Cmos 1-4 Throw Swith

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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 16, NO. 3, MARCH 2006

A CMOS Single-Pole-Four-Throw Switch Kwangchun Jung and Kenneth K. O, Member, IEEE

Abstract—An one-pole-four-throw switch that can be used to switch between the band select filters of four cellular bands and a single input programmable low noise amplifier has been demonstrated in a 0.18- m complementary metal oxide seminconductor process. Its insertion losses are 0.39, 0.61, 0.66, and 0.75 dB in the GSM900, DCS1800, PCS1900, and wide-band code division multiple access system bands. IIP3’s of the switch are 27 dBm and isolations are greater than 22 dB. Index Terms—Complementary metal oxide seminconductor (CMOS) integrated circuits, metal oxide semiconductor field effect transistor (MOSFET) switch, 1P4T switch, radio frequency (RF) switch.

Fig. 1. input.

Interface between off-chip filters and a multiband LNA with a single

Fig. 2.

Schematic of the 1P4T RF switch.

I. INTRODUCTION

W

ITH the evolution of wireless communication systems to the third generation, the need for low cost multifrequency band and multistandard integrated transceiver concepts has increased [1]. A high level of integration with a reduced number of off-chip components is a key to low cost. Traditionally, to support the multifrequency band operation, two or more sets of key radio frequency (RF) blocks are integrated on a single die [2]–[4]. However, these increase the die area, complexity of test, and cost. As an approach to mitigate these, a multiband programmable low noise amplifier (LNA) with a single input is proposed [5]. The LNA can potentially handle global system for mobile communication (GSM900), digital cellular system (DCS1800), personal communication system (PCS1900), and wide-band code division multiple access system (WCDMA). For a radio using such an LNA, an RF switch is needed to connect the required off-chip filter for a chosen frequency band to the LNA. As shown in Fig. 1, in order to support the four cellular standards, an one-pole-four-throw switch (1P4T) is needed. The switch does not need to handle large power like a T/R switch, which makes it ideally suited for implementation in complementary metal oxide seminconductors (CMOS). This letter reports such an 1P4T switch implemented in a 0.18- m CMOS process. II. DESCRIPTION OF THE RF SWITCH The 1P4T switch consists of four transistors and four gate resistors, and is shown in Fig. 2. The simplest topology (e.g., without shunt transistors for improving isolation) is employed in order to lower the parasitic capacitances at the input and output nodes, which in turn lowers insertion loss. Despite this, because four sources are connected to the RFOUT node, instead of two Manuscript received September 27, 2005; revised November 22, 2005. The authors are with the Silicon Microwave Integrated Circuit and System Research Group (SiMICS), Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL 32611 USA (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/LMWC.2005.869857

sources in a single pole double throw switch, insertion losses of 1P4T switches are expected to be higher than those of the previously reported 1P2T switches [6]–[8]. Dealing with these additional parasitic capacitances is a challenge for realizing low insertion loss 1P4T switches. The transistors, M1, M2, M3, and M4 perform the switching function. A dc voltage of 1.2 V is applied at the sources and drains of transistors using bias-Ts to lower junction capacitance, which lowers insertion loss. Applying the same dc voltage to all the sources and drains makes the dc power consumption almost zero. The gate bias resistors, R1, R2, R3, and R4 are implemented using polysilicon resistors. The resistors ac isolate the gates of transistors for improved linearity. The bias Ts could also be replaced with on-chip resistors like the gate bias resistors. The circuit requires no external matching networks. The control voltages are switched between 0.8 and 3.0 V. The channel resistance of transistor is one of the dominant factors determining insertion loss. Increasing the gate width of transistors reduces channel resistance. However, it increases the drain-to-body and source-to-body junction capacitances. This

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JUNG AND O: CMOS SINGLE-POLE-FOUR-THROW SWITCH

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Fig. 3. (a) Micrograph of the 1P4T RF switch and (b) photograph of PCB and bonded switch chip.

