Class G/H Amplifier

What Is Class G/H Amplifier?

There are a variety of amplifier designs that enhance class-AB output stages. They have more efficient techniques to achieve greater efficiency with low distortion. These designs are common in large audio amplifiers. Without the efficiency increases, the heatsinks and power transformers would be prohibitively large (and costly). The terms "class G" and "class H" (G/H amplifier) are used interchangeably to refer to different designs. They vary in definition from one manufacturer or paper to another.  

Class G Amplifiers

Class-G amplifiers (which use "rail switching" to decrease power consumption and increase efficiency) are more efficient than Class AB amplifiers. These amplifiers provide several power rails at different voltages and switch between them as the signal output approaches each level. Thus, the amplifier increases efficiency by reducing the wasted power at the output transistors. Class-G amplifiers are more efficient than class AB but less efficient than class D. However, they do not have the electromagnetic interference effects of class D.  

Class H Amplifiers

Class-H amplifiers create an infinitely variable (analog) supply rail. They are sometimes referred to as rail trackers. This is done by modulating the supply rails to be only a few volts larger than the output signal, "tracking" it at any given time. The output stage operates at its maximum efficiency all the time. This is due to the circuit's ability to keep the rail transistors (T2 and T4) in cutoff until a music voltage peak is sufficient to require the additional voltage from the + and - 80 V supplies. The class H amplifier is two amplifiers in series. 

G/H Amplifiers Enhancements

Classes G/H amplifiers offer enhancements to the basic class AB design. Class G uses multiple power supply rails of various voltages. They switch to a higher voltage when the audio signal wave has a peak value that is higher than the supply voltage level. They also switch back to a lower supply voltage when the peak value of the audio signal reduces. Switching the supply voltage to a higher level only when it reaches the largest output signals and then switching back to a lower level reduces average power consumption and heat caused by wasted power. Class H improves on class G by continually varying the supply voltage when the audio signal exceeds a particular threshold level. The power supply voltage tracks the signal's peak level to be only slightly higher than the instantaneous value of the audio wave. It returns to its lower level once the signal peak value again falls below the threshold level. Therefore, classes G and H require considerably more complex power supplies. This adds to the cost of implementing these features.  

Source Texts

• Power Amplifier Classes: Classes G and H
• Class G and Class H Amplifiers