LOW NOISE AMPLIFIER

LOW NOISE AMPLIFIERMarium Saleem 08-TE-02Zarmina Amir 08-TE-05Marriam Imtiaz 08-TE-08Sadaf Talha 08-TE-30Irum Jafri 08-TE-33ContentsIntroductionConfigurationParametersConstructionApplicationsAnalysis

Introduction

Low-noise amplifier(LNA) is anelectronic amplifier that combines a low noise figure, reasonable gain, and stability without oscillation over entire useful frequency range.

Its main function is to amplify extremely low signals without adding noise, thus preserving the required Signal-to-Noise Ratio (SNR).

The Low Noise Amplifier always operates in Class A, typically at 40-50% efficiency.

Thisactive antennaarrangement is frequently used inmicrowavesystems likeGPS.

Configuration Of Low Noise Amplifier

Low noise amplifier is a key component which is placed at the front-end of aradio receivercircuit.

It is usually located very close to the detection device to reduce losses in thefeed line.

For low noise configuration, the amplifier needs to have a high amplification in its first stage. Therefore, JFETs andHEMTs are often used, anddistributed amplifiers could be used.

Current should be properly chosen that will reduce the relative amount of noise.

Biasingis done by large resistors, a large resistor prevents leakage of the weak signal out of the signal path or noise into the signal path.

Input and outputmatchingcircuits enhance the gain.

Using an LNA, the effect ofnoisefrom subsequent stages of the receive chain is reduced by thegainof the LNA.

While the noise of the LNA itself is injected directly into the received signal. Thus, it is necessary for an LNA to boost the desired signal power while adding as little noise and distortion as possible, so that the retrieval of this signal is possible in the later stages in the system.

A good LNA has a low NF (like 1dB), a large enough gain (like 20dB). Receiver

Parameters

Here are some important parameters of low Noise Amplifier:

Operating Supply VoltageOperating Supply CurrentNoise FigureGainOperating FrequencyOperating Temperature RangeInput And OutputVoltage Standing Wave Ratio(VSWR).Reflection CoefficientAmplifier Dynamic Range

1. Operating Supply Voltage

Usually LNA require less operating voltage in the range of 2 V to 10 V.

MAX 2640 operate at +2.7 V to +5.5 V.2. Operating supply current

LNA require supply current in the range of mA, the supply current require for LNA is dependent on the its design and the application for which it has to be used.

MAX 2640 which is used for satellite application requires a supply current of nearly 6mA.

3. Noise FigureNoise figure is one of the important factors which determines the efficiency of a particular LNA. Hence, we can decide which LNA is suitable for a particular application.

Noise factor is defined as the ratio of input SNR and output SNR of amplifier.

Contd..nf=(SNR)in/(SNR)out=(Sin/Nin)/(Sout/Nout)nf=Noise Factor

Noise Figure (NF) is Noise Factor expressed in dB.

NF=10*log(nf).

Contd..Insatellite communication, an LNA having a very low noise figure is required.

Noise figure varies according to the operating frequency for a same LNA. e.g. for MAX 2640,

NF 0.5 dB for 900 MHzNF 1.2 dB for 1575 MHzNF 1.3 dB for 1900 MHz NF 1.5 dB for 2450 MHz

Low noise figure results in better reception of signal.

4. Gain

An amplifier gain is defined as the ratio of its output and input power. G=Pout/ Pin

According to requirement high gain LNA are designed for application by manufacturer.

The noise figure of a system can be lowered by taking advantage of higher gain of LNA.

Contd..If LNA will not have high gain then the signal will be affected in by noise in LNA circuit itself and maybe attenuated so high gain of LNA is the important parameter of LNA

However, too high gain of LNA will limit receiver dynamic range, thus we generally determine LNA gain by trade-off of system noise figure, receiver dynamic range and associated factors.

Contd..Like NF gain of LNA also varies with the operating frequency. e.g. for MAX 2640

15.1 dB at 900 MHz,15.7 dB at 1575 MHz 14.4 dB at 1900 MHz,13.5 dB at 2450 MHz

5. Operating Frequency

Frequency is the basic thing for any electronic device specially among the ones used in communication systems.

Range of LNA operation is very wide, it operate from 500 KHz to 50 GHz.

MAX 2640 operate in the frequency range 300 MHz - 1500 MHz.6. Operating Temperature Range

Every electronic device have some limit on operating temperature, LNA also has some limit on its operating temperature.

General range for faithful operation of LNA is -30C to +50C.

MAX 2640 operates on very wide temperature range i.e. -40C - +125C.

7. Input And OutputVoltage Standing Wave Ratio(VSWR).

LNA input and output VSWR reflect the matching of input and output circuit respectively.

Input matching circuit is designed to achieve minimum noise but not maximum output power.

Whereas output matching circuit is designed for maximum output power and minimum VSWR. Contd..Therefore, there always is certain mismatching at LNA input port and it may lead to instability for system in some situations.

Usually, in order to reduce the influence which is caused by LNA input mismatching for system, some components such as a low insertion loss isolator may be used to figure it out.

8. Reflection CoefficientAn amplifier achieves the lowest noise figure NFminwhen s= opt

opt= Optimum complex reflection coefficientRn= Noise resistances= Complex source reflection coefficient

Contd..Its input port is mismatched in the view of point of power transmission. The amplifier power gain is consequently decreased, but it is a method we use frequently in our design to achieve lower noise by trade-off between gain and noise figure.

9. Amplifier dynamic range For LNA design, the receiver dynamic range should be carefully considered to avoid to serious nonlinear distortion made at the end state of receiver.

Operating frequency, bandwidth, pass band gain flatness and related parameters should be carefully considered as well.

LNA Construction

The steps required in constructing a LNA are as follows:

Transistor TransducerStability CheckStability Enhancement

S-parameters describe the response of an N-port network to voltage signals at each port. The first number in the subscript refers to the responding port, while the second number refers to the incident port. Thus S21 means the response at port 2 due to a signal at port 1

If we assume that each port is terminated in impedance Z0, we can define the four S-parameters of the 2-port as:

1. TransistorTransistor selection is the first and most important step in an LNA design, as the working is based mainly upon transistors, which are categorized as:

S-parameter :It is a built-in device which does not require any type of external biasing because it has fixed S-parameters.Normal devices:They are like other transistors to which external bias can be applied. In designing a LNA, the S-parameter design is mostly used.Contd..The primary requirements of a transistor used in microwave low noise amplifier are:

High GainLow Noise Sufficient Dynamic RangeHigher Operating Frequency

2. TransducerOne of the crucial stages in designing a Low Noise Amplifier is proper selection of a transducer.

The transducer selected should have a maximum gain and minimum noise figure(NF).

Some examples of transistors that can be selected are-ATF-34143andATF-351433. Stability Check

While designing any amplifier, it is important to check the stability of the device chosen, or the amplifier may function as anoscillator.

For determining stability, calculate Rollet's Stability factor, (represented as variable K) using S-parameters at a given frequency.

For a transistor to be stable, parameters must satisfy K>1 and ||