The final amplifier is the power source of every audio installation. Its job is to convert a small alternating voltage into a powerful signal for driving loudspeakers with as little distortion as possible.
Delivering a large amount of power is not a simple task for an amplifier. Voltage amplification and current amplification are both necessary in order to provide sufficient power to speakers connected to the amplifier. To produce power amplification in a final amplifier, various concepts have been developed for using transistors or FET’s to generate high-quality output signals and to improve the efficiency of the output stage.
Final amplifiers are normally classified according to the configuration of the output stage. This determines their efficiency and quality since the output stage is where the actual power amplification takes place.
Class A amplifiers
A class a amplifier has very low efficiecy but it has almost no crossover distortion. Class A final amplifier is the simplest configuration and also one of the best configuration for high quality audio reproduction and can be implemented using a standard emitter follower. The quiescent current through the transistor is equal to the peak AC output current, which means that the transistor is biased in the middle of its working range and simply conducts more or less current when driven by an alternating voltage.
The efficiecy of a class A amplifier is very low: 25% at maximum output amplitude and even less at low signal levels. The efficiency can be improved by using a symmetrical design with 2 transistors, but even so the highest effic. is 50%.
Class B amplifiers
The class B configuration employs 2 transistors, each of which conducts for exactly half of the signal cycle. In the quiescent state, no current at all flows through the transistors. The efficiency of the a class B output stage is around 78% but the primarily disadvantage of this configuration is the transfer distorsion that occurs each time the load must be transferred from one transistor to the other. This leads to crossover distorsions, which is a quite audible degradation of the signal waveform. To solve this, class A and class B were combined to produce class AB, this is a class B configuration in which a masll quiescent current flows, causing the output stage to work in class A at low power levels.
Class D amplifiers
Class D amplification it refers to a switchig amplifier that uses pulse-width modulation. The input signal is compared with a triangular waveform and the signal from the comparator switches the output stage to the positive or negative supply voltage. This is done using a very swithing frequency which is usyally ten times or more than the audio bandwidth (200kHz or above). As the output stage only has to switch, its efficieny is very high. However, there also a number of drawbacks to this approach. It is rather difficult to keep the signal waveform free of distorsion, a hefty output filter is required and drastic measures must be takes to limit radiated interference. For low-distorsion amplification, it is always necessary to use negative feedback.
Class G and H amplifiers
In both classes the supply voltage is adjusted according to the magnitude of the output signal. In class G the supply voltage is continuosly adjusted to match the desired amplitude of the output signal. Such a tracking supply voltage can be implemented using modern switching power supplies, have good regulator circuit to allow the supply voltage to respond sufficiently quicly to changes in the amplitude of the signal generated by the output stage.
In class H is essentially the same think as in class G, except that here the supply voltage is switched betwwen several distinct levels instead of being continuously varied. This allows the dissipation in the output stage to be considerably reduced, especially when large amounts of power are involved.
Class S and T amplifiers
Sony developed its S-Master technology and Tripath has devised its class T amplifier. In its S-Master technology, Sony combined several techniques to make the class D configuration suitable for domestic hi-fi applications. Here the process of converting the incoming signal into a corresponding pulse-width signal is called complementary pulse length modulation.
Tripath has developed a technique that combines the signal quality of class A and AB amplifiers with high efficiency ( around 80-90%). This is done using a combination of analogue and digital circuitry, togheter with digital algorithms that modulate the input signal using a high-frequency switching waveform.