Class G Amplifier

A class B amplifier has relatively high efficiency for a rail-to-rail sinusoid. This figure is relatively good because the output signal is close to the supply lines a considerable part of the time, with a limited voltage drop across the output transistors. The output signal for audio, however, is close to zero most of the time, with only few excursions to higher levels. Thus the average voltage drop across the output transistors is large, causing the poor efficiency figures for audio.

An amplifier in class G uses multiple supply voltages. At lower power levels, the lower supply voltage is used. When the signal becomes too large for this supply, the higher power supply takes over, and delivers the output power. In this way the average voltage drop across the output transistors is reduced and the overall efficiency can be improved. There are two basic ways in which class G amplifiers are realised. The difference is the way of switching between the supply voltages. Picture 1 shows the upper half of a possible output stage. The upper transistor is switched on during signal peaks increasing the power supply of the lower transistor that controls the output voltage from Vdd1 to Vdd2. Another way to use this circuit is opening the lower transistor totally during signal peaks, giving the higher MOST the role of output transistor.

Picture 1: Serial Class G amplifier



A disadvantage of this circuit is that there are always two elements in series. At low output voltages, the diode decreases the efficiency. During signal peaks the two transistors are in series, so that the output current has to pass two VDS voltage drops. Picture 2 shows a parallel topology that does not suffer from these problems. It needs special precautions in the driver circuitry, however, to prevent high VGS reverse voltages across the upper left output transistor.

Picture 2: Parallel Class G amplifier


In general, the need for multiple supplies may be a problem. If a transformer is used in the power supply, multiple taps are a good solution, but if a car battery is used, it is more problematic. Another problem with this type of amplifiers is the distortion caused by the switching between the two amplifiers. By using a comparator with hysteresis and delay to decide between the supplies, the number of changes can be reduced, but this is a very inelegant way to reduce the total distortion. Another way to limit switching distortion is by switching between the two amplifiers gradually. However, this cuts down the efficiency a little.