A SiGe low noise amplifier for 2.4/5.2/5.7GHz WLAN applications

ISSCC 2003 / SESSION 20 / WIRELESS LOCAL AREA NETWORKING / PAPER 20.8

20.8

A SiGe Low Noise Amplifier for 2.4/5.2/5.7GHz

WLAN Applications

Po-Wei Lee, Hung-Wei Chiu, Tien-Ling Hsieh, Chih-Hsien Shen, Guo-Wei Huang1_{, Shey-Shi Lu}

Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China

1_{National Nano Device Laboratory, Hsinchu, Taiwan, Republic of China}

Wireless communication has evolved into a world of multi-standards/multi-services with operating frequencies of 900MHz/1.8GHz/1.9GHz bands for GSM, 1.5GHz band for GPS and 2.4GHz/5.2GHz/5.7GHz bands for WLAN. Therefore, it is desirable to combine two or more standards in one mobile unit [1-2]. The primary challenge in designing multi-band trans-ceivers is increasing the functionality of such communication systems while minimizing the number of additional hardware such as low noise amplifiers (LNAs). Typical design strategies [2-4] have used different LNAs for different frequency bands. However, this inevitably increases cost, power consumption and form-factor. Recently, concurrent dual-band LNAs with excellent performance have been devised, but off-chip capacitors and inductors are required [5]. In this work, an integrated SiGe LNA, which can handle the 2.4/5.2/5.7 GHz triple bands, is reported. The change of frequency bands is accomplished by switching between two different bias currents.

Figure 20.8.1 shows the schematic of our LNA. Basically, it con-sists of two common-emitter amplifiers. Transistor Q1 is preced-ed with an inductor LBfor input impedance matching. Transistor Q2 provides additional gain as well as output matching, if nec-essary, by using a local shunt-shunt feedback resistor R3around it. Note that resistive loads R1and R2instead of LC tuned loads, are used to minimize the die area.

By extending the previous work [6], it can be shown that the input impedance ZINlooking into the base of a common-emitter amplifi-er with a parallel RL-CLload, as shown in Fig. 20.8.2, is equivalent to a series RIN–CINnetwork over some frequency range with the formulas indicated in Fig. 20.8.2. In our circuit, the parallel RL-CL load arises from the parallel combination of the resistive load of the first stage and the input impedance of the second stage. RIN can be adjusted to be nearly 50 ohms by appropriate values of RL, CL, transconductance gmand base-collector capacitance CBC. CINis the sum of base-emitter capacitance CBE and Miller capacitance CM, and both are functions of bias current. Hence CIN is varied when bias current is changed. The resonant frequency determined by CINand LBis changed accordingly, and as a result, the resonant frequency for input matching also varies. Thus, one application band can be switched to another application band by simply switching the base current (or collector current) from one bias cur-rent to another bias curcur-rent as indicated in Fig. 20.8.1. Note that the change of CIN (or CM) with bias current can be substantial because of the beneficial Miller multiplication effect. Although in this paper bias current is used to change CIN(or CM), other means can also be adopted. For example, a series combination of a capac-itor and a switch can be connected between the base and collector of Q1 to achieve the same goal.

The triple band LNA shown in Fig. 20.8.1 was implemented in a standard 0.35µm SiGe BiCMOS process by the commercial foundry TSMC. The circuit parameters are: LB= 1.39 nH, R1= R2 = 300_{Ω, R}3= 600Ω, and C1= 3pF. Both transistors Q1and Q2have the same emitter size of 0.3µm x 20.3µm x 2 emitter fingers. Note that the circuit (excluding the patterns for testing) occupies a

very small area of 355µm x 155µm because only a small value (1.39nH) inductor LBis used. The noise and scattering parame-ters were measured on wafer using an automated NP5 measure-ment system from ATN Microwave Inc.

