High Power Broadband RF Power Amplifier Design

Use MOS FET (MOSFET), take a push-pull power amplifier class AB mode, with broadband transmission line matching transformer technology, to design a broadband high power RF pulse power amplifier module, the output pulse power up to 1200W, the work of band 0. 6M ~ 10MHz. Debugging and practical results show that. The amplifier stable and reliable performance.

Broadband linear RF power amplifier modules are widely used in electronic countermeasures, radar, detection and other important communications system, the broadband, high power generation technology is the wireless electronic communications system, a critical technology. With the development of modern wireless communications technology, broadband high-power technology, broadband frequency hopping, spread spectrum technology to solid-state linear power amplifier design is put forward higher requirements, that the frequency of broadband RF power amplifier output power greater of The overall modular equipment.

Typically, the HF ~ VHF band design of broadband RF power amplifier, field effect transistor (FET) design than the use of conventional power transistor designed to facilitate simple, is based on a relatively high input impedance FET, and the relative frequency input impedance change will not have too much bias, easy to impedance matching, another relatively simple bias circuit, the design of high gain amplifier, good linearity.

This high-power wideband linear RF amplifier is to use MOS field effect transistor (MOSFET) to design and adopt AB class push-pull power amplifier, and its work frequency band O 6M ~ 10MHz, the output pulse power is 1200W. After debugging use, amplifier stability and reliable performance. Debugging, testing and practical use of test instruments are shown crossing, spectrum analyzers, power juice, high-power coaxial attenuators, network analyzer and RF signal generator.

A pulse power amplifier design

1.1 Circuit Design

Design of wideband high-power pulse amplifier module requires the work of more than four octave band, and the output power of higher harmonics and clutter suppression; other harmonics are at work as the band, thus requiring amplifier module has a high linearity.

Design requirements for the design of RF power amplifiers using three FET amplifier chain, all the selected MOSFET. Each level are used to enlarge class AB power amplifier mode, and both use push-pull, in order to ensure the broadband power amplifier module can work. Taking into account the positive voltage power supply is usually more convenient to use, so use enhanced MOS FET. In addition to development of broad-band and output power, use of broadband transmission line matching technology and the feedback circuit, in order to meet the design requirements.

High Power Broadband RF Power Amplifier Design

The establishment of launching channels are produced in the RF signal source through a few levels of the middle-level amplification only then input to the power amplification stage, the final RF signal transmission through the antenna to go out.

Figure 1, the input signal is 20 ~ 21dBm, 50Ω input; working voltage of 15V and a 48V, 15V which is the first, second amplifier to provide operating voltage, 48V power amplifier provides the final level of working voltage; 6V regulated output can be carried out using 15V or 48V voltage regulator transformation, circuit design uses the whole class AB power amplifier, designed for standing wave ratio of 1.9. Amplified through the middle-level signal, the first by Tl (4:1) impedance transformation amplifier backward people. In the first half cycle Q1 signal conduction, signal data later this week conduction of Q2; then turns through T2 (16:1) impedance transformation into the second stage amplification, the same signals in the first half cycle of Q3 turns on, the data later this week of Q4 turn, complete the full cycle of the energy signal amplification; into the last one to enlarge the use of T3 (4:1) impedance transformation in order to continue to increase the work of current drive power MOSFETMRFl57. To ensure the 50Ω output impedance of the output transformation for the T4 (1:9).

Circuits using negative feedback circuit is aimed at the entire bandwidth of frequency response to generate a relatively stable power gain, maintain gain linearity, while the introduction of negative feedback circuits help to improve input return loss and low frequency signal power amplifier stability sex.

Additionally, each level of circuit design, use the sliding rheostat to set the bias voltage of each tube, this greatly reduces the occurrence of crossover distortion, where possible, zoom in on the signal, data later this week of the wave distortion.

1.2 Circuit Board (PCB) and the transmission line transformer design

To ensure the consistency of band signal amplification, reducing clutter and harmonic, broadband high power RF amplifiers using class AB power amplifier, to ensure the symmetry of the circuit. In the design of PCB, the copper film as much as possible to ensure the alignment of symmetric form, the same length. To facilitate the selection of PcB board dielectric constant, the PCB board for the tin plate light. The signal input and output using the Smith Chart Software Calculation and Simulation of copper film traces the shape, size, to ensure good impedance matching.

High-power broadband RF pulse power amplifier design in a dark area on behalf of copper area. Copper tube through the magnetic ring after the first copper film through both ends of the plate and welded together to complete the secondary coil. Tl T2 basically similar design, but using coaxial SFF-1.5-l of the core lines of primary line winding circle the number is different.

Some changes in the production of secondary coil 73, the purpose is to enhance the adoption of low-frequency signal level. Do not use copper pipes, using copper foil bent in an arc. Shown in Figure 3.

High Power Broadband RF Power Amplifier Design

In each bead hole through two copper pieces, each with both ends of the copper film plate welding, so that the coil secondary coil is two laps, and then complete the primary coil impedance than the winding. The aim is fixed impedance ratio increases with the primary and secondary turns of to improve low frequency performance of the amplifier.

1.3 Thermal Design

Any RF power amplifier, the output power of large power tubes are large, very high heat, heat must be on the tube. According to PD at every level of power tubes and pipes of the thermal characteristics of targets, the heat index reached the devices, including devices die case thermal resistance ROJC, the device allows the junction temperature of T1, the work environment such as temperature TA can be calculated need to use the cooling material size and type. The design of the device working temperature 55 ℃, the use of aluminum heat sink size is 290mm × 110mm × 35mm, and the need to use a MOSFET DC fan for cooling the final deal.

2 pulse power amplifier assembly and commissioning

Amplifier tubes used in the design of all MOSFET, because of its anti-static performance is very poor, too easy for welding equipment will be on static burn the tube, especially the last one of the power MOSFET (MRFl57), so pipe installation Be careful when. Circuit design before, you can use Multisim software or device model Pspice software to use familiar IRFSIO and IRF530.

Circuit start debugging, you can not start the last level MOSFETMRFl57 for bias voltage setting. First by testing the amplification effect of the former two to set MRF157 static operating point, the first two tests are the results of signal amplification 100VVp-p (high impedance input) or so. Debugging the operating point voltage of each tube should not be too high, slightly higher than the turn-on voltage VC (TH) can be. Supply side must be monitored in the operating current to prevent overcurrent. Extreme fine-tuning each tube grid by a transformer to adjust the static working point, so as to minimize waveform distortion. At this point you can use the oscilloscope waveform monitor output. According to the former two circuit debugging of the actual results, first-class major impact on the enlarged range, while the second stage affected the amplified wave.

Debugging last a power amplifier, because MRFl57 too expensive, must be very careful. Every time debugging, as first set up a static voltage of each tube, do not dynamically change the static working point. Terminal access to 5011 high-power coaxial attenuator and input to the spectrum analyzer. Through the spectrum analyzer in frequency domain waveforms can be output power, and harmonics.

This broad-band high-power amplifier designed in a lab environment, complete assembly and test, and a long coil with the launch of a joint trial. Experimental and practical that the normal operation of the amplifier, reliable, able to complete high-power broadband RF pulse amplification to meet the design requirements, and working under the band play a significant role in a detection device with good results.