13.1 Introduction

In part I of this book we introduced noise mechanisms, paying attention to noise modeling in passive and active devices. In the second part, emphasis was given to various techniques to reduce interference caused by signals arriving from outside the investigated circuit.

We now attempt to use these topics in the context of low-noise circuit design. It must be remembered that, traditionally, this field is concerned with principles and techniques to reduce the intrinsic noise level only. It is assumed that extrinsic noise is not a threat, since circuit immunity to interfering signals has been increased as much as possible. Therefore, a typical formulation of the low-noise design problem is the following [205]: Given a signal source with known electrical characteristics (impedance, bandwidth, noise, etc.), how do we optimize the amplifier design in order to achieve the minimum noise of the system source-amplifier?

Methodology. The design of any electronic circuit is an iterative process. When an amplifier is designed for a specific application, many requirements concerning the DC power supply, gain, bandwidth, input and output impedances, feedback, stability, noise, and cost must be met. To begin, the designer makes strategic decisions, including circuit topology, the technology adopted to fabricate the amplifier, the type of active devices (bipolar or field-effect devices), their bias points, etc.