The weak signal received from the antenna is amplified by the amplifier at the front end of the RF product receiver, so the amplifier is required to have a certain gain and small noise figure.
In this paper, with the help of Agilent’s RF product circuit design software ads (advanced design system), a high gain low noise amplifier (LNA) is designed and verified by simulation.
1 Composition of RF product amplifier
The composition of single-stage RF product amplifier is shown in Figure 1, including RF product transistor amplification circuit and input and output matching network.
2 design of RF amplifier
2.1 transistor selection
The selection of transistor devices is very important for the design of low noise amplifier.
According to the working frequency, gain, noise figure and other index requirements, and considering that it is easy to obtain the corresponding component model during design and simulation, Avago’s high electron mobility transistor (E-PHEMT) atf-58143 is finally selected for design (the component model of atf-58143 can be downloaded from Avago’s website).
2.2 design of bias circuit
To design LNA, first determine the static working point and use the “DC” in ads_ FET_ The template of “t” can easily simulate its output characteristic curve. Referring to the datasheet of atf-58143, it can be determined that when VDS = 3V and IDS = 35mA, all design indexes meet the requirements.
After the static operating point is determined, the form and parameters of the bias circuit must be determined. Without manual calculation, it can be easily completed with the help of the design wizard tools in ads (designguide → amplifier → tools → transistorbiology). Because the component value provided by ads is non nominal, the designer needs to replace it with a nominal component close to the value provided by ads. The bias circuit and static parameters of each point are shown in Figure 2.
2.3 stability analysis and improvement
The conditions for the absolute stability of the transistor are k “1, △ 1. Of which:
If these two conditions cannot be met at the same time, the circuit will have the possibility of potential instability and oscillation. The stability simulation analysis of the transistor under the above bias conditions shows that its stability coefficient K1 does not meet the condition of absolute stability within the required operating frequency band.
The introduction of negative feedback can not only improve the stability of the circuit, but also expand the working bandwidth. Negative feedback is introduced into the RC circuit in series between the output and input, where r needs to meet the following conditions:
At the same time, a small inductance is added to the two sources to introduce negative feedback to further improve the stability. The value of the inductance can be determined only after repeated adjustment.
The circuit with negative feedback is simulated again, and the stability coefficient K “1 in its working frequency band meets the absolute stability condition.
2.4 design of input matching circuit with minimum noise figure and output matching circuit with maximum gain
If the input matching circuit and output matching circuit convert the input impedance Zin and output impedance zout of RF product devices to the standard system impedance Zo, that is, Zin = Zo, zout = Zo (or, as shown in Fig. 1), the transmission gain of the device can be maximized. However, when the input and output match, the noise is not the best. When Γ S= Γ Opt, the minimum noise figure can be obtained.
Equal power gain circle and equal noise coefficient circle can be easily drawn by ads, as shown in Fig. 3. It can be seen from the figure that if the M2 point is matched to the standard system impedance, the circuit can obtain the maximum gain; If the standard system impedance is matched from m3 point, the minimum noise figure can be obtained. Obviously, the maximum gain and minimum noise figure cannot be obtained at the same time. For low noise amplifier, the first thing is to consider the minimum noise figure, so m3 points are matched. By using the built-in tool “smithchartutilitytool” of ads, as long as the frequency, source impedance and target impedance values are set, the required input matching circuit can be designed.
After the input matching is completed, add an impedance measurement control in the schematic diagram to measure the output impedance, and then use the “smithchartutilitytool” to match the output impedance to the standard system impedance again to obtain the output matching circuit with the maximum gain.
When the matching of the output is completed, the return loss of the input will become worse because the equivalent impedance Zin looking inward from the input is changed. Therefore, the matching circuits at the input and output can be optimized and improved simultaneously by using the optimization control, or adjusted by using the tunig tool.
2.5 final circuit and simulation result analysis
The matched and optimized circuit is shown in Figure 4. The functions of each element in the circuit are: C6 and L6 are input matching circuits; C7 and L7 are output matching circuits; L1, L5, C3 and R5 are feedback elements; L3 and L4 are choke inductors; C4 and C5 are isolated direct coupling capacitors; C1 and C2 are bypass capacitors.
It should be noted that the feedback inductors L1 and L5 and the elements C6, L6, C7 and L7 in the matching circuit are often replaced by microstrip lines in engineering because of their small values.
The simulation results are shown in Figure 5. Its working bandwidth is 500MHz, the gain at the center frequency is close to 20dB, the input and output reflection loss is less than – 10dB, the noise coefficient is less than 0.5dB, and the stability coefficient is greater than 1. If the feedback circuit is disconnected and simulated again, it will be found that the gain increases, but the stability coefficient will be less than 1, and the amplification circuit will not work normally.
Through the design and Simulation of low noise amplifier of RF products, it can be seen that the use of ads aided design circuit has the advantages of simple theoretical calculation, fast design process, easy parameter modification and convenient verification, shortens the design cycle, improves the design accuracy, and has practical value in engineering.
Responsible editor: GT