The high-power wide-band linear RF amplifier module is extensively utilized in critical communication systems such as electronic warfare, radar, and detection. Its broadband and high-power generation technology plays a vital role in modern wireless communication systems. As wireless communication technology continues to evolve, the demand for broadband high-power solutions, wideband frequency hopping, and spread spectrum techniques has increased significantly. This has placed greater challenges on the design of solid-state linear power amplifiers, requiring them to operate over wider frequency ranges and deliver higher output power while maintaining modular and compact configurations.
In general, broadband RF power amplifiers designed for the HF to VHF bands typically employ FETs rather than simple power transistors. This choice is based on the high input impedance of FETs, which allows for easier impedance matching and simpler biasing circuits. Additionally, FETs offer higher gain and better linearity, making them ideal for such applications.
This paper presents a high-power wide-band linear RF amplifier designed using MOSFETs, operating in class AB push-pull mode. It functions within a frequency range of 6 MHz to 10 MHz and delivers an output pulse power of 1200W. After thorough debugging, the amplifier demonstrates stable operation and reliable performance. The testing process involved various instruments such as a signal multiplier, spectrum analyzer, power meter, high-power coaxial attenuator, network analyzer, and RF signal generator.
### 1. Pulse Power Amplifier Design
#### 1.1 Circuit Design
The designed broadband high-power pulse amplifier module requires a working frequency band exceeding four octaves, with a large output power and strong suppression of harmonics and clutter. Since harmonics may fall within the operating frequency range, the amplifier must maintain a high level of linearity.
To meet these requirements, the RF power amplifier chain in this design uses three stages of FETs, all implemented as MOSFETs. Each stage operates in class AB mode, using a push-pull configuration to ensure broad bandwidth operation. A positive voltage supply was chosen for convenience, and enhanced MOSFETs were used. To further broaden the frequency range and achieve high power output, transmission line broadband matching techniques and feedback circuits were incorporated.
Given the requirement for high-power pulsed emission, the first and second stages of the amplifier use MOSFETs (IRF510 and IRF530) with fast switching characteristics to preserve the integrity of the pulse modulation signal's rising and falling edges, reducing clutter and harmonic interference. For the final stage, which needs to deliver 1200W of pulse power, the MOSFET MRFl57 was selected without employing power synthesis techniques. The schematic diagram of the RF pulse power amplifier circuit is shown in Figure 1.
The signal path begins at the source, passes through several intermediate amplification stages, and is finally amplified by the power stage before being transmitted via an antenna. In Figure 1, the input signal is 20–21 dBm at 50Ω, with a working voltage of 15V and 48V. The 15V supplies the first two power amplifier stages, while the 48V powers the final stage. A 6V regulated output can be derived from either voltage source. The overall design uses class AB amplification, with a standing wave ratio of 1.9.
After intermediate stage amplification, the signal undergoes impedance transformation via T1 (4:1), then enters the power amplifier. During the upper half of the signal, Q1 is activated, while Q2 is turned on during the lower half. This is followed by T2 (16:1) for the next stage. Similarly, Q3 is active during the upper half of the signal, and Q4 during the lower half. The signal is then transformed via T3 (4:1) into the final stage, where the current is increased to drive the high-power MOSFET MRFl57. Finally, T4 (1:9) ensures a 50Ω output impedance.
Negative feedback is used to stabilize the power gain across the entire frequency band, maintain gain linearity, and improve the stability of the input return loss and low-frequency signal amplification. Additionally, each stage incorporates a variable resistor to set the bias voltage of the transistors, minimizing crossover distortion and ensuring waveform symmetry.
#### 1.2 PCB and Transmission Line Transformer Design
To ensure consistent signal amplification across the entire frequency band and reduce the impact of clutter and harmonics, the design employs class AB power amplification to maintain circuit symmetry. When designing the PCB, care was taken to make the copper traces symmetrical and of equal length. A lead-tin light board was used to facilitate dielectric constant selection, and Smith chart software was employed to simulate and optimize the shape and size of the copper traces for optimal impedance matching.
A key aspect of the design is the fabrication of transmission line transformers, which are essential for impedance matching between the signal source and the power MOSFETs. These transformers help maximize the bandwidth potential of the tubes. Two critical aspects in their design and use are the impedance matching between the source, load, and transmission line, as well as proper connection and grounding methods. Given the class AB configuration, the primary and secondary coils should be as symmetrical as possible.
Four transmission line transformers (T1, T2, T3, and T4) were used in the design. The secondary coils of T1 and T2 had one turn, while T3’s secondary coil had two turns to prevent magnetic saturation at low frequencies. T1, T2, and T3 used the core of a coaxial line SFF-1.5-1 for the primary coil, with copper foil for the secondary. T4 was an imported high-power transformer (model: RF2067-3R). The design of T1 is shown in Figure 2.
The secondary coil of T1 was constructed by passing a copper tube through a magnetic ring and welding it to copper plates at both ends. The design of T2 was similar but with a different number of turns on the primary coil. The secondary coils of T3 were modified to enhance low-frequency signal passage by bending copper foil into an arc shape, as shown in Figure 3.
#### 1.3 Thermal Design
RF power amplifiers generate significant heat due to their high output power and power consumption. Proper thermal management is crucial to ensure reliable operation. Based on the power dissipation (PD) of each stage and the thermal characteristics of the components, factors such as thermal resistance (ROJC), allowable junction temperature (TJ), and ambient temperature (TA) were considered to determine the appropriate heat sink material and size.
In this design, the operating temperature was set to 55°C, with an aluminum heat sink measuring 290mm × 110mm × 35mm. A DC fan was also used to cool the final stage MOSFET (MRFl57).
### 2. Pulse Power Amplifier Assembly and Commissioning
All amplifier tubes in the design are MOSFETs, which are highly sensitive to static electricity. Special care was taken when handling the last-stage high-power MOSFET (MRFl57), as it is expensive and easily damaged. Before circuit design, simulations using Multisim or PSpice software were conducted to familiarize with the behavior of IRF510 and IRF530.
During initial debugging, the bias voltage for the last-stage MOSFET (MRFl57) was omitted, and the static operating point was set using the amplification effect of the first two stages. The test results showed an output of about 100V p-p (high-impedance input). The operating point voltage of each transistor was kept slightly above the threshold voltage (VGS(TH)) to avoid excessive current. Fine adjustments to the gate-end transformer helped minimize waveform distortion, and an oscilloscope was used to monitor the output waveform.
When debugging the final stage, extra caution was required due to the high cost of MRFl57. Static working voltages were set, and dynamic adjustments were avoided. The output was connected to a spectrum analyzer through a 5011 high-power coaxial attenuator to measure output power and harmonic components.
The wideband high-power amplifier was assembled and tested in a laboratory environment over an extended period using a transmitting coil. The tests confirmed that the amplifier operated reliably, meeting the design specifications. It successfully handled high-power broadband RF pulses and demonstrated excellent performance in a detection device operating within the target frequency range.
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