Design Of CMOS Low Noise Amplifier For WLAN Application

As the fast development of RF wireless communication, Wireless Local Area Networks (WLAN) has extended in school, enterprise and family with a incredible speed, being as an tool to increase working efficiency. CMOS RF wireless receiver front-end circuit has a vast market and a good prospect due to its nice character.In this paper, LNA which is the key point of RF communication has been researched.On the base of analyse of MOSFET noise theory, matching theory and traditional Cascade LNA noise module, a kind of LNA working under WLNA IEEE802.11b/g standard is designed using process of TSMC CMOS 0.18um.This LNA is Cascade structure with source of negative feedback. First, input and output matching is achieved of LC matching net. The small-signal and the noise figure of LNA are analyzed. The circuit is simulated by ADS2008 software. The result shows that: S(1,1)=-23.541dB, S(2,2)=-23.281dB,S(2,1)=19.638dB, nf(2)=2.461dB, 1dB compress point is -4.013dBm and with good stability. The LNA is satisfied with the demand. But the inductor used in input matching has a bad effect to the noise character of LNA because of its parasitic effect.Use the combination matching net of micro-strip and capacitance to design LNA instead of LC matching net. The result of simulation shows that: S(1,1)=-21.026dB, S(2,2)=-21.283dB , S(2,1)=20.368dB, nf(2)=1.007dB, 1dB compress point is -8.262dBm and with good stability . Under the working frequency of 2.4GHz, the noise figure has been reduced obviously, but the size of micro-strip is a little large. Therefore, this measure should be taken into consideration in practice.Because of the size of micro-strip is reduced as the increase of working frequency, this measure is suitable for high part of radio frequency. A combination match LNA is designed under WLAN IEEE802.11a standard. The result of simulation demonstrates that the size of micro-strip has reduced as 1/3 of which in 2.4GHz, and the noise character is good. Therefore, this measure has an important value in high frequency.