370 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 13, NO. 9, SEPTEMBER 2003 A Microwave Doherty Amplifier Employing Envelope Tracking Technique for High Efficiency and Linearity Youngoo Yang, Member, IEEE, Jeonghyeon Cha, Bumjae Shin, and Bumman Kim, Senior Member, IEEE Abstract—In this letter, we have demonstrated a microwave Doherty amplifier employing input signal envelope tracking tech- nique. In the amplifier, gate bias of peaking amplifier is controlled according to the magnitude of the envelope. A 2.14-GHz Doherty amplifier has been implemented using 4-W PEP LDMOSFETs and an adaptive controlled gate bias circuit has been constructed and the control shape is optimized experimentally. The perfor- mance of the microwave Doherty amplifier has been compared with those of class AB amplifier using one-tone, two-tone, and forward-link wideband code-division multiple access (WCDMA) signals. For a forward-link WCDMA signal, the measured power added efficiency (PAE) of the microwave Doherty amplifier is 39.4% at dBc adjacent channel leakage ratio (ACLR), while that of the comparable class AB amplifier is 24.2% at the same ACLR level. Index Terms—Adjacent channel leakage ratio (ACLR), envelope tracking, microwave Doherty amplifier, power added efficiency (PAE), wideband code-division multiple access (WCDMA). I. INTRODUCTION F OR THE POWER amplifiers of code-division multiple access (CDMA) base stations, linearity is the most im- portant figure of merit. To meet the stringent requirements for linearity, the power amplifiers usually operate at class A or AB mode and is backed-off of output power to accommodate a high peak-to-average power ratio. However, there is tradeoff between linearity and efficiency, and the amplifiers with a high linearity have low efficiency. As the power level of amplifier increases and the size becomes more compact, the lower effi- ciency causes severe thermal problems. Hence, the efficiency of the high power amplifier has become an important issue. The Doherty amplifier is a promising solution for high efficiency with good linearity. Also, it has a very simple circuit structure and is easy to implement [1]–[5]. The previously reported microwave Doherty amplifiers without envelop tracking circuits deliver more output power compared with their counterparts of class AB amplifiers at the same linearity [4], [5]. However, the drain current of the Manuscript received November 10, 2002; revised March 20, 2003. This work was supported in part by the Agency for Defense Development and by the Ko- rean Ministry of Education under the Brain Korea 21 Project. The review of this letter was arranged by Associate Editor Dr. Arvind Sharma. Y. Yang was with Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea. He is now with Skyworks Solutions, Inc., Newbury Park, CA 91320 USA. J. Cha, B. Shin, and B. Kim are with the Department of Electronic and Elec- trical Engineering and Microwave Application Research Center, Pohang Uni- versity of Science and Technology (POSTECH), Kyungbuk 790-784, Korea. Digital Object Identifier 10.1109/LMWC.2003.817130 Fig. 1. Structure of a microwave Doherty amplifier employing envelope tracking technique. peaking amplifier is slightly lower than that of the carrier amplifier at a high power level because of the fixed lower bias of the peaking amplifier. Thus, the load impedance of this microwave Doherty amplifier cannot be fully modulated to the value for a high power match. As a result, the carrier and peaking amplifiers cannot generate their respective full powers. The reduced output powers directly affects the improvement of efficiency as well as cost. To solve the problem, we propose a new microwave Doherty amplifier employing an envelope tracking technique. In the pro- posed Doherty amplifier, gate voltage of the peaking amplifier is controlled according to the input signal envelope to max- imize efficiency with the desired linearity. For verification, a microwave Doherty amplifier is implemented and tested using one-tone, two-tones, and forward-link wideband code-division multiple access (WCDMA) signals. The Doherty amplifier is compared with the class AB amplifier, and the tested results show superior performances with the Doherty amplifier. II. IMPLEMENTATION OF THE PROPOSED DOHERTY AMPLIFIER In the previous section, we proposed a new microwave Do- herty amplifier. Fig. 1 shows a block diagram of the proposed Doherty amplifier. The Doherty amplifier consists of two-part circuits; a fully matched microwave Doherty amplifier and an adaptive gate bias control circuits for the peaking amplifier using an envelope tracking technique. Because of the adaptive gate bias control circuits, the Doherty amplifier can be fully modulated. Furthermore, linearity as well as efficiency can be improved by optimizing the shape of gate voltage of the peaking amplifier. The mechanism for the improvement of linearity has been explained in our earlier works [4], [5]. We have designed the microwave Doherty amplifier with a target of maximizing the power added efficiency (PAE) at 1531-1309/03$17.00 © 2003 IEEE