Class D audio power amplifier evaluation and application

Today several of the basic amplifier topology, AB power amplifier has been widely used in various audio products, including mobile phones, mp3 players and PMP system. Taking into account the class AB power amplifiers provide high-quality signal amplifier performance, so it is suitable for headphones and speakers of some low-power applications. But as more and newer portable multimedia products with higher demand, stereo audio, 3D surround sound and large volume export has gradually become a new generation of portable multimedia products essential to the function. Smaller form factor design, use thinner battery, low efficiency of a fatal weakness AB power amplifier. In order to solve the energy and power conflict between the audio output, D, class AB power amplifier efficiency than has been greatly improved, has been gradually applied in a number of high-end products. The following briefly introduces the class D amplifier works, and basic testing.

By ON Semiconductor NCP2820 example, D, Class AB power amplifier uses a radically different work pattern, shown in Figure 1, NCP2820 interior consists of three parts: pre-amplification, sampling, and pulse width modulation and power output of some H- .

Pre-amplification, by a fully differential analog amplifier constitutes the input of the differential signal amplification accordingly, the internal fixed gain of resistance by the resistance of Rf divided by Ri come. Ci and Ri to form a low pass filter circuit, isolated DC input signal does not need some. As the NCP2820 uses a fully differential op amp, so if the input is fully differential signal, Ci can be omitted.

Sampling and pulse width modulation, to prevent large after a 250kHz signal and the triangular wave comparison pulse width modulation to form a 250kHz square wave signal. The width of each pulse reflects the real-time input signal range.

H-output circuit, shown in Figure 1 by two MOSFET structure, generated by the pulse signal level before the driver. Produce a power output of the pulse signal to the load speaker. Moving coil is now widely applied 8Ω speaker, the internal coil with approximate inductance 15mH and itself constitutes a demodulation circuit impedance of 8Ω, you can restore the pulse square wave analog signal. This is why the output does not require additional low-pass filters demodulation circuit reasons.

NCP2820 uses a 1.5mm × 1.5mm flip-chip in a small package, and few external components, so that a more simple circuit design, circuit board layout is also more convenient. But there needs to be emphasized is because Class D amplifier's output is approximately 250kHz pulse waveform, so cabling, amplifiers close to the speaker as far as possible, to avoid unnecessary noise and interference.

Engineers in the class AB power amplifier testing, you can capture directly to the oscilloscope probe to the actual input and output waveforms. However, Class D amplifier in the test, because the output is not analog audio signal, but a certain frequency square wave digital pulse, which would require additional filters to be tested. Figure 2 D audio power amplifier test configuration.

Class D audio power amplifier evaluation and application

From the engineer's point of view, THD + N (total harmonic distortion + noise) and PSRR (power supply rejection ratio) is relatively important two parameters, these two parameters directly reflect the performance of audio amplifiers.

Figure 3 shows the harmonic distortion + noise test circuit configuration. Use of audio analyzer signal generator can produce a standard for testing differential or single-ended sinusoidal signal input to the NCP2820, NCP2820 and test the output. Can be set in accordance with the requirements of test input and output signals, and in accordance with the set, showing test results.

Class D audio power amplifier evaluation and application

If you set the test output THD + N performance of the relative output power, audio signal generator analyzer in a fixed point on the adjustment of the frequency sine wave input to the NCP2820 increases, and test the output at different input signal amplitude harmonic wave value, calculated based on the given load power. This power value divided by the baseband get THD + N power test results, and graphical forms, and in Figure 4 for the NCP2820 in 4.2V supply voltage, 8Ω load, get the test results. NCP2820 can be seen in the 1W output power, THD + N <1% to ensure the quality of the output music

If you need to test the output THD + N performance relative signal frequency, signal generator, audio analyzer in a fixed rate to the NCP2820 point to adjust the input sine wave frequency from 20Hz ~ 20KHz, while the analyzer to accept port test out the output signals of different frequency input of harmonic value, the value of each test requires comparison with the base-band frequency and then displayed in graphical form in Figure 5 for the NCP2820 in 8Ω load THD + N performance by the relative frequency of the characteristic curve.

Class D audio power amplifier evaluation and application

THD + N performance of the decision of the output audio quality. As the Class D amplifier works, so performance will be weaker than class AB amplifier.

Another important performance parameter is the relative PSRR power supply rejection ratio, this parameter reflects the power amplifier supply voltage fluctuations by the impact. In the portable wireless communications products, is particularly important.

Figure 6 shows the power supply rejection ratio test of basic configuration. Harmonic distortion can be seen and test different input at this time NCP2820 is received, the ideal case of BTL output differential signal should be zero. However, because power fluctuations can cause fluctuations in the output signal generated corresponding to the noise generated may be heard. In measuring the relative power fluctuations PSRR frequency, the signal generator to generate a range of fixed-frequency standard from 20Hz ~ 20KHz sinusoidal signal, by providing NCP2820 BUFFER drive required for DC component. Analyzer test port to accept a 20Hz ~ 20KHz frequency in the output signal of the RMS, and then compare the input rms signal and be expressed graphically in Figure 7, expressed in units of-dB power supply rejection ratio characteristics NCP2820 Bode plots.

Class D audio power amplifier evaluation and application

Class D audio power amplifier evaluation and application

In summary, the audio amplifier test configuration to rely on professional audio analyzer, engineers will often test the system as a whole, and not for the amplifier IC to do these tests in detail. ON Semiconductor provides all the audio amplifier unit includes NCP2890, NCP2892, NCP2809, NCP4894, NCP2820, NCP2821 are available in the datasheet of the corresponding test results to ensure product quality.