RF Circuit Design, 2nd Edition

Chapter 17

LNA (Low-Noise Amplifier)

17.1 Introduction

In a wireless communication system, the LNA (low-noise amplifier) is the first circuit block in the receiver. It is one of the most important blocks in RF circuit design, because of the following:

• The sensitivity of the receiver is mainly determined by the LNA noise figure and power gain. The noise figure of the LNA significantly impacts the overall noise performance of the receiver. On the other hand, the power gain of the LNA significantly suppresses noise contributions from subsequent stages, so that it as well impacts the overall noise performance of the receiver.
• The LNA plays an important role in the linearity of the entire system. Its nonlinearity must be reduced as much as possible.
• In a CDMA (code division multiplex access) wireless communication system, the LNA takes care of AGC (automatic gain control) in the entire system as well.

This chapter covers the following:

• Typical Design Procedures These include selection of device size, raw device testing, input and output impedance matching, stability checking, and linearity examination.
• Cascode LNA As the wireless bandwidth is raised up to gigahertz (GHz) or tens of GHz, the performance of the LNA is restricted by the input Miller capacitance. Increasing isolation between the input and output in an LNA would be helpful to an advanced communication system. The cascode LNA would improve the isolation performance from single-ended LNA.
• AGC (Automatic Gain Control) Without ...