Introduction to four types of amplifiers

　　1. B-class power amplifier

　　The conduction angle of this type of amplifier is 180 °, with an efficiency of 78.5% and half cycle operation. The method of adding positive and negative half cycles can restore the waveform of the input signal. To compensate for the error caused by the opening voltage of the tube during stacking, Class AB can be used.

　　2. Class C power amplifier

　　Class C amplifiers are widely used in high-power CW and FM outputs, and in AM amplifiers, bias can be changed to adjust amplitude changes. As mentioned earlier, when the conduction angle of a Class C power amplifier is less than 180 °, the nonlinear output signal is not a simple multiplication of the input signal. Class A amplifiers use one tube, Class B amplifiers use two tubes, and Class C amplifiers use one tube, but the bias point is different. In the output circuit of Class B amplifiers, signal quality can be improved by adding filters. Class C output circuits must have resonant circuits to recover the fundamental signal. The biggest advantage of Class C amplifiers is high efficiency.

　　3. Class D power amplifier

　　The efficiency of Class C power amplifiers can reach 100%, but the output power is zero. Changing the bias of a Class B power amplifier so that the output is not half cycle linear but non-linear clipping, resulting in a positive and negative square wave output, which is then restored to sine through a resonant circuit. In a Class D amplifier, the transistor is close to being in a switching state. If the switching time is zero, then the leakage source voltage is zero and the leakage current is maximum, theoretically achieving 100% efficiency. In fact, BJTs can operate at several megahertz, while FETs can operate at tens of megahertz, and cannot be infinitely fast.

　　4. Class E power amplifier

　　The aforementioned power amplifier can be referred to as a switching power amplifier. In order to avoid parallel capacitor discharge of switching devices and reduce instantaneous power loss during switching, a load network can be designed for the amplifier to determine the voltage at both ends of the device after shutdown, which is called a class E power amplifier.

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