Based FLM3135-18F S-band microwave power amplifier design

S-band microwave power amplifiers for radar transmitter, wireless communications, measurement equipment, systems key components. Microwave amplifier gain, output power, nonlinear and other parameters directly affect the system performance. S-band microwave power amplifier is the core of the development work under the conditions of large signal power amplifier input and output broadband matching circuit. High-power amplifier output impedance is very low, usually less in the 5 Ω, which transforms the impedance matching circuit than the large, leading to difficulties in the direct design of broadband matching circuit. Meanwhile, the power amplifier intermodulation and harmonic non-linear also with the matching circuit, the circuit design must be taken into account.

The key is to input and output microwave power amplifier matching circuit design. The power amplifier matching circuit can use approximate linear dynamic impedance matching method of large-signal S parameters of simulation, harmonic balance method can also be non-linear methods such as simulation. This article describes the internal impedance matching is based on the nature of the S-band FET amplifier design, no need to design matching circuits, reducing the optimal design of the power amplifier module, thus shortening the development cycle, reducing design costs and improve the technical indicators.

2 Power Amplifier System

2.1 system and the principle

PA System indicators determine its composition and design of linear power amplifier design with emphasis on intermodulation. Analysis of third-order intermodulation characteristics, ignoring the amplifier memory, the transmission characteristics of third-order Taylor formula can be approximated as:

Based FLM3135-18F S-band microwave power amplifier design

The formula: Vout (t) for the amplifier output voltage, Vin (t) for the power amplifier input voltage. When the inputs are two-tone signal that Vin = A1cosω1t + A2cosω2t, in addition to amplified signal frequency components ω1 and ω2, falls band amplifier also generate third order intermodulation 2ω1-ω2 and 2ω2-ω1, so that A1 = A2 = 4, substituting into equation (1), available:

Based FLM3135-18F S-band microwave power amplifier design style in: IM3 that third-order intermodulation. Third-order intermodulation generally can not filter filter, you must select the appropriate amplifier and the design of appropriate matching circuit. From (2) can be seen, increasing the input power amplifier 3dB, while the third-order intermodulation is back to back 6 dB.

Use FLM3135-18F single-stage gain of 10 dB ~ 15 dB, in order to meet the targets proposed by 25 dB, to be cascaded amplifier. Cascaded amplifier drive level on the final output of the third-order intermodulation can be calculated by the following formula:

Based FLM3135-18F S-band microwave power amplifier design

Type in: dIM3 is driving the introduction of the power amplifier amplifier output IM3 IM3 change in volume; IM3 (driver) and IM3 (final), respectively, the final stage driver amplifier and amplifier IM3 (dBc).

Therefore, from (3) available:

Based FLM3135-18F S-band microwave power amplifier design

About 2.2 FLM3135-18F

GaAs FET not only for small signal amplification, can be used to power amplifier, the operating frequency can be expanded to millimeter wave band, combine multiple single device to achieve higher output power. Determine the capacity of FET output power depends on three factors: the drain - gate breakdown voltage, maximum channel current and thermal characteristics. To get large output power in addition to the above three factors, but also to avoid the introduction of resistive and capacitive parameters, increasing the gate width can be any increase in channel current, but increase the gate width will increase the number of parasitic parameters, especially the increase gate-source capacitance and gate resistance, so the gain will decrease with the increase of gate width, therefore, lower power FET power gain, the actual work of FET power amplifier into saturation, and 1 dB, the gain is lower. Another source drain series resistance and inductance of the existence of power gain are so down.

FLM3135-18F is FUJITS produced the band of 3.1 GHz ~ 3.5 GHz microwave FET, the internal integrated

Input and output impedance matching network. In the 50 Ω system, the standard communications band can have better power and gain. LM3135-18F of the basic performance parameters such as listed in Table 1.

Based FLM3135-18F S-band microwave power amplifier design

2.3 The first stage driver amplifier

S-band power FET integrated amplifier gain and the gain is generally 8 dB ~ 12 dB, to meet the design specifications of the output power requirements need to add before the final amplifier stage driver amplification. Driver amplifier must not only have enough bandwidth, gain and output power, but also high enough linearity does not impinge on the system intermodulation, harmonic impact. From (4) know that, in order to drive the overall level of the index-modulation effect is less than 1 dB, 6 dB in the output regression testing conditions, IM3 should be less than -53 dBc. Mainly adopted two measures to ensure the linearity: First drive to enlarge the work of the class A amplifier. A class of best linear amplifier does not introduce a large distortion, while working in Class A FET power amplifier input and output impedance of Q values generally low, easy-to-broadband matching; Second, choose the required output power is greater than the high-power linear GaAs power amplifier tubes, designed to take redundancy.

Drive-level amplification selected Motorola's 1 W GaAsFET. Amplifier matching circuit microstrip line and high-Q 值 ceramic capacitor and a half set of the total circuit form, with the S Shen Shu Fang Zhen approximate GaAsFET features, then adjust the output matching circuits. The actual measured output of 24 dBm, driver amplifier IM3 is less than -60 dBc, does not influence the system output spectrum. Figure 1 shows the schematic design of driver amplifier.

Based FLM3135-18F S-band microwave power amplifier design

2.4 The last stage power amplifier

General narrow-band power amplifier within the tube impedance parameters are known, power amplifier matching circuit design is the key to the whole system. The design of a two-port linear passive network, a port load is 50 Ω, another port tube amplifier output impedance and input impedance of the output conjugate match. Impedance directly affect the results of the output gain and power amplifier. In order to achieve the desired match results, often using microstrip lines and shunt capacitance of the hybrid power amplifier matching network circuit, input and output circuit topology similar to the low-pass circuit. However, due to the high-power GaAs FET's total gate Kuan very, very low impedance device, resulting in input and output impedance by Fengzhuang parasitic capacitance and inductance in governing foreign Pipei Fangda Qi Dian Lu Ke very difficult, particularly high frequency, design Daikuan power FET amplifier is the most direct way to use the microwave package to resolve within the matching low input impedance of the device.

S-band FLM3135-18F microwave tube field effect transistor with internal impedance matching network, therefore, focus on design of power supply design only configure the network, while the input and outputs can be lumped components and distributed components as the matching network. Broadband and power levels greater than 5 W, it is usually lumped elements the state selected power FET as the input matching circuit, the use of metal wire bonding to achieve the lumped inductance and capacitance is to use high dielectric constant ceramic metal "insulator" metal type. Capacitor parasitic inductance and resistance must be small, and has sufficient thermal and mechanical strength, small temperature coefficient, 40 V or higher breakdown voltage. Since the output impedance much higher than the input impedance of the output matching network with a set of components to achieve the total distribution. Figure 2 is a schematic end amplifier.

Based FLM3135-18F S-band microwave power amplifier design

3 Conclusion

With matching circuit within the microwave FET FLM3135-18F input and output characteristics of a good, in-band power gain characteristic flat element need to design complex input and output circuit, the circuit reliability. Final test results meet the specification, to meet user requirements, has been successfully used for target recognition and the perception of a model platform.