Class D-amplifiers

A "class D"-amplifier is a power amplifier where the power transistors in the output stage are used as switches. Due to this there is no voltage across them while they are conduciting. As a consequence of this, there are no losses in such an amplifier, which can have a very high efficiency (in practice up to more than 95 %).
The transistors are always switched off or on. This way the instantaneous output voltage is either high or low. By suitable switching, it is possible to set the average of the output signal to the desired value. The circuit that controls this switching, is called the modulator. The instantaneous deviation from the desired value is then considered as switching noise. In practice this switching noise has to be filtered by a filter circuit. Obviously this filter must be lossless to retain the high power efficiency. In practice 2nd order inductor-capacitor filters (LC-filters) are used.

A major problem in the design of class D amplifiers is the compensation of non-ideal effects of the output filter. The solution adopted by the CAS-group is to embed the output filter in the control loop of the analog modulator (see figure).

An alternative approach implements the the modulator completely digital. A stereo 2x100 Watt demonstration system is pictured to the left. Distortion by the switching transistors present a significant problem. However, using predistortion, the signal quality can be improved: a dynamic range of 90 dB and and peak SNDR of 62 dB was demonstrated.