Class A amplifier is the simplest type of amplifier. For any output waveform, the transistors of the output stage are always in the on state (not completely off). This type of amplifier has excellent linear characteristics, but the efficiency is very low.
The output stage transistors of the Class B amplifier are only turned on during a half cycle (180 degrees) of the signal waveform. In order to amplify the entire signal, two transistors are used, one for the positive output signal and the other for the negative output signal. The efficiency of the class B amplifier is much higher than that of the class A amplifier, but due to the crossover point between the two transistors from on to off, the distortion is larger.
The combination of class A and class B is called class AB amplifier, which has higher efficiency than class A amplifier and lower distortion than class B amplifier. By biasing the two transistors in the circuit, the two transistors are turned on when the signal is close to zero (the class B amplifier introduces a non-linear operating point); when the signal is large, the transistor switches to the class B operating mode. It can be seen that both transistors maintain effective operation when the signal is small, similar to a class A amplifier; when the signal is large, only one transistor remains active for each half cycle of the waveform, similar to a class B amplifier.
The output of the Class D amplifier is a switching waveform, and the switching frequency is much higher than the highest frequency of the audio signal that needs to be restored. After low-pass filtering, the average value of the output waveform is consistent with the actual audio signal. Since the output stage transistors are in a fully on or off state during operation, they will not enter the linear operating region of the transistor (this is the reason for the inefficiency of other types of amplifiers). Class D amplifiers have extremely high efficiency (up to 90%, even higher). Modern class D amplifiers can achieve the same level of fidelity as class AB amplifiers.
Class G amplifier is the same as Class AB amplifier, but it uses two or more power supplies to work at a small signal level, and the amplifier uses a lower power supply voltage. As the signal level increases, the amplifier automatically switches to the appropriate power supply voltage. Because high-voltage power supply is used only when necessary, and class AB amplifiers always use high-voltage power supply, the efficiency of class G amplifiers is higher than that of class AB amplifiers. Guide to Class A-Class H Amplifiers (continued)
Class H amplifiers minimize the voltage drop of the output stage by adjusting their own power supply voltage. Multiple discrete voltages can be used, or continuously adjustable voltages can be used. Although similar to the Class G amplifier technology and aimed at reducing the power consumption of the output stage circuit, the Class H amplifier technology does not require multiple power supplies. The design of Class H amplifiers is more complicated than other amplifiers, and additional control circuits are needed to predict and control the power supply voltage.