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Integrated Switched Differential Inductor

IP.com Disclosure Number: IPCOM000016834D
Original Publication Date: 2003-Jul-17
Included in the Prior Art Database: 2003-Jul-17
Document File: 6 page(s) / 57K

Publishing Venue

Motorola

Related People

Nihal Godambe: AUTHOR

Abstract

Integrated RFICs often utilize several integrated inductors within circuits such as LNAs, mixers, VCOs etc. These inductors occupy a significant amount of silicon die area and we hence seek methods to either increase the inductance per unit area or share certain inductors. Also, differential circuits are commonly used in RFIC implementations and therefore require the use of differential inductors. Furthermore, applications that need to switch frequency bands require either capacitors or inductors that can be switched. This article describes a novel method in which an integrated differential inductor can be switched to provide different inductance values by varying the mutual inductance and lengths of its turns.

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Integrated Switched Differential Inductor

Nihal Godambe

Abstract:

Integrated RFICs often utilize several integrated inductors within circuits such as LNAs, mixers, VCOs etc. These inductors occupy a significant amount of silicon die area and we hence seek methods to either increase the inductance per unit area or share certain inductors. Also, differential circuits are commonly used in RFIC implementations and therefore require the use of differential inductors. Furthermore, applications that need to switch frequency bands require either capacitors or inductors that can be switched. This article describes a novel method in which an integrated differential inductor can be switched to provide different inductance values by varying the mutual inductance and lengths of its turns.

Introduction

        � � � � � � � � � � � Integrated RFICs often use several integrated inductors for applications such as tuned loads, emitter degeneration, impedance matching and baluns.� These inductors occupy a significant die area (proving to be costly) and often do not scale with IC technology. The advent of differential circuit topologies (now commonplace) would require twice the number of inductors. However, the use of a differential coil decreases the effective area required and gives us a better ‘Q’ when compared with 2 separate spiral inductors. Most applications often require several frequency bands to be covered (over process variation), and this can be achieved by using a switched bank of capacitors or inductors. This idea has been applied to an integrated LC VCO design in [1], in which a bank of capacitors and several discrete VCO inductors are switched to produce a large tuning range in order to cover IC process variations.

Switched capacitors are commonly used since it is more difficult to switch multiple inductors (since using multiple instances of a given inductor has an area penalty). However switching inductors may be more optimum when they are used for emitter degeneration and as tuned loads, since they occupy less voltage headroom. Also in a switched LC VCO tank, switching inductors may prove to be more optimal than switching capacitors as was shown in [2].� Hence an area efficient solution is needed to implement such a switchable inductor.

To address this problem, this article describes a differential topology in which multiple inductances can be electronically switched without any significant area penalty. Furthermore, this electronically switched differential inductor can be integrated in any IC process, which has low-loss MOS switches and inductors available.        � � � � � �

Topology

The inductor topology consists of a balanced/symmetrical differential coil with MOS SPST and SPDT switches in series with different turns. By turning these switches ON or OFF we produce turns of different lengths, and these turns have different mutual inductances between each other- to in effect produce a programmable or switchable inductor.

The example used below is for...