Decade Bandwidth High Efﬁciency GaN HEMT Power Ampliﬁer Designed With Resistive Harmonic Loading Christer M. Andersson ∗ , Junghwan Moon † , Christian Fager ∗ , Bumman Kim † , Niklas Rorsman ∗ ∗ Microwave Electronics Laboratory, Chalmers University of Technology, Kemiv¨ agen 9, SE-412 96 G¨ oteborg, Sweden † Department of Electronics and Electrical Engineering, Pohang University of Science & Technology, Pohang, South Korea Abstract—The use of resistive loading at higher harmonics in wideband power ampliﬁer design is proposed. Although the theoretical efﬁciency of such operation is lower than other classes the signiﬁcantly simpliﬁed load network design potentially allows for multi-octave realizations. A decade bandwidth (0.4-4.1 GHz) GaN HEMT power ampliﬁer was thereby designed, delivering more than 40 dBm output power with 10-15 dB gain and 40- 62% drain efﬁciency. Linearized modulated signal ampliﬁcation was then successfully demonstrated at multiple frequencies (0.9 to 3.5 GHz), using various downlink signals (LTE, WCDMA, WiMAX), with resulting ACLR lower than -46 dBc. Index Terms—Broadband ampliﬁers, gallium nitride, power ampliﬁers, wideband. I. I NTRODUCTION The increasing complexity of wireless infrastructures, in the form of an increasing number of standards and frequency bands, is creating a demand for wideband high efﬁciency power ampliﬁers. Wide bandgap GaN HEMT transistors are considered an important enabling technology in this develop- ment. This is mainly attributed to signiﬁcantly higher transistor impedance levels, which simpliﬁes wideband matching and ampliﬁer design. Technology aside, decisions on the ampliﬁer architecture level will affect the ﬁnal ampliﬁer bandwidth, power and ef- ﬁciency performance. For high efﬁciency classes of operation it is difﬁcult to maintain appropriate higher harmonic loading conditions over more than one octave of bandwidth. Although the range can be somewhat extended by a balanced design, inevitably the highest fundamental frequencies will overlap the lowest second harmonic frequencies, e.g. in a class-B design the fundamental load will become short circuited. In this work we propose to use resistive loading at the higher harmonics, equal to that at the fundamental. The efﬁciency (58%) of this type of operation is lower than for other high efﬁciency classes. However, by not requiring special load conditions at the higher harmonics the bandwidth limiting harmonic overlap issue is avoided. Potentially this can lead to multi-octave bandwidths, as there are few other design considerations besides the maximization of the resistive load network bandwidth. In Table I a summary of some recently published wideband GaN ampliﬁer results are listed. It can be seen that the resis- tively loaded GaN ampliﬁer designed in this work demostrates high efﬁciencies and excellent bandwidth performance. TABLE I WIDEBAND GANPOWER AMPLIFIER COMPARISON ( ∗ PAE) Ref. Freq. (GHz) BW (%) Gain (dB)  (dBm)  (%) [1] (2011) 0.55 - 1.1 67 9.5-12 40 65-80 [2] (2011) 0.9 - 2.2 84 10-13 40-43 63-89 [3] (2010) 1.9 - 4.3 77 9-11 40-41.8 57-72 [4] (2009) 0.5 - 2.5 133 15 39.5-41.3 45-63 [5] (2009) 0.35 - 8.0 183 8-10 38-40 20-33 ∗ This work 0.4 - 4.1 164 10-15 40-42 40-62 II. AMPLIFIER THEORY AND DESIGN In the proposed wideband resistively loaded ampliﬁer the goal of the output matching network is to present a resistive load by a conjugate matched load admittance (  ) over a desired bandwidth, i.e.   ()=1/  −   , (1) where   is the load resistance, and   the transistor output capacitance. The ampliﬁer properties are found theoretically by Fourier analysis, assuming a rectiﬁed current waveform (class-B type gate biasing at pinch-off). As a result of the resistive loading, the voltage waveform will also have the shape of an inverse rectiﬁed sinusoid. For maximum efﬁciency the fundamental load resistance (  ), the peak voltage swing, and output power are equal to class-B operation provided the drain bias voltage (relative the I(V)-knee) is increased by a factor (1 − 1/) = 36%. Without such a drain bias increase a 1.3 dB decrease in output power is expected. The drain efﬁciency  =  2 /(8( − 1)) = 58% is signiﬁcantly lower than other classes however (e.g. class-B 78.5%) and is the cost of a potentially higher bandwidth. Finally, the ampliﬁer should have good linearity performance given that only even harmonics are generated at the output. A 15W bare die GaN HEMT (CGH60015D) from Cree, Inc. was considered for such a design at a drain bias of 30 V and direct wire-bonding to the input- and output matching network PCBs. Load-pull simulations (Agilent Advanced Design Sys- tem) of a transistor model provided by the manufacturer gave an estimated output capacitance (  ) of 0.9 pF and a load resistance (  ) of 30 Ω that maximized the efﬁciency. It is often found, however, that a slightly smaller load resistance 978-1-4673-1088-8/12/$31.00 ©2012 IEEE