Power amplifier is an indispensable important part in wireless communication system. With the development of communication systems, power amplifiers have entered a stage of rapid development. At present, the main application of power amplifiers tends to be microwave frequency bands, especially the civilian blowout development. Relatively few 1.6-30 MHz power amplifier applications are used, and the development of new products for optional power amplifier tubes in this frequency band is also reduced. Under the premise that the indicators are constantly required to be improved, this causes increased difficulty in the design of power amplifiers in this frequency band, among which the design of broadband low-voltage high- power power amplifiers is the most difficult.
1 Index requirements
This project is an important part of the military short-wave transmitter part. According to the specific use occasion and environmental characteristics of the project, according to the GJB regulations and the requirements of the whole machine, the following indicators are required for the power amplifier:
(1) Frequency range: 1.6 ～ 29.9999MHz;
(2) Output power: single tone power is 100 W (AVG), dual tone peak envelope power is 100W;
(3) Excitation amplitude: single tone 0 dBm (50 Ω impedance);
(4) Gain: 50 dB;
(5) Gain flatness: ± 1dB;
(6) Power consumption: at an output power of 100 W and a power supply voltage of 13.8V, the current is less than 16A;
(7) Working voltage: 11 ～ 17 V can work normally;
(8) Test voltage: 12.5 V, 13.8V;
(9) Third-order intermodulation: less than or equal to -32 dB (test conditions are: dual signal, interval 0.67 kHz, average power 50 W, PS = 13.8V;
(10) Harmonics: Even harmonics are less than or equal to -20 dB, and odd harmonics are less than or equal to -13 dB.
2 Line plan
The use of tube push-pull circuit, negative feedback circuit and transmission line transformer can meet the requirements of gain flatness. The power amplifier unit is composed of excitation amplifier stage, last pre-amplifier stage, last-stage power amplifier stage, type II attenuation input network, bias circuit, total current control and temperature control circuit. Calculate the total gain of the power amplifier according to the requirements of the RF input signal amplitude and output power:
VIN = 0 dBm; POUT = 50 dBm; GP = 50 dB
According to the total gain of the power amplifier and the frequency band range (1.6 ~ 30 MHz), the power amplifier is designed in three stages. The gain distribution and the circuit form adopted by each stage are as follows: the first stage adopts the class A working state, the gain is greater than 25 dB, and the output power is about 25 dBm; the last front stage adopts the class A and B working state, the gain is greater than 16dB, and the output power is about 41 dBm ; The final stage adopts Class A and B working conditions, the gain is greater than 12 dB, and the output power is about 53 dBm.
The transmission line transformer coupling is used between all levels, and the imported magnetic double-hole magnetic ring is used as the magnetic material. The magnetic flux density is consistent well, which is good for the gain flatness of the entire frequency band. The transmission line transformer is used to transmit high-frequency energy in a wide frequency band, and the matching between the two-pole amplifier and the impedance matching between the final amplification and the load are realized. Due to the limited number of low-voltage power amplifier tube markets, the final stage uses a transistor with an output power of 80 W, and the final stage uses a push-pull output to the tube, thereby ensuring power surplus. The advantages of Class A power amplifiers are good linearity, low distortion, and good noise figure. It has an almost undistorted impulse response below the 1 dB compression point. The passband fluctuations at different output levels are small and those at different output levels The phase and gain are unchanged, and the Class A working state is adopted in the first stage in order to obtain good linearity.
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