increases the return loss and the loss through associated parasitic substrate resistances. Because of these, there are optimal transistor widths for minimum insertion loss. The parasitic capacitances also degrade insertion loss of a transistor with frequency as well as isolation [6], [7]. Since the transistors are intended for operation at different frequencies, their sizes must be accordingly chosen (1 296 m, 2 378 m, 3 378 m, 4 422 m). This is another unique requirement not to be considered for the design of previously reported switches. In order to get comparable insertion losses among four transistors, only M4 for WCDMA was chosen to have the optimum width while the other transistors were made narrower than the optimum. M1 for GSM900 has the smallest width and M4 for WCDMA has the largest width. The minimum channel length of 180 nm is exclusively used to reduce the channel resistance. The multifinger interdigitated transistor layout is used to reduce the junction capacitances. The parasitic interconnect capacitance between a drain and a source is an important factor determining isolation and its effects become more critical as the transistor length decreases. Hence, only the metal1 and metal2 are used for the drain and source connections. III. EXPERIMENT RESULTS A microphotograph of the 1P4T RF switch is shown in Fig. 3(a). To reduce the lengths for the interconnections between M3 and bond pad and between M4 and bond pad, the transistors are located between the pads. R1-R4 resistors are chosen to be (\sim) 70 k to float the gate terminals. Li and

Fig. 4. (a) Measured return loss and (b) insertion loss of 1P4T RF switch.

O [8] showed that RF switches with low substrate resistances have better insertion loss and isolation. Therefore, all of the die area except the four transistors, four resistors, and twelve pads, are occupied by substrate contacts. The die size including the bond pads is 0.3 mm . Fig. 3(b) shows a photograph of the switch mounted on a printed circuit board (PCB). Fig. 4 shows the measured return loss and insertion loss versus frequency at the four bands. The return losses for GSM 900, DCS 1800, PCS 1900, and WCDMA are 23 dB, 14 dB, 16 dB, and 18.5 dB, respectively, which are excellent. For GSM 900, the insertion loss varied between 0.37 dB (935 MHz) and 0.39 dB (960 MHz). For DCS 1800 and PCS 1900, the maximum insertion losses are 0.61 dB at 1880 MHz and 0.66 dB at 1990MHz, respectively. Last, for WCDMA, the maximum insertion loss is 0.75 dB at 2170 MHz. These should be adequate for cellular applications. Actually, the simulated insertion losses were all less than 0.5 dB. The differences between simulation and measurement are attributed to the fact that the actual bond-wire inductances ( 3 nH) on the RF pats are 50% higher than that used for the design

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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 16, NO. 3, MARCH 2006

further reduce the insertion loss by using a more scaled CMOS process. Isolations in the GSM900, DCS1800, PCS1900, and WCDMA bands are 29, 24, 23, and 22 dB, respectively. The linearity requirement of the 1P4T RF switch in a receiver chain is not stringent. Fig. 6 shows the input third-order intercept points (IIP3) measured using two tones. IIP3s for all four bands are 27 dBm. These are more than adequate since the IIP3 specifications of four standards are less than 0 dBm. IV. CONCLUSION

Fig. 5. Simulated insertion losses for varying bond wire inductances.

An one-pole-four-throw switch for a multiband receiver is implemented using 1.8-V 0.18- m NMOS transistors. Its insertion losses are 0.39, 0.61, 0.66, and 0.75 dB for the GSM900, DCS1800, PCS1900, WCDMA bands, and its IIP3s of 27 dBm should be sufficient for this application. The insertion losses are lowered by eliminating the shunt transistors normally found in one-pole-two-throw switches. Different width transistors are used in the four bands to balance the insertion loss. REFERENCES

Fig. 6. Measured IIP of 1P4T RF switch.

( 2 nH). Fig. 5 shows the simulated insertion loss versus frequency for four different values of bond-wire inductance on the RF paths. The insertion losses increase with the bond-wire inductance primarily due to the increases of input impedance, which increases the return loss. It should also be possible to

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