Design of L-Band Low Noise Amplifier

Design of L-Band Low Noise Amplifier

introduction

Low noise amplifier (LNA) is a key microwave component in electronic systems such as radar, communication, electronic countermeasure, telemetry and remote control, and has a wide range of applications. Since the noise factor of the microwave system basically depends on the noise factor of the preamplifier, the quality of the LNA noise factor will directly affect the performance of the entire system. The design of the low noise amplifier mainly includes the design of the input and output matching networks and the DC bias network, as well as measures to improve the stability of the transistor.

This paper first introduces the source series negative feedback principle of the amplifier to improve stability, then designs an L-band low noise amplifier example, and gives the simulation results of the amplifier input, output return loss, gain, noise figure and other parameters.

Design of Low Noise Amplifier

The performance indicators of the low noise amplifier designed in this paper are: in the frequency band of 1.90GHz to 2.10GHz, the power gain Gp ≥ 30dB and the noise figure NF ≤ 1dB. Considering the index requirements, it is proposed to adopt a two-stage amplifier cascade technology to achieve it. The noise figure of the n-stage amplifier can be expressed as:

Among them, NF is the noise factor of the whole amplifier; NF1, NF2...NFn are the noise factors of the first, second to nth stages of the amplifier respectively; G1, G2,...Gn-1 are the power gains of the first, second to n-1th stages of the amplifier respectively. It can be seen from formula (1) that the noise factor and gain of the first-stage amplifier will directly affect the noise factor of the entire amplifier. To obtain a low noise factor for a cascaded low-noise amplifier, the selected first-stage transistor of the amplifier should have a low noise factor and a high gain at the operating frequency.

When designing LNA, we should first select the appropriate device according to the design index, and then design the input and output matching networks according to the impedance characteristics of the device at the operating frequency. Since the gain index of the designed low-noise amplifier is greater than 30dB, it is necessary to use a multi-stage cascade method to achieve it. Agilent's ATF54143 E-PHEMT transistor has the characteristics of high gain and low noise, and is suitable for various LNA circuits in wireless systems with a frequency range of 450MHz to 6GHz. The noise figure of the tube at the 2GHz frequency point is 0.5dB, and the gain is 17dB, so this transistor is selected as the first stage of the amplifier; in order to achieve the gain index of the amplifier, MGA86576 is selected as the second stage.

Effect of Source Series Feedback Inductance on Stability

Stability is something that must be considered in LNA circuits. The stability of an amplifier refers to the level of suppression of oscillations. The stability of the amplifier must be guaranteed to avoid possible self-excitation. In a two-port network of a transistor amplifier, its two ports are connected to the signal source and the load respectively. With the S parameters of the two-port circuit, the stability coefficient K of the amplifier can be easily calculated. The stability criterion is as follows:

Where △ = |S11S22-S12S21|. When K>1 and △<0, the amplifier is in an absolutely stable state; K<1, the circuit is potentially unstable.

Since AFT54143 is not absolutely stable within the operating frequency band, a 100Ω resistor is connected in parallel at the output end to improve the stability of the amplifier. To ensure that ATF54143 remains stable within the widest possible frequency band, the source electrode is introduced into the method of series inductive feedback. The inductor is replaced by a very thin microstrip line. The equivalent circuit after the transistor is connected to the series feedback inductor is shown in Figure 1.

After the feedback inductor LS is connected in series, the input impedance Zin of the transistor can be expressed as:

When wLs<

It can be seen from formula (4) that after the source is connected in series with the inductor, the real part of the input impedance of the transistor two-port network can be increased, while the imaginary part remains basically unchanged, so that it gradually coincides with the impedance of the best noise matching; on the other hand, adding a passive component will not deteriorate the noise performance of the transistor.

