Design of microwave low noise amplifier and simulation design with ADS

0 Introduction

With the rapid development of the communication industry, people's requirements for various wireless communication tools are getting higher and higher. Low power radiation, long range and large coverage have become the common pursuit of various operators and even wireless communication equipment manufacturers, which also puts higher requirements on the receiving sensitivity of the system.

1 The role of microwave low noise amplifier

In general, the receiving sensitivity of a receiving system can be expressed by the following calculation formula: As



can be seen from the above formula, in various specific (bandwidth B _W , demodulation S/N are fixed) wireless communication systems, the key factor that can effectively improve the sensitivity is to reduce the noise factor NF of the receiver, and the key component that determines the noise factor of the receiver is the low noise amplifier at the front end of the receiver.

Figure 1 shows the principle block diagram of the receiver RF front end. As can be seen from Figure 1, the main function of the low noise amplifier is to amplify the weak signal received by the antenna from the air and reduce noise interference so that the system can demodulate the required information data. Therefore, the design of the low noise amplifier is crucial to the entire receiver.



2 Main Technical Indicators of Microwave Low Noise Amplifiers

2.1 Noise Factor The

noise factor is defined as the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio of the amplifier, that is,



for a single-stage amplifier, the noise factor is calculated as:



where Fmin _is the minimum noise factor of the transistor, which is determined by the amplifier tube itself, Γopt _, Rn _and Γs _are the optimal source reflection coefficient, transistor equivalent noise resistance and source reflection coefficient at the input of the transistor when Fmin is obtained. _For

multi-stage amplifiers. The noise factor should be calculated as:



where _NFn is the noise factor of the nth stage amplifier, and Gn _is the gain of the nth stage amplifier.

For systems with high noise factor requirements, since the noise factor is very small, it is inconvenient to express it with the noise factor, so it is often expressed by noise temperature. The conversion relationship between noise temperature and noise factor is:


where Te _is the noise temperature of the amplifier, T0 ₌ 2900K ^, and NF _is the noise factor of the amplifier.



2.2 Amplifier Gain

The gain of an amplifier is defined as the ratio of the amplifier output power to the input power:

G=P _out ／P _in (7)

Generally, increasing the gain of a low noise amplifier is very beneficial to reducing the noise figure of the whole device, but too high a gain of a low noise amplifier will affect the dynamic range of the entire receiver. Therefore, in general, the gain of a low noise amplifier should be determined in combination with the overall noise figure of the system, the dynamic range of the receiver, etc.

2.3 Reflection Coefficient

From equation (3), it can be seen that when Γ _s =Γ _opt , the noise figure of the amplifier is the smallest, NF ₌ NFmin _, but at this time, from the perspective of power transmission, the input end will be mismatched, so the power gain of the amplifier will be reduced, but sometimes, in order to obtain the minimum noise, appropriately sacrificing some gain is also a method often used in the design of low noise amplifiers.

In addition, the input-output standing wave ratio, dynamic range, operating frequency, operating bandwidth and in-band gain flatness of the low noise amplifier are also very important indicators and need to be considered during design.

3 Circuit simulation design

The frequency range required for this circuit design is 1.95～2.05GHz, the noise figure: Nf should be less than 2 dB, the in-band gain is G greater than 10 dB, and the input and output impedances are 50 Ω. The above indicators are used to select the circuit transistors and ADS simulation.

3.1 Transistor selection

According to the performance requirements of the amplifier, this design uses HP's AT-41511 as the core device for design. Since the device model of this type of transistor is included in the ADS software, it can be used directly in the design and simulation process without having to build the device model yourself.

3.2 ADS simulation comprehensive index realization

During simulation, the noise figure, amplifier gain, and stability factor can all be added to the optimization target for optimization, and the gain flatness index can be met by limiting the in-band amplifier gain, and finally meet the requirements of each index. The input matching network can also be optimized by repeatedly adjusting the optimization method and optimizing the weight (Weight) in the target. However, the optimization of some circuit indicators may also lead to the deterioration of some other indicators. At this time, some optimization variables can be added as needed.

Figure 2 shows the S parameter diagram after a random optimization.


The simulation results show that the circuit has basically achieved relatively good performance, and has good input-output matching, high gain and stability factor, and good noise factor.

3.3 Package model simulation design

After completing the sp model design, it is necessary to replace the sp model with the package model for further design. The specific work to be done is as follows:

(1) Replace the sp model with the package model;

(2) Select the DC operating point and add the bias voltage;

(3) Design the feeding circuit (use of resistor divider, fan-shaped line, high resistance line, etc.);

(4) After replacing with the package model, the various parameters may change. If the technical indicators are not met, the schematic diagram of the package model can be simulated and optimized.

When designing the package model. The circuit shown in Figure 3 can be used to simulate the IV characteristics of the device to select its DC operating point. When designing the



bias circuit, in order to prevent the influence of the AC signal on the DC power supply, a 1/4 wavelength high resistance line can be added between the power supply and the feeding point to curb the AC signal. If there is a microstrip line with a short circuit at the end of the circuit, in order to avoid DC short circuit, a DC blocking capacitor should also be inserted at the ground end.

4 Conclusion

From the simulation design process, we can see that it is very convenient to use Agilent's ADS software for RF circuit design, simulation and optimization. It contains a rich schematic model library, multiple simulation analysis methods and a series of easy-to-use and powerful design tools. This can make complex RF circuit design work simple and fast, save a lot of manual calculation design process, and improve design work efficiency. The design of the microwave low-noise amplifier given in this article is relatively successful and basically meets the index requirements.