Circuits Syst Signal Process (2011) 30:705–720 DOI 10.1007/s00034-011-9309-6 COGNITIVE RADIO-BASED WIRELESS COMMUNICATION DEVICES Design and Analysis of Frequency-Tunable Ampliﬁers using Varactor Diode Topologies Tayfun Nesimoglu Received: 7 February 2010 / Revised: 10 September 2011 / Published online: 10 May 2011 © Springer Science+Business Media, LLC 2011 Abstract The design of frequency-tunable ampliﬁers is investigated and the trade- off between linearity, efﬁciency and tunability is revealed. Several tunable ampliﬁers using various varactor diode topologies as tunable devices are designed by using load- pull techniques and their performances are compared. The ampliﬁer using anti-series distortion-free varactor stack topology achieves 38% power added efﬁciency and it may be tuned from 1.74 to 2.36 GHz (about 35% tunable range). The ampliﬁer using anti-series/anti-parallel topology is tunable from 1.74 to 2.14 GHz (about 23% tun- able range) and provides 42% power added efﬁciency. It is demonstrated that tunable ampliﬁers using distortion-free varactor stack topologies provide better power added efﬁciency than the tunable ampliﬁers using reverse biased varactor diodes and their linearity is similar to that of a conventional ampliﬁer. These ampliﬁers may facili- tate the realization of frequency agile radio frequency transceiver front-ends and may replace several parallel connected ampliﬁers used in conventional multimode radios. Keywords Ampliﬁers · Tunable ampliﬁers · Impedance matching · Reconﬁgurable matching networks · Reconﬁgurable radio · Software deﬁned radio 1 Introduction The objective of Software Deﬁned Radio (SDR) is to provide a ﬂexible radio that is capable of operating over a continuously evolving set of communication standards. This work was initiated at University of Bristol within a Toshiba Research Europe Ltd. project. Much of the work reported here was carried out at Middle East Technical University, Northern Cyprus Campus (METU-NCC) and funded by the Scientiﬁc and Technical Research Council of Turkey (TUBITAK) under the project code 110E105 and partly funded by the METU-NCC under the project code FEN-1. T. Nesimoglu ( ) Middle East Technical University, Northern Cyprus Campus, Kalkanli, Guzelyurt, Mersin 10, Turkey e-mail: ntayfun@metu.edu.tr