The measured S parameters of the LNA at room temperature when biased with IB1bcurrent source are shown in Fig. 20.8.3. The IB1bbase current results in a value of collector current for Q1 IC1 = 3.8mA. VCC is 1.5V. The input return loss S11 is below –27.6dB from 2.4GHz to 2.5GHz. Power gain S21is 13.8dB at 2.4GHz. S22shows a very broadband matching. Note, however, for low-IF/ zero-IF applications, S22does not have to be matched to 50 ohms. When IC1is decreased to 3mA by switching the base current of Q1 from IB1bto IB1a, CINis reduced and hence the quency band for input matching is switched to the higher fre-quency bands as shown in Fig. 20.8.4. Now S11is below – 34.2dB from 5.1GHz to 5.4GHz and below – 21dB from 5.7GHz to 5.9GHz. S21is 14.4 and 13.3dB at 5.2 and 5.7GHz, respectively. The noise performance is shown in Fig. 20.8.5. Noise figures of 3.18, 3.42 and 3.21dB are obtained at 2.4, 5.2 and 5.7GHz, respectively. The power consumptions are only 8.7 and 7.5mW for 2.4 and 5.2/5.7GHz bands, respectively.

From the experimental results, it is clear that a miniaturized fully monolithic 2.4/5.2/5.7GHz triple band LNA with low power consumption for WLAN applications is realized by a simple bias switching technique. The characteristics of the S parameters at temperatures other than room temperature are also shown in Figs. 20.8.4 and 20.8.5. From Fig. 20.8.4, it is clear that the res-onant frequency of S11 at room temperature with IC1 = 3.8mA shifts to lower frequencies of 2.1 and 1.4GHz at 50 and 100O_C, respectively. Nevertheless, even at 100O_{C S}

11 is still below –15.6dB from 2.4GHz to 2.5GHz. From Fig. 20.8.5, the resonant frequency of S11at room temperature with IC1= 3mA shifts to a lower frequency of 5.1GHz both at 50 and 100O_{C. S}

11is still below – 34dB from 5.15GHz to 5.35GHz and below – 21dB from 5.725GHz to 5.825GHz. Noise figures at VCC= 2V and room tem-perature were also measured and are summarized in Fig. 20.8.6. To our knowledge, the noise figure (2.73dB) achieved is a state-of-the-art result among all C-band silicon-bipolar based LNAs with on-chip matching network.

Acknowledgements

The authors thank the chip implementation center and nano device labo-ratory for technical support.

References

[1] T. Antes and C. Conkling, “RF Chip Set Fits Multimode Cellular/PCS Handsets,” Microwave RF, pp. 177-186, Dec. 1996.

[2] S. Wu and B. Razavi, “A 900-MHz/1.8-GHz CMOS Receiver for Dual-Band Applications,” IEEE JSSC, vol. 33, no. 12, pp. 2178-2185, Dec. 1998. [3] R. Magoon, I. Koullias, L. Steigerwald, W. Domino, N. Vakillian, E. Ngompe, M. Damgaard, K. Lewis, A. Molnar, “A Triple-Band 900/1800/1900 MHz Low-Power Image-Reject Front-End for GSM,”

ISSCC Dig. of Tech. Papers, pp. 408-409, Feb. 2001.

[4] K. L. Fong, “Dual-Band High-Linearity Variable-Gain Low-Noise Amplifiers for Wireless Applications,” ISSCC Dig. of Tech. Papers, pp. 224-225, Feb. 1999.

[5] H. Hashemi and A. Hajimiri, “Concurrent Dual-Band CMOS Low Noise Amplifiers and Receiver Architectures,” Dig. of Symposium on VLSI

Circuits, pp. 247-250, June 2001.

[6] S. S. Lu, C. C. Meng, T. W. Chen and H. C. Chen, “The Origin of the Kink Phenomenon of Transistor Scattering Parameter S22,” IEEE Trans.

ISSCC 2003 / February 12, 2003 / Salon 9 / 12:00 PM

20

Figure 20.8.1: LNA schematic. Figure 20.8.2: Method for the input matching.

Figure 20.8.3: Measured S parameters (for 2.4GHz band).

Figure 20.8.5: Measured noise figures. Figure 20.8.6: Performance summary.

Figure 20.8.4: Measured S parameters (for 5.2/5.7GHz band).



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