A class-D puts out a pulse-width modulated (PWM) signal instead of the linear signal that is typical in class-AB amplifiers. The PWM signal contains the audio signal and the PWM switching frequency plus harmonics. The class-D audio amplifier is much more efficient than a class-AB amplifier because the output MOSFETs switch from very high impedance to very low impedance, operating nanoseconds in the active region. With this technique very little power is lost in the output stage. Furthermore, the inductive element of the LC filter or speaker stores energy from cycle to cycle and ensures that switching power is not lost in the speaker.Introduction
Although class-D amplifiers have been around for a while, many people still do not understand the basics of how a class-D amplifier works and what makes it very efficient. This article explains how the pulse-width modulated (PWM) signal is created and how you hear the audio frequencies and not the switching frequencies of the PWM waveform. This article goes into detail on how outputting a PWM waveform is much more efficient than outputting a linear waveform, and why some class-D amplifiers require LC filters while others do not.
B>How does a class-D output signal (PWM) contain an audio signal?
The TPA3001D1 block diagram (shown in Figure 1) helps explain how the PWM signal is formed. First, the analog input class-D uses a pre-amplifier to gain the input audio signal and ensure a differential signal. Next, the integrator stage low-pass filters the audio signal for anti-aliasing and stability. The audio signal is then compared to a triangle wave to create a pulse-width modulated (PWM) signal. The gate-drive circuitry uses the PWM to drive the output FETs, which creates a high current PWM signal on the output.
Figure 1. TPA3001D1 block diagram.
Figure 2 shows how a typical PWM signal is formed from the comparator block in Figure 1. The audio input is compared to the 250-kHz triangle wave. When the audio input voltage is greater than the 250-kHz triangle wave voltage, the non-inverting comparator output is high, and when the 250-kHz triangle wave is greater than the audio signal, the non-inverting comparator output is low. The inverting comparator output is low when the non-inverting comparator output is high and high when the non-inverting comparator output is low. The average PWM non-inverting output voltage, VOUT+(avg) is the duty cycle times the supply voltage, where D is the duty cycle, or "on" time, t(on) / total period, T.
VOUT+(avg) = D * Vcc (1)
D = t(on) / T (2)
The duty cycle of the inverting output, VOUT-, is the 1 -- the duty cycle of VOUT+. If the input is at mid-supply, the duty cycle of VOUT- and VOUT+ is 0.5.
VOUT-(avg) = (1-D) * Vcc (3)
Figure 2: Comparator's inputs and PWM outputs of a typical class-D amplifier.
The TPA3001D1 and TPA3002D2 use the filter-free modulation scheme that is used in the TPA2005D1. With this modulation scheme, the positive output, VOUT+, is the same as the typical class-D PWM, but the negative output, VOUT-, is not just the inverse of VOUT+. In this case there are two comparators and the positive integrator output is compared to the triangle wave to create VOUT+'s PWM, and the negative output of the integrator is compared with the triangle wave to create VOUT-'s PWM. Figure 3 shows the Comparator inputs and PWM outputs for the filter-free modulation scheme, where the audio signal is assumed to be a dc voltage since the audio signal contains much lower frequencies than the 250 kHz triangle wave. Figure 3 also shows the differential output voltage.
Figure 3: TPA3001D1 and TPA3002D2 inputs outputs and PWM.
Figure 4 shows the TPA3001D1 PWM outputs with a 20 kHz audio input signal. Notice how the duty cycle increases as input voltage increases.
Figure 4: Scope plot showing input signal, output before filter, and output after filter (sine wave and PWM).
An audio signal in the PWM waveform is much easier to see in the frequency domain. The PWM signal is made up of the input frequency, the switching frequency and the harmonics of the switching frequency plus side bands. Figure 5 shows amplitude versus frequency of the input, PWM output and filtered output. Figure 5 shows how the audio signal is extracted from the PWM by low pass filtering. The filtered output has the 1 kHz sine wave frequency component plus any of the 1 kHz harmonics that show up in the audio band as distortion plus any remaining ripple voltage from the switching frequency. The speaker cannot reproduce the switching frequency and its harmonics, and even if it could, the ear could not hear it. A listener would not be able to tell the difference between the filtered and unfiltered PWM signals shown in Figure 5 if they were both sent directly to the speaker.
Figure 5: Amplitude versus frequency plot showing input signal, output before filter, and output after filter.
*This is article is in two parts. Look for part 2, which examines efficiency and filtering in detail, next week.