Class D power amplifiers
Historically, audio-amplifiers have been configured as class A, class B or class AB and the art of design is well known. Also well known is the poor efficiency of these amplifiers compared to that of class D amplifiers. Whereas the theoretical best efficiency for class B amplifiers is 78.5%, the practical upper limit is more nearly 70 % when driving a purely resistive load. But when driving real speaker loads which can have power factor angles of 60° or more, efficiency can come down to 55 % or less. Class D amplifiers, however, can attain efficiencies of 90 %, and with careful component choices can exceed 95 % even. Moreover, the power factor of the load does not affect the on-state power losses in the MOSFET switches normally used in such amplifiers. Using class D techniques, amplifiers capable of delivering several hundred watts to the load can be designed using small, inexpensive, stamped heat sinks. Efficiency of class D amplifiers opens the possibility for powerful, small, light amplifiers with good sound quality to be designed today.
A class D amplifier is designed to operate with digital or pulse-type signals. It is necessary, however, to convert any input signal into a pulse-type waveform before using it to drive a large power load and to convert the signal back to a sinusoidal-type signal to recover the original signal.
Above block diagram illustrates how a sinusoidal signal may be converted into a pulse-type signal using some form of saw tooth or chopping waveform to be applied With the input into a comparator type op-amp circuit so that a representative pulse type signal is generated. While the letter D is used to describe the next type of bias operation after class C, the D could also be considered to stand for “Digital” since that is the nature of signals provided to the class D amplifier.
Shows a block diagram the unit required to amplify the digit signal and then convert back to tl sinusoidal-type signal employing a low-pa filter. Since the amplifiers transistor devices used to provide the output are basically either off or on, they provide current only when they are turned on, with little power loss due to their low on-voltage. Since most of that power supplied to the amplifier is transferred to the load the efficiency of the circuit is typical very high. Power MOSFET devices has been quite popular as the driver device for’ the class D power amplifier.
Class D Power amplification Waveform-Digital