The application of broadband power amplifier began to expand from military to civil. At present, it has broad application prospects in the fields of wireless communication, mobile phone, satellite communication network, global positioning system (GPS), live satellite reception (DBS), its communication technology and millimeter wave automatic anti-collision system. Broadband power amplifier also plays an important role in optical transmission system.
In the fields of wireless communication, electronic warfare, electromagnetic compatibility testing and scientific research, there is a great demand for RF and microwave broadband amplifiers, and these fields have different requirements for broadband amplifiers, especially in the application of communication system and electronic warfare system, there are special requirements for the performance indexes of broadband low noise and power amplifier. In the design of uplink narrowband amplifier, the port matching is generally designed according to low noise or conjugate matching, so as to obtain low noise amplifier or maximum output power. However, under the condition of broadband, the change of input / output impedance is relatively large, so the concept of conjugate matching is not appropriate. Because of this, the matching circuit design method of broadband amplifier is also different from that of narrowband amplifier. The circuit structure of broadband amplifier can be divided into the following types:
Balanced amplifier; Feedback amplifier; Distributed amplifier; Lossy matching amplifier; Active matching amplifier; Darlington on structure. Various structures have their own characteristics and applicable conditions. Reasonable selection should be made according to the performance index requirements of specific amplifiers in the design.
1 Structure and principle of broadband power amplifier
1.1 index analysis of broadband power amplifier
Many indexes of broadband power amplifier are the same as those of ordinary power amplifier, such as saturated output power, p1db compression point, power efficiency, intermodulation distortion, harmonic distortion, microwave radiation, etc., but broadband power amplifier also has its particularity.
1.1.1 working band width
The operating frequency band usually refers to the continuous operating frequency range in which the amplifier meets all its performance indexes.
1.1.2 gain flatness and fluctuation slope
Gain flatness refers to the difference between the highest gain and the lowest dB in the frequency band. The gain flatness of multiple frequency range amplifier is generally ± 1 ~ ± 3 dB. In the microwave system, more than two broadband amplifiers are sometimes required to be cascaded, and the gain flatness of the cascaded amplifier will deteriorate, which is caused by the inconsistency between the output standing wave ratio of the front stage amplifier and the input standing wave ratio of the rear stage amplifier. Especially in the wide frequency band, the reflection phases between stages are sometimes superimposed and sometimes offset, increasing the fluctuation. Therefore, it is generally necessary to add a matched attenuator between the stages of the cascade amplifier. Ambient temperature, DC bias voltage and time aging have great influence on the gain value, but have little influence on the gain flatness.
1.1.3 standing wave ratio and reflection loss
The VSWR index of broadband amplifier is more difficult to guarantee than that of narrow-band amplifier. The octave amplifier can reach VSWR “2”. When the requirements are high, ferrite isolator can be used to improve the standing wave ratio. However, in the case of multiple octaves, applicable ultra wideband isolator cannot be obtained, so the standing wave ratio cannot be very good.
Late double diffusion MOS (LDMOS) adopts double diffusion technology to conduct two boron and phosphorus diffusion successively in the same window. The channel length can be accurately determined by the difference of transverse junction depth between the two impurity diffusion. The channel length l can be made very small and is not limited by the lithography accuracy. Due to the short channel effect of LDMOS, the transconductance, drain current, working frequency and speed are much higher than those of general MOSFET; In RF applications, LDMOS has better linearity, larger linear gain, high efficiency and lower cross modulation distortion. At the same time, LDMOS is based on mature silicon process devices, which can reduce the cost several times compared with other microwave transistors.
1.3 structure of lossy matched amplifier
The lossy gain compensation matching network can achieve an “important” compromise between gain, emissivity and bandwidth. Moreover, the impedance characteristics of this matching network can also improve the stability of the amplifier and reduce its size and price, because the scheme of the lossy matching circuit is simple. In many practical cases, in order to improve broadband matching with minimum gain fluctuation and input reflection coefficient, it is very effective to parallel resistive elements at the input of transistors. For higher frequencies, the use of inductive reactance elements in series with resistors has additional matching improvement than the basic type. For broadband lossy matched MOSFET high power amplifier, it is best to use series lumped parameter inductance. The structure used in this design is shown in Figure 1. A capacitor connected in parallel with the resistor can improve the power gain at the high end of the frequency band.
1.4 broadband impedance matching circuit
After using the lossy matching network to flatten the gain and solve the stability problem, it is necessary to match the input and output impedance of the tube to 50 Ω, which requires a broadband impedance matching circuit. When designing the input impedance matching circuit, it is necessary to consider stability, gain, flat gain, input standing wave ratio, etc. when designing the output matching circuit, it is necessary to consider harmonic suppression, output standing wave ratio, loss, etc. before designing the output matching circuit, it is necessary to carefully analyze whether to select the output impedance parameters according to the maximum power output or the rated power output, In order to obtain the required output power. In the design, a hybrid matching circuit composed of microstrip and capacitor is selected. The circuit structure is composed of N R-type circuits in series.
Design, simulation and optimization of broadband power amplifier
The working frequency band of the power amplifier designed for the project is 700 ~ 1100 MHz, the gain is greater than 30 dB, the port standing wave ratio is less than 1.5, the output power is greater than 33 DBM, and the gain flatness is ± 1 dB. In order to meet the design requirements, two-stage amplification is adopted. The front stage amplifier adopts mmicpower amplifier hmc481mp86, a 6dB resistance attenuator is added in the middle, and the last stage adopts Freescale LDMOS field effect transistor mw6s004n.
2.1 circuit diagram of broadband power amplifier
In Figure 2, the front stage Amplifier MMIC power amplifier hmc481mp86 uses the large signal S-parameter file hmc481mp86 embedded.s2p provided by the manufacturer to replace the simulation. The last stage adopts the LDMOS power tube model provided by Freescale, and its input and output impedance values are obtained by the load traction method using ads, When matching, the impedance value at each frequency point in the whole frequency band shall be fully considered.
2.2 ADS simulation and optimization results
Figure 3 (a) shows the curve of S21. When the input is 0 DBM, the power gain is 34 dB, and the gain meeting the design requirements is greater than 30 dB; When the output power is greater than 33 DBM, it can be seen from several points in Figure 3 that the gain flatness is ± 1 dB; Figure 3 (b) shows the stability coefficient; Figure 3 (c) shows the input standing wave ratio; Figure 3 (d) shows the output standing wave ratio.
2.3 test results
Physical test and commissioning, use spectrum analyzer to measure power, use network analyzer to see the gain flatness and input / output standing wave ratio of the whole frequency band, and add an isolator behind the output port according to customer requirements. When the input signal power is o DBM, the test data are shown in Table 1.
Broadband power amplifier has broad application prospects. The design requirements are different from general power amplifier, and the requirements for impedance matching are more strict. In this paper, the gain flatness of power transistor is improved by using lossy matching network, which makes the structure of impedance matching circuit simple. The indexes of the whole power amplifier meet the user’s design requirements and have been delivered for use.
Responsible editor: GT