L Band Low Noise Amplifier

Junlin Song School of Electronic Engineering

University of Electronic Science and Technology of China Chengdu, China

693516274@qq.com

Haoquan Hu School of Electronic Engineering

University of Electronic Science and Technology of China Chengdu, China

huhq@uestc.edu.cn

Abstractlow noise amplifiers are the key components of the receiver front end, it has been widely used in microwave communication, radar, electronic warfare, and a variety of high-precision microwave measurement system and its performance directly affects the performance of entire receiving system, so it is essential to develop excellent low noise amplifier. In this paper, it has designed a LNA which has good performances at 2GHz. The measurement results performs a high gain (>15dB), fine flatness(

It has a wide bandwidth and well VSWR, however the noise figure is bad, the output range is narrow, and it is hard to achieve.

(3) Balanced amplifier

The biggest advantage of the balanced amplifier is it can both obtain the minimum noise figure and good input matching characteristics, as well as greater reliability and greater improvements in a wide bandwidth, however, a wide band coupler is relatively difficult.

(4) Negative feedback amplifier

Put negative feedback between FET drain and gate or source to lower the low frequency gain, make the input matching better, the principle of this structure is easy and have a relative wide bandwidth, well linearity, work stably, and the VSWR in band is well.

III. DESIGN OF LNAThe RF section of the LNA includes: appropriate transistor,

input matching network and output matching network. The block diagram of LNA is shown as below:

Inputmatchingnetwork

transistorOutput

matchingnetwork

Fig. 1 The block of LNA

Choosing an appropriate is important, in this paper it use FET ATF54143.

The noise figure is defined as the ratio of input SNR and output SNR[1].

In the formula, NF is noise figure; Sin is the input power and Nin is input noise figure; Sout is output power and Nout is output noise figure.

in in

out out

S NNF

S N (3.1)

For single-stage amplifier, the noise figure is

min 224

(1 ) 1

s soptn

s sopt

F F R

(3.2)

In the formula, Fmin is the minimum noise figure of the transistor, it is determined by the transistor only, sopt is the best reflection coefficient, s is the reflection coefficient of source and Rn is the noise resistance of transistor, in the multi-stage circuit the noise figure is determined by the formula shown in the below :

321

1 1 2

11 FFF FG GG

(3.3)

In the formula F is the total noise figure, F1 , F2 , F3 are noise figures of the first stage, second stage and third stage amplifiers, G1, G2 are gain of the first stage and second stage

of amplifiers, from the formula we can know the total noise figure of the circuit is determined by the first stage. So in the designing the first stage should design should let the circuit have best noise figure.

When the reflection coefficients of the input and output are all less than one (|1|

Fig.2 simulation model

The simulation results are shown in the figures at below. The fig.3 shows the noise figure and stable fact. In fig.4 it shows the gain.

Fig.3 The noise figure and StabFact

Fig.4 Gain of the LNA

From the simulation results we can get the conclusions that the circuit is absolutely stable, and the noise figure is less than 1dB, the LNA achieves the goals we make.

V. THE MEASUREMENTIn the figure.5 it shows the sample we make, from figure.6

to figure 8, they show all the index of the LNA. All the results are measured by signal analyzer FSV make by R&S.

Fig.5 The sample of LNA

The VSWR are measured by vector network analyzer HP8756A, Fig.6 is the input VSWR and Fig.7 is the output VSWR.

Fig.6 Input VSWR

Fig.7 Output VSWR

The noise and gain is measured by FSV made by R&S, Fig.8 is the gain and noise figure.

Fig.8 Gain and noise figure

VI. CONCLUSIONMeasurements and simulation results are fit well, we can

conclude that all the theories in the above is right, but the input and output VSWR is a little big, the SMA connector results a little, and transistors are different one by one. But in all the measurements are acceptable. The LNA in the paper is better than others at gain and flatness and NF. It was used in the receive system, the lower the noise figure is, the better the system performs, so sometime we satisfy the gain to get low noise.

REFERENCES

[1] Kuo-Hua Cheng, A novel 2.4GHz LNA with Digital Gain control using 0.18um CMOS, IEEE,2005.

[2] Dale D. Henkes, LNA design uses series feedback to achieve simultaneous low input VSWR and low noise Applied Microwave&WirelessOctober 1998 pp.26-32..

[3] Andrei Grebennikov, RF and Microwave Power AmplifierDesign, McGraw-Hill,New York,2005,pp.315-339

[4] David M. Pozar, Microwave Engineering, Third Editon, John Wiley Inc. New York, 1998, pp .612-617brev., in press.

[5] Bal S. Virdee, Avtar S. Virdee,Ben Y. Banyamin, BroadbandMicrowave amplifiers,Artech House,Boston,2004,pp.8-10