Design and Implementation of Broadband Low Noise Amplifier Circuit Based on GSM

A multi-mode wideband low noise amplifier (LNA) suitable for base station GSM was designed by using ADSP software from Agilent's ADS software by selecting AT41486 transistor with low noise figure, good performance and low price. The amplifier operates in the frequency range of 1700MHz to 2000MHz, the noise figure is not more than 1.8dB, the gain is not less than 13dB, the gain flatness in the band is not more than 0.5-dB, the input voltage standing wave ratio is less than 2.0, and the output voltage standing wave The ratio is less than 3.3. The circuit has the characteristics of small noise figure, large gain, small gain flatness in the frequency band, and excellent performance.

0 Preface

Low-noise amplifier (LNA) is a key circuit in wireless communication equipment. It is located at the forefront of the receiving equipment and amplifies the weak RF signal received from the antenna. Its noise performance and gain have an important impact on the performance of the whole machine. Therefore, it is required. The circuit has as high a gain as possible with less than the required noise figure. In general, the minimum noise figure and maximum gain cannot be achieved at the same time, and their indicators are mainly determined by the input matching circuit. In order to meet the design requirements, the principle of balancing is generally adopted.

In the domestic GSM system, the operating frequency covers the range of l710MHz to 1990MHz. If a narrowband LNA circuit is used, it can only be used in one system, and the application is limited. The wideband LNA circuit designed in this paper covers the above frequency range with its operating frequency, which makes the LNA circuit at the front end of the receiver very flexible.

1 Design simulation and optimization of low noise amplifier circuit

1.1 Performance indicators and device selection

The design requires an operating frequency of 1700MHz to 2000MHz, a noise figure of less than 1.8dB, a gain of more than 13dB, an input voltage standing wave ratio of less than 2.0, and an output voltage standing wave ratio of less than 3.5.

The design uses Agilent's AT41486 tube. The device is a general-purpose NPN bipolar silicon tube with excellent characteristics such as low noise, high gain, and good frequency characteristics. The model uses the Pb parameter model that comes with ADS.

1.2 Circuit stability

Circuit stability is both an important indicator of the amplifying circuit and a prerequisite for the normal operation of the circuit. Circuit stability is the most basic requirement, and any RF circuit has the potential to oscillate in certain frequency bands and certain terminal conditions. There are many factors that cause the circuit to be unstable. There are mainly S parameters of the pipe, input and output matching circuits, and selected working points. Stability metrics are usually expressed by the stability factors K, B, Δ, which are functions of the device small-signal S-parameters. When using the ADS tool, it is very convenient to calculate the K and B parameters, so they are used here to measure the stability of the circuit. Calculated as follows:

As long as K"l,|B|"0, the circuit can work stably

The stability factor of the transistor AT41486 was simulated by ADS, and the results are shown in Table 1.

It can be seen from Table 1 that in the range of 1600MHz to 2100MHz, the requirements of K"1 and B"0 are satisfied, indicating that the transistor is absolutely stable in the frequency band in which it operates, and no stabilization measures are required.

1.3 Noise figure

The noise figure of the RF system is related to the noise figure F of each stage of the circuit and the power gain G other than the final stage. If the RF system includes an N-stage circuit in which the power gain of the i-th stage is Gi and the noise figure is Fi, the noise figure F of the radio system can be expressed as:

It can be seen from equation (5) that the noise figure F1 and the gain G1 of the first stage have the greatest influence on the overall noise index of the circuit, and the LNA is just the first stage of the system. Therefore, its design is very important and requires a sufficiently small noise figure. , as high as possible gain. For a single-stage LNA circuit, the noise figure can be calculated by the following equation (6).

Among them, the minimum noise figure of the transistor, the optimum source reflection coefficient when obtaining the minimum noise figure, and the equivalent noise resistance of the transistor are respectively determined by the characteristics of the tube itself, and the values ​​are shown in Table 2. Is the reflection coefficient of the input source. It can be seen from equation (6) that at the time, the noise figure F of the amplifying circuit is the smallest, equal to Fmin.

Since the gain Ggain of the amplifying circuit is to be taken into consideration, the actual matching can be appropriately deviated in order to satisfy the requirement of the noise figure F, so as to obtain a larger gain and satisfy the overall performance index. It can be seen from Table 2 that as long as the design is reasonable, a noise figure of less than 1.8 dB can be fully realized in the range of 1700 MHz to 2000 MHz. At design time, the input matches the initial selection and then optimizes based on other metrics.

1.4 LNA structure analysis and design

The circuit structure is shown in Figure 1. The design focus is on the input and output matching circuit. Considering the wide operating frequency range, the input matching uses a microstrip double-branch stub and the output is a microstrip single-segment stub. The DC offset is based on the tube data, using a 10V power supply, and the operating point is set to Vce=8.0V, Ic=10mA.

1.4.1 Input matching circuit

The optimum input impedance when obtaining the best source reflection coefficient by simulation is 11-j*14.2. In order to obtain the minimum noise figure, the input matching circuit is designed according to the impedance, and the noise figure is the smallest, but at this time, the input terminal is mismatched. Therefore, the gain is not the maximum, and the voltage standing wave ratio is not the smallest.

The design uses PTFE material, εr=2.65, H=0.8mm, T=35μm. In order to achieve wideband amplification, the input end uses a microstrip double stub. The input matching circuit that meets the requirements can be obtained by using the calculation tool as shown in Fig. 2.

1.4.2 Output Matching Circuit

Using the same method, the output impedance can be obtained as 78.05-j*42.65. The output uses a microstrip single stub to achieve conjugate matching. The single stub obtained by simulation is W1=W2=-W3=2.188mm, L4=29.33mm, L5=15.62mm.

1.5 Simulation Results and Analysis

Figure 3 is a complete circuit schematic. It is repeatedly optimized by ADS, and the results are shown in Figures 4 and 5 and Tables 3 and 4.

(1) As can be seen from Fig. 4, the gain of the circuit is greater than 13 dB in the frequency range of 1700 MHz to 2000 MHz, wherein the maximum gain point is 13.5 dB at the 1800 MHz position, and the gain fluctuation is 0.5-dB, which meets the performance requirements.

(2) According to the noise figure curve of Fig. 5, in the operating frequency band, the maximum noise figure is 1.999dB at 2000MHz, which is less than 1.8dB.

(3) It can be seen from Table 3 that the input voltage reflection coefficient VSWRl is less than 2.0, and the output voltage reflection coefficient VSWR2 is more than 2.5 below 1750MHz, not more than 3.0, which meets the design requirements.

(4) Table 4 shows that the requirements of K"1 and B"0 are met in the required frequency band, indicating that the circuit is absolutely stable.

In summary, this circuit meets the design requirements.

2 Conclusion

It can be seen from the above results that the low-noise wide-band amplifying circuit designed by using the Pb model achieves the design requirements by using the ADS software, and can be used for the front end of the GSM base station, has a wider working bandwidth than the conventional LNA, and has low gain and noise figure. For other products, it has important practical significance and application prospects. Due to the complexity of the RF circuit design and the accuracy of the requirements, the implementation of the circuit performance indicators is subject to a variety of factors, which require repeated adjustments and optimizations.