Doherty Power Amplifiers in Software Radio systems F.M. Ghannouchi 1 , and K. Rawat 2 iRadio Laboratory, Department of Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada T2N1N4, fadhel.ghannouchi@ucalgary.ca 1 , krawat@ucalgary.ca 2 Abstract Power amplification in software defined radio needs reconfigurabilty as well as optimum performance in terms of linearity and efficiency to handle different modulation standards (and hence carrier and modulation bandwidth). Since digital predistortion is now widely accepted as highly suitable solutions for linearization in a reconfigurable perspective. Hence, this technique in conjunction with multi-band Doherty power amplifiers finds a potential solution for reconfigurable-multi-band software defined radio transmitter. This paper demonstrates the current state-of art for such transmitters, including a brief discussion about the available design methodology for the dual-band Doherty power amplifier. 1. Introduction In the perspective of software-defined radio (SDR) architectures, the demand of multi-band transmitters is becoming important in order to provide reconfigurability and multi-operability. Moreover, for high data rate handling capability with low power consumption, the transmitter architecture should also be selected in order to obtain high- efficiency and linearity to maintain the quality of service [1]. Since linearity and efficiency are the two main benchmarks for any transmitter topology [1], an optimal solution is being sought. Among these solutions, the dual-band predistortion based transmitter is effective, in terms of high linearity and digital baseband implementation, which provides reconfigurability [1]. For such transmitters, the power amplifier (PA) is operated at a certain average power backoff, which depends on the peak-to-average power ratio (PAPR) of the predistorted signal. To enhance the efficiency in this back-off region, Doherty PA is studied extensively and adopted in base station [3-4]. Based on these motivations, a multi-band transmitter with multi-band DPA and a digital predistortion can be sought as optimal solutions in terms of high efficient and linear architecture that can be implemented easily in the current RF/digital platform for the base-stations [5-6]. In this paper existing methodologies for the development of dual-band Doherty PA have been discussed including the implementation issues. Some results are also presented for the dual-band Doherty PA design and its linearization while operating at 1.96 GHz and 2.425 GHz bands using WCDMA and WiMax signals respectively. 2. Multi-Band Transmitter Architecture & Design Consideration The multi-band transmitters as proposed herein, need multi-band Doherty PA and reconfigurable digital predistortion techniques. A. Dual-band Doherty PA design The main efforts in Doherty PA design is the development of Doherty combiner and dual-band carrier and peaking amplifiers. Such structure utilizes dual-band transformers or impedance inverters analogous to their single-band architecture [3-4]. Three possible dual-band transformers can be used in passive circuits associated with the Doherty PA. Among these, Pi-type and T-type stub loaded transmission lines represents 90° type transformers [7-8] as the electric length at both bands are 90° or its odd multiple. However, Monzon transformer is a non-90° type, since the electric length at the two bands depends on the frequency ratio (f 2 /f 1 ) of the dual-band operation, where, f 1 and f 2 represents the first and second frequency of operation respectively [9]. For the T-type or Pi-type stub loaded transmission lines, the image impedance and overall electric length are the function of the characteristic impedance and electric length of the respective loaded transmission line and the stub loading it [7-8]. These image impedances and overall electric length of the Pi-type or T-type structures can be derived using their corresponding transmission line matrices (ABCD matrix) which can be found in [7], [8] respectively. Thus, the T-type or Pi-type structure to a certain extent of frequency-band represents a 90° transmission line, whereas on the other hand Monzon transformer act as impedance inverter in a certain band of frequency. For a dual-band Doherty PA associated circuitry ,the use of these type of structures are attempted in some designs [10-11]. 978-1-4244-6051-9/11/$26.00 ©2011 IEEE