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It is important to note that the frequency of the PWM output is the same as the carrier frequency. Moreover, the PWM output is plotted by doing the comparison explained previously.įig 3 : PWM input and output. In the Figure 3 below, an input signal of frequency 2 Hz is plotted along with a carrier signal of frequency 20 Hz. If the sine is above the carrier signal, the output is equal to 1ĭuring the course of this tutorial, the transformation of the signal will be tracked by plotting every step of the amplification with the MatLab ® software.
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The output of the modulator is obtained by doing the following comparison between these two signals : In order to be consistent with Shannon’s theorem, the frequency of the carrier signal must be at least twice as high as the sinusoidal signal frequency. This technique consists in comparing the input sinusoidal signal with a high frequency triangular signal commonly called carrier obtained from an independent generator. A simple graph representing the PWM is shown in Figure 2 below : fig 2 : Principle of a PWM modulator
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Many modulations techniques exist, however the most common and widely used for many applications is the Pulse Width Modulation (PWM). Moreover, this final stage eliminates undesirable harmonics than may have been generated during the amplification process. We present in detail in the section dealing with this stage that the transistors work in a particular regime and a complementary configuration in order to amplify correctly the rectangle signal.įinally, a low-pass filter is used in order to restore the sinusoidal shape of the signal. The switching stage is where the amplification takes places thanks to transistors. We will see in the dedicated section that the view proposed in Figure 1 concerning the modulation is oversimplified. While classic amplifiers accept as an input a sinusoidal signal, Class D amplifiers previously transform it through a modulator into a rectangular signal. The signal path along with the succession of these different modules is presented in Figure 1 below : fig 1 : Flowchart of a class D amplifier Presentation of the Class D amplificationĬlass D amplifiers generally consists of three different modules : a modulator, a switching stage and a low-pass filter. Finally, this information is synthesized in a conclusion that summarizes the global transformation of the signal. A little note about the efficiency of this amplifier is given in the last section. The next sections are therefore focused on each of these modules to understand how the signal is transformed during the class D amplification process. As we will see during this section, class D amplifiers are composed of three different main modules. In the first section, the simplified architecture of a class D amplifiers along with its general functioning are presented. However, as we have seen with the class C amplifiers, this cannot be implemented since no power is delivered to the load.Ĭlass D amplifiers precisely solve this problem by functioning with a different method than traditional classes A, B, AB or C amplifiers. As the conduction angle is decreased, high efficiencies are reached such as with the class C amplifiers.Ī conduction angle that tends to 0° is therefore desirable to achieve 100 % of efficiency. Indeed, high conduction angles-based amplifiers offer a very good linearity such as the class A Amplifiers but present a very limited efficiency, generally around 20 to 30 %. In the previous tutorials, an important link has been established between the conduction angle of an amplifier and it’s efficiency.