blocks as shown in Fig. 1.1. At the input we have the antenna, which transforms the electromagnetic

1. Introduction

In the era of wireless communications, devices that are able to exchange information through modulated radio frequency waves acquire more and more importance. These are required to be portable, low cost, with low power consumption and must meet several specifications given by the telecommunication standard they refer to. In the recent years a great progress has been made in all these fields. Great importance assumes the advent of digital treatment of information, which enables far better perfo rmances over the analog counterpart. Many advances have been made in the signal modulation techniques too, thus allowing an increase of the communication distances with the minimum possible error. A device that is able to communicate with others must be equipped with a transmitter and a receiver. Our work will focus on the latter, in all its aspects.

IF Filter Preselect

Filter LNA

MIX1 MIX2

ADC Base-band

circuits fRF

1

fOL

fI F

2

fOL

Antenna

Figure 1.1 Simplified scheme of a receiver

1.1. Typical RF receiver (superheterodyne and direct conversion)

A typical receiver that works in the radio- frequency range can be schematized as a chain of main

blocks as shown in Fig. 1.1. At the input we have the antenna, which transforms the electromagnetic

field in the air in an electrical signal at frequency f

. Then we have a band-pass preselect filter,

whose role is to separate the useful band from the unwanted signal. Following the filter we have the

key-component of this work, that is the Low Noise Amplifier, then the first Mixer, that translates

the carrier frequency to a lower and fixed frequency, called intermediate frequency (f

), that is the

difference between f

and the frequency of the local oscillator f

. The next stages are, in that

order: a selective BP filter centered on f

, a second mixer, which translates the signal to base-band,

which means that the frequency spectrum at its output is centered on zero. The architecture above

described is called “superheterodyne” and makes use of at least two mixers and at least one

intermediate frequency. Some receivers make a direct conversion from the RF carrier to the base- band signal, with just one mixer, whose local oscillator is set to the same frequency as the input carrier. This direct conversion technique is called “homodyne”. The next stage is, in case of digital information, an Analog-to-Digital Converter followed by the digital circuits working in base band that decode and eventually treat the digital signal received. With this architecture we do not need any IF filter and just one mixer: this implies very low power consumption and an easy implementation of an integrated single-chip receiver. However

1.2. What is an LNA?

A low noise amplifier is the first amplifier that we find in a receiver. This block is the main subject

of this work. Looking at Fig 1.1 we see that the LNA works with a radio-frequency signal, generally

in the range of a gigaHertz or more. Moreover, this received signal comes from the antenna and

then passes through the input filter. This means that we have to cope with a signal whose amplitude

can vary within a range of several orders of magnitude. Indeed it is unpredictable how strong the

input signal will be, hence we must design this block with the ability to treat with very different

signals. We have only information about the frequency band of such signal. Another aspect is the

noisy nature of the received signal and the fact that we have to amplify this signal without

degrading it.