After connecting to the source negative feedback, the ATF54143 is simulated. Figure 2 shows the simulation results before and after the transistor stability is improved. (a) shows that when the source negative feedback is not connected, the transistor is unstable in the frequency band less than 3.5GHz; (b) shows that after connecting to the source inductor, the ATF54143 is stable in the frequency band of 1.9GHz to 2.1GHz. It can be seen that after adopting the source series negative feedback technology, the stability factor K is greater than 1 in the required frequency range, meeting the absolute stability condition requirements.

Bias Circuit Design

The DC bias circuit provides the amplifier with appropriate voltage and current, so that the transistor works at the required static operating point and keeps the static operating point constant within the range of transistor parameters and temperature changes. The optimal conditions are selected according to the device characteristics. Here, the typical DC operating point parameters of ATF54143 are selected: VDS=3V, ID=60mA; the bias method uses resistor bias, which has good temperature stability. Combining the design principles of the above bias circuit and the S parameter data of ATF54143, the DC bias circuit shown in Figure 3 can be designed.

Where Vdc is the feed voltage, and its value is 5V; Vds is the drain-source operating voltage of ATF54143, which is 3V; Idc is the drain current required by the static operating point of ATF54143, which is 60mA; IBB is the current flowing through the R3 and R4 resistor divider, which is generally at least 10 times the gate leakage current, and is selected as 2mA here; R2 is calculated based on Vdc and Idc. From formulas (5), (6), and (7), it can be calculated that R2=32.3Ω, R3=1200Ω, and R4=300Ω. R5 is a high-resistance resistor, and the R5 value is selected as 12KΩ here. Taking a larger resistance value can improve the efficiency of the transistor.

Design of input and output matching networks

In the design of low noise amplifier, the design of input and output matching network is to transform the input impedance and output impedance of transistor to standard 50Ω respectively under the premise of obtaining noise figure and gain index. Usually, the maximum gain and optimal noise figure of low noise amplifier cannot be obtained at the same time. Therefore, in the design process, it is necessary to compromise between gain and noise figure to meet the requirements of design index. The substrate used in the design is F4B dielectric board with a thickness of 1 mm and a relative dielectric constant of 2.7.

The design of the first-stage amplifier circuit is based on the small signal S parameters of ATF54143 to calculate the input and output impedance of the amplifier center frequency. The input impedance Zin=50×(0.999+j3.928E-4), the output impedance Zout=50×(1.017+j0.064), the conjugate matching method, that is, ZS=Z*in, ZL=Z*out, is used to design the input and output matching circuits, and the ADS software is used to optimize and simulate the size of the matching network microstrip circuit. The noise coefficient frequency response of the amplifier is shown in Figure 4. It can be seen from the figure that the noise coefficient of the amplifier is about 0.6dB at the center operating frequency; Figure 5 is the amplifier gain/frequency curve, it can be seen that the gain of the designed first-stage amplifier is greater than 10dB within the operating frequency range.

The MGA86576 in the second stage of the amplifier circuit is an internally matched gain module and can be directly cascaded with the first stage. However, in order to reduce the input/output return loss, a matching circuit design is added to this stage of the amplifier circuit. The steps are similar to the first stage, and the stages are connected with capacitors.

Simulation Results

After the circuit design is completed, the designed low-noise amplifier is simulated using ADS software. Figure 6 is the noise figure simulation result of the low-noise amplifier. It can be seen that the noise figure NF≤0.67dB in the 1.90GHz~2.10GHz frequency band; Figure 7 is the gain characteristic frequency response of the low-noise amplifier. At the 2.0GHz frequency point, the gain is approximately 38dB; Figure 8 is the input and output return loss frequency response of the entire low-noise amplifier, where the dotted line is the input return loss and the solid line is the output return loss. In the entire working frequency band, the input return loss is less than -15dB, and the output return loss is less than -22dB.

Conclusion

This paper introduces a method of improving the stability of a low-noise amplifier by using source series negative feedback, and designs an L-band low-noise amplifier with a center frequency of 2GHz and a bandwidth of 200MHz. The simulation results show that source series negative feedback can improve the low-frequency stability of the amplifier.