An amplifier operating by means of pulse-width modulation (see pulse modulation). The input signal produces a square wave modulated with respect to its mark space ratio. Push-pull switches are then operated by the modulated square wave so that one switch operates with a high input level and the other with a low input level. The resultant output current is proportional to the mark space ratio and hence to the input current. In theory class D amplifiers are very highly efficient but they require an impractically high speed of switching to avoid distortion.