Increasing Gain of Low-Noise Microwave Amplifier Design

Sir

I, Devesh, M.Tech II-year, SVNIT Surat, wants to know how we can improve the gain of
LNA without inceasing stages of amplifier. I am giving some details of LNA design.

Thanks

PERFORMANCE ANALYSIS 0F LOW NOISE

MICROWAVE AMPLIFIER

Abstract: The following paper presents designing of 2.4 GHz - 2.5 GHz frequency band low noise microwave amplifier using AT-31011 transistor with minimum noise figure and good(low) VSWR. The purpose of the amplifier is to amplify the received RF path of a Wireless local area network (WLAN). The design methodology required the analysis of the transistor stability and proper matching network selection. The design of an LNA in Radio Frequency (RF) circuits requires the trade-off of many importance characteristics such as gain, noise figure (NF), stability, power consumption and complexity. This situation forces designers to make trade-offs in the design of RF circuits. The designed simulation process is done using Ansoft Designer SV, while FR4 strip board is used for fabrication purposed. A single stage LNA has been successfully designed with 12 dB forward gain and 2 dB noise.

1: Introduction

Amplifiers are used to increase the voltage, current, and/or power of a signal, and are used in most in both transmitters and receivers. Low noise amplifiers are used at the front end of receivers. They are usually approximated as small signal devices, and are usually tuned (i.e; they use networks at their input and output to provide a match and gain over a relatively narrow bandwidth). Usually there is a trade-off between the two important parameters of LNA namely noise figure and VSWR in the design of LNA but we can remove the trade-off and presents good VSWR along with low noise figure. In this paper to obtain better results microstrip lines are used for matching networks.

Generally to attain good VSWR in LNA, isolators with low loss are used at its input which increases the cost of the amplifier but QH (Quadrature hybrid) is used in order to attain good VSWR and to decrease the cost of the amplifier. These QHs are cascaded with amplifiers [4].

Power amplifiers are used at the output of transmitters. They provide a high output power, and so cannot be approximated as small signal. They are designed using different techniques than small signal amplifiers.

2: LNA Block Diagram

2.1 Generic LNA Block Diagram

A generic single stage LNA configurat- ion. is shown in figure 1.

Figure 1: Generic LNA (Reinhold , 2002)

Input and output matching networks are needed to reduce undesired reflections and improve the power capabilities. In figure the amplifier is characterized through its S matrix at a particular DC bias point.

2.2 Design Specifications

• Gain : 12 dB
• Noise Figure: 2.0 dB
• Use microstrip matching networks
• VSWRin:1.5: VSWRout:1.75
• BW: 100 MHz from 2.4 GHz to 2.5 GHz

3: Schematic LNA Circuit

3.1 Schematic LNA Circuit Diagram:

LNA circuit is simulated using Ansoft Designer SV and is shown in fig. 2 given below.

As wavelength becomes significantly small compared with the characteristic circuit component length the influence of parasitics in discrete elements becomes more noticeable. Under these circumstances microstrip lines are widely used for source and load matching purpose due to their superior performance characteristics. Microstrip lines are one of the most important medium of transmission in microwave transistor amplifiers in the microwave integrated-circuit technology. In this paper for LNA designing, double stub matching networks are used to solve difficulties in tunning.

3.2 LNA Circuit Simulation Results

3.2.1 Scattering parameters of LNA

TABLE 1: S₁₁ and S₂₂

TABLE 2: S₁₂ and S₂₁

3.2.2 Maximum Gain

Figure 3: Maximum Gain

From fig. 3 simulated maximum gain is 14.35 dB at 2.4 GHz freq. which is greater than designed value.

3.2.3 VSWR Plots

Figure 4: VSWR Plots

From fig. 4 simulated VSWRin (1.52 abs.) and VSWRout (1.72 abs.) at 2.4 GHz is satisfactory and is kept within the typical VSWR limits (1.5-2.5).

3.2.4 RETURN LOSSES

Figure 5: Return Loss

From fig. 5 simulated return losses at input and output ports at 2.4 GHz is satisfactory and equal to 0.20 abs. and 0.27 abs..

3.2.5 SATBILITY ANALYSYS

Figure 6: Source and load stability circles

From figure 6 we see that both the input and output stability circles lie completely outside the Smith Chart for the range of frequencies 2 GHz to 3 GHz; hence the transistor is unconditionally stable for the desired frequency range.

3.2.6 STABILITY FACTOR

Figure 7: Stability Factor K

The idea of stability can also justfy using Stability factor K. From fig. 7 simulated stability factor K is greater than unity for the range of frequencies 2 GHz to 3 GHz hence the transistor is unconditionally stable for the desired frequency band.

3.2.7 SUmmary of the results (center frequency: 2.4 GHz)

Table 3

Parameter	Designed	Simulated
Gain	12	14.35
VSWRin	1:1	1.52:1
VSWRout	1.75:1	1.76:1
Noise Figure	2.0	2.12
Bandwidth	100 MHz	100 MHz

4: Conclusion

Low noise amplifier is designed for low NF, high gain, good VSWR using design tool Ansoft Designer SV. The degree of success of LNA design is quite satisfactory. The results show that the simulated gain is greater than designed value and simulated noise figure is close to designed value. The VSWR return loss is also kept within limits.