The first stage of a Class-D amplifier (after an initial amplification stage) is the audio modulation stage which changes the analogue input signal to a constant frequency, varying duty cycle, PWM signal, as shown below.
This seemingly complex operation is accomplished with a ramp generator and comparator. First, the ramp generator is set to a fixed frequency, 250kHz, for example. The resulting 250-kHz triangle waveform, from the ramp generator, and the analogue input signal are connected to a comparator. The comparator's output switches each time the analogue input signal and triangle waveform cross. The resulting output from the comparator is a 250kHz PWM waveform, which contains the analogue input signal, as shown. The input modulation stage drives the DMOS transistors in the output stage which transfer power from the power supply to the low pass filter as "packets of energy". Transistors in Class-D amplifiers operate in the cut-off or saturation regions (i.e. "on" or "off", like a switch). This mode of operation is very similar to switch-mode power supplies. Operating the DMOS transistors in this way minimises power losses commonly associated with transistors in linear power amplifiers. The final stage of a Class-D amplifier is a low pass filter comprised of an inductor and a capacitor. Much of the gains in efficiency will be lost in the filtering stage if it is not properly designed. A 2nd order LC, Butterworth low-pass filter is typical.
The Class-D design approach can produce an amplifier with better than 90% efficiency but the resulting amplifier is more complex than its linear counterpart and there is an important tradeoff: the higher the frequency of operation, the easier it is to design the filter that removes the carrier frequency; but the higher the switching losses in the output transistors which eliminate the Class-D amplifier's efficiency advantages. In the 1980s, MOSFETs became available that could meet both the switching-speed and conduction-loss requirements to effectively implement Class-D amplifiers. In a typical, practical Class-D amplifier, the configuration is the bridge type in which two output stages and two Butterworth filters supply the amplified audio signal to a floating load, as shown in the figure below.
When driving 100W into a 4(ohm) load, typical, commercial first-generation amplifiers had a THD+N across the audio spectrum of a few percent. These figures were roughly maintained at low power levels too meaning that this type of design was not yet really of interest for high-quality monitoring and listening applications. However, this level of performance - coupled with high efficiency - was immediately of very considerable interest in audio amplifiers for cars and public address. However, it has been discovered that THD may be practically reduced by (amongst other things); filter selection, higher grade op-amps in the circuit, careful design of the feedback compensation network, and improving the linearity of the comparative triangle waveform. In particular, matching the output filter to the speaker's impedance reduces peaking in the amplifier's closed-loop response and improves the distortion characteristics of the amplifier. These gradual improvements have resulted in the development of the latest generation of integrated IC switching amplifiers which feature a power output range of 20 to 150 W and up to 95% efficiency with only 0.01% THD; a performance that may be said to rival that of a conventional amplifier, but with the advantage of much greater efficiency.
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