Brief introduction of RF power amplifier
In the front-stage circuit of the transmitter, the radio frequency signal power generated by the modulated oscillation circuit is very small, so it takes a series of amplification to obtain sufficient radio frequency power before it can be fed to the antenna for radiation. In order to obtain sufficient RF output power, a RF power amplifier must be used. RF power amplifier (Rf pa) It is an electronic circuit that comprehensively considers the problems of output power, excitation level, power consumption, distortion, efficiency, size and weight, and is an important part of various wireless transmitters. Its main technical index is output power and efficiency, which is the key to studying RF power amplifier. In addition, the harmonic components in the output should be as small as possible to avoid interference with other channels. The requirements for power transistors mainly consider parameters such as breakdown voltage, maximum collector current and maximum pipe consumption. In the transmitting system, the output power range of the RF power amplifier can be as small as mW and as large as several kW, but this refers to the output power of the final power amplifier. In order to achieve high-power output, the last stage must have a high enough excitation power level. The RF power amplifier can be divided into a (A)B (B)C (C)Three types of working conditions. Class A amplifiers have a conduction angle of 360, Class B amplifiers have a conduction angle equal to 180, and Class C amplifiers have a conduction angle less than 180. Class A is suitable for low-power amplification of small signals; Both Class B and Class C are suitable for high-power working conditions, and the output power and efficiency of class C working conditions are the highest among the three working conditions. Therefore, most RF power amplifiers work in class C, but the current waveform distortion of class C amplifiers is too large and can only be used to amplify the load resonant power by using a tuning loop. Due to the filtering capability of the tuning circuit, the circuit current and voltage are still close to sine wave shape and the distortion is very small. In addition to the above working state classified according to the current conduction angle, there are also D (D)Class amplifier and E (E) Class amplifier, Class D amplifier is more efficient than Class C amplifier.