Introduction
A Class D amplifier, also called a switching amplifier, is a type of electronic amplifier where the active devices—typically MOSFET transistors—function as on/off switches rather than operating in their linear gain region as in conventional amplifiers. They are also called power amplifiers since they deliver significantly more power to the load than the source. They have the best power efficiency among classes of amplifiers.
Working principle
The operation of a Class D amplifier relies on high-speed switching and filtering rather than traditional linear amplification. The analog input signal is first converted into a high frequency Pulse Width Modulation (PWM) output, where the pulse width is proportional to instantaneous amplitude of the audio signal. This PWM signal controls the output transistors (typically power MOSFETs), which alternate between fully on and fully off states. Because the devices conduct with negligible resistance when on and carry no current when off, power loss is extremely low. This results in very high efficiency. The amplifier’s raw output is a PWM waveform rather than a direct analog signal. An LC low-pass filter at the output stage removes the fast rise/fall edges, leaving behind the reconstructed audio waveform to drive the loudspeaker.
Open loop class D amplifier
An open-loop Class D amplifier is a basic type of Class D (switching) amplifier that operates without any global feedback loop from the output back to the input. It relies strictly on the accuracy of its modulation and switching stages to track the input signal, with no error correction or compensation for non-idealities, power supply noise, or device nonlinearities. It is very simple to implement.
Closed loop class D amplifier
Many Class D amplifiers employ negative feedback from the PWM output back to the input. This closed-loop design improves linearity and significantly enhances the power supply rejection ratio (PSRR). In contrast, open-loop Class D amplifiers lack feedback and inherently exhibit 0 dB PSRR.
Deploying negative feedback bring noise shaping in Class D amplifiers. This effectively suppresses in-band noise caused by the nonlinearities of the pulse-width modulator, output stage, and variations in supply voltage. This approach is conceptually similar to the noise-shaping techniques employed in sigma-delta modulators.
