio analyzer, engineers will usually test the system as a whole, but will not do the above for detailed testing amplifier IC. All audio amplifier parts ON Semiconductor provided by the company include NCP2890, NCP2892, NCP2809, NCP4894, NCP2820, NCP2821 provides corresponding test results in the datasheet to ensure product quality.
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