A fully integrated transformer-based front-end architecture for wireless transceivers

Abstract

Broadcom, Irvine, CAAs wireless transceivers for standards such as Bluetooth and 802.11 become more prevalent, the need for lower-cost solutions becomes increasingly important. In this paper, a fully integrated RF front-end architecture is presented; it offers good RF performance, while requiring no off-chip components. This design uses one on-chip transformer to perform the functions of matching, single-ended to differential conversion, and transmit/receive (T/R) switching. While this architecture can be used in many time division duplex systems, the focus of this paper is on CMOS implementations for the Bluetooth standard.The initial architecture of this front-end is based on an earlier design [1], and uses several off-chip components as shown in Fig. 5.8.1. There are various ways to reduce the number of off-chip components. First, the T/R switch can be brought on chip, although the best reported CMOS switches introduce around 1dB of loss [2]. Also, on-chip transformers can replace the off-chip baluns and matching elements [3]. Another approach shorts the LNA input and PA output with a common matching circuit, although some compromise between the LNA and PA impedance match may have to be made [4].Figure 5.8.2 shows the architecture presented in this paper. It uses a multi-tap integrated transformer with one set of ports on the primary side, and two sets of differential ports on the secondary side. One end of the primary connects to the antenna, acting as an RF input/output (I/O) port, while the other end connects to AC ground. On the secondary side, one set of ports connects to the input of the LNA, while a second set of ports connects to the output of the PA. The center tap of the secondary is connected to V DD , providing both AC ground and a DC current path to the PA. The TX and RX front-ends are coupled to the RF I/O port through their connections to the transformer. T/R switch functionality is achieved by powering down one path when the other is in use. The transformer also performs differential to single-ended conversion for both paths, eliminating the need for separate on-or off-chip baluns.For proper operation of the transformer, each port must be tuned appropriately. The simultaneous loading of the PA, LNA, antenna, as well as die and package parasitics must be factored into the overall transformer design and tuning. Furthermore, the matching at each port must be maintained in both RX and TX modes. At the RF I/O port, series and shunt capacitors are used to bring the impedance to 50Ω. For best performance, both the primary ground and the secondary center tap must provide a good AC ground.The LNA schematic is shown in Fig. 5.8.3, with the core design carried over from previous work [1]. A transformer turns ratio of 7 to 2 is selected in order to provide an appropriately large voltage gain at the LNA input. The transformer windings also act as gate inductors, replacing the off-chip inductors in the old architecture [1]. A high-Q transformer design is used to minimize the series resistance of the winding...

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