Class D amplifiers, also known as a switching amplifiers, are electronic amps in which the transistors operate as electronic switches. These transistors are usually MOSFETs and they operate by switching back and forth between the supply rails. They are fed by a modulator using diverse techniques that encode the audio input into a pulse train. The audio escapes into the amplifier through a simple low-pass filter that blocks the high-frequency pulses. This process is precise and efficient (and we love it).
A Brief History on Class D Amplifiers
Alec Reeves invented the first Class D amplifier in the 50s. The name of its first commercial version was X-10, and the company that launched it was Sinclair Radionics. Sinclair, years later, produced the the X-20—but it had many inconsistencies and limitations that made it impractical and fairly unsuccessful. Regardless, the development MOSFET technology solved those inconsistencies and limitations.
The silicon-based MOSFET technology empowered the Class D amp. The TA-N88 was the first Class D amp that used powered MOSFETs, as well as a switched-mode power supply. After that, the VDMOS technology launched and it became quite popular, successful, and at a low-cost. Many years after this the first Class D amplifier based on an integrated circuit launched.
How They Work
So, getting back to business, Class D amps work by generating a train of rectangular pulses of fixed amplitude. They might vary in width and separation or in number per unit time. These pulses represent the amplitude variations of the analog input signal and they are able to sync the modulator clock with an incoming digital audio signal, converting it to analog. The output of the modulator is able to switch the transistors on and off consecutively. But be careful: the transistors can never conduct together because, if they did, it would cause a short circuit between the supply rails that run through the transistors.
The transistors are extremely efficient because they are only able to be on or off—there is no halfway. This means that they spend very little time in the linear zone and don’t waste much power. And the process is fairly simple. A low-pass filter offers a path for the low frequencies of the audio signal, which ends up leaving the high-frequency pulses behind. Some applications skip the output filter altogether. In these cases, the circuit trusts the inductance of the amplifier so that the HF component doesn’t heat the voice loop.
And if you’re looking for more…
You can compare this class of amplifiers to others. If this interests you, in our opinion, the one you should look for first is the Class T amp. Of course, other types are available in our website, so feel free to go on exploring!
