Analog PWM Amplifier

PWM stands for pulse width modulation. The engineer may want to discuss the “analog” because, strictly speaking, PWM is a digital modulation type. Another name for this type of amplifier is “Class-D.” However, this is often equated with “digital amplifier,” which doesn’t do it justice. This term describes only one – in our case, not present – special case. In fact, the generated pulse width signal is an analog signal with infinite resolution in the time domain. And the input signal of our pow-amp is also an analog one. At no point does the conversion to a digital data word take place. Hence the choice of the name “analog PWM amplifier”.

Simply described, the input signal in the form of a modulated oscillation is translated into a sequence of square-wave pulses with a pulse width modulated by the input signal. This pulse sequence is then amplified and, at the end, converted back into a modulated oscillation by an analog low-pass filter.

However, the amplitude of the pulse train knows only two states, either 0 or 100%. Therefore, the amplifying power transistors only switch between the off and the maximum amplitude states. The directly recognizable benefit is the very high efficiency or the low waste heat. Conventional amplifiers – especially Class-A – burn up more power than is converted into a driving force in the loudspeaker. With the PWM amplifier, the always full switching of the transistors, instead of the intermediate stages guided by the amplitude of the input signal, makes this high efficiency possible. However, in terms of music, this also ensures extremely fast response times and thus very high accuracy in the dynamic behaviour. Extremely high power is available within a very short time. The finest nuances in the music, which make the difference between simply “nice-sounding” and “deeply moving”, are thus authentically reproduced.