A concurrent dual-band low noise amplifier with gain enhancement topology for 2.4/5.2 GHz applications

: This paper presents a dual-band low-noise amplifier (LNA)operating at 2.4/5.2 GHz for wireless local area network (WLAN) applications. The LC parallel resonance and LC series network are adopted to achieve input impedance matching and noise matching simultaneously at dual-band. Besides, a small-size capacitor with a coupled inductor is added for gain-enhancement. The simulation results show that the LNA has gains(S21) of 17.1 dB and 8.5 dB with a noise figure of 3.6 dB and 3.3 dB at 2.4 GHz and 5.2 GHz, and input return loss(S11) of -15.6 dB and -13.8 dB while output return loss(S22) of -11.7 dB and -20.7 dB at two operating frequencies, respectively. Therefore, the proposed LNA structure is an attractive alternative to WLAN applications.


Introduction
With the development of the multiple standardization and miniaturization of a communication system [1], the multi-band RF front-end suitable for the interconnection between various applications receives noticeable attention.
As the first active stage of the RF receiver, a low noise amplifier (LNA) has a great impact on the performance of the RF front-end.Most of the articles about LNA published recently are based on reconfigurable LNA [2,3], broa dBand LNA [4] and concurrent LNA [5][6][7].The advantages of reconfigurable LNA and broa dBand LNA are area consumption and bandwidth respectively, but their performance will deteriorate dramatically when applied in dual-band applications.Therefore, dualband concurrent LNA is an attractive choice to make a trade-off between area consumption, noise, linearity and gain.
Generally, matching networks implemented in concurrent LNA are narrowband matching at input/output or inter-stage [6~9], and an external large resistor should be added for DC isolation.However, the thermal noise of the isolation resistor combining with the gate noise of MOSFET will significantly deteriorate the noise figure of LNA at high frequency.In this paper, a trap network accompanied by a series LC network is applied in the LNA for DC isolation and dual-band impedance matching, removing the effect of thermal noise of isolation resistor.Besides, a small-size capacitor with a coupled inductor is used to improve the gain.
The paper is organized as follows.Section II describes the design of the proposed circuit topology.Section III shows the layout simulation result of the LNA and section IV concludes this paper.

Circuit design
As shown in fig. 1, the cascade structure is adopted in the proposed LNA to alleviate the Miller effect and obtain better reverse isolation.Meanwhile, the series LC combining with a parallel LC works as a dual-band impedance matching network.Besides, a capacitor with a coupled inductor is inserted to enhance the gain.

Dual-band input and output impedance
Fig. 2 shows the simplified small-signal equivalent circuit, and the input impedance,, can be expressed as follows: In the equation, g m1 is the transconductance of M 1 , Ls is the negative source fee dBack inductor, C gs is the gatesource capacity of M 1 , and Ω is the angular frequency related to 2.4 GHz or 5.2 GHz.To obtain dual-band impedance matching, the real part and imaginary part of Z in should be equal to 50 Ω and zero respectively.And the real part of dual-band input impedance is given by: the imaginary part is given by: As shown in Equation ( 4) and ( 5), the dual-band input impedance can be presented by the following parameters.
Output impedance is described by equation( 6):

Noise analysis
This section describes the combination of impedance matching and noise optimization.And the equivalent circuit for noise analysis of the proposed LNA is shown in Fig. 4.

Figure 4: the equivalent circuit for noise analysis
The noise factor is described by equation [7]: In equation [7], R n represents the source resistance, and G s , Y s , and Y opt are the source conductance, the source admittance and the optimum source admittance respectively.And the parameters can be expressed as the following equations.
In the equation, α is the constant related to the process, which α = g m /g do , g do is the drain-source conductance when the drain-source voltage Vds is equal to 0.γ is the channel noise figure of the transistor, which is related to the channel length.
When Y s is equal to Y opt , the minimum value of F is equal to F min .According to Thomas theory [12], the noise factor F min of the LNA can be expressed as: ( ) In the equation, δ is the gate noise figure , and c is the correlation coefficient between the gate current noise and the leakage current noise, which is approximately equal to j0.395 in theory [13], reflecting the coupling capacitance between the channel and the gate induced noise source.
The optimum noise impedance is given by: Therefore, noise matching and impedance matching can be obtained simultaneously.

Gain enhancement technology
Generally, the LNA consists of two stages or more to obtain high gain.To reduce area consumption, a cascade structure with gm enhancement was implemented in the proposed single-stage LNA.Traditional technology adopts a cascade structure to eliminate the Miller effect and provide better reverse isolation [14].Base on the small-signal analysis, the gain of the LNA without LC resonance network can be derived as: ( ) In the equation, r o1 is the output resistance of M 1 , r o2 , is the output resistance of M 2 , g m2 is the transconductance of M 2 , g mb2 is the substrate transconductance of M 2 , r o2 is the output resistance of M 2 , and parameter k, Χ, β, are given by: ( ) ( ) When the LC resonance network is added, parameter k is expressed as k1, which is given by: ( ) ( ) where, obvious that the gain can be enhanced by making an adjustment of L3/C3, which is presented in the simulation results.

Simulation results
This section presents the simulation results of the proposed dual-band LNA operating at 2.    The performance of the proposed LNA is summarized in Table 1, which is compared to other papers published working at 2.4 GHz and 5. 2 GHz.The performance shows a good trade-off between impedance matching, noise, linearity and gain.However, the proposed LNA consists of one-stage, which is designed under the limitation of minimum noise figure.And multi-stage cascade structure can be adopted to reduce the design difficulty without the limitation of area consumption and power dissipation.Besides, folded cascade structure with lower supply voltage is an attractive choice for low power applications.

Conclusions
A dual-band concurrent LNA operating at 2.4 GHz and 5.2 GHz with gain enhancement technology is presented in this paper.The LNA designed shows a noise figure below 3.6 dB and the input /output return loss is better than-10 dB.And the power consumption is 9.8mW with a power supply voltage of 1.2V.As the performance mentioned above exhibits, the proposed concurrent dual-band LNA is suitable for multi-frequencies RF systems like WLAN.

Conflict of interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Figure 1 :
Figure 1: Schematic of the proposed dual-band LNA

4
GHz and 5. 2 GHz.The layout of the proposed LNA is shown in Fig.5.S-parameters, NF and P1 dB of the LNA are shown in Fig 6-11 respectively.The proposed LNA is implemented on the SMIC 0.13um 1P8M CMOS process, and the chip size of the layout is 642.6 x910.1 .The simulation results of the LNA exhibits S11 of -15.6 dB and -13.8 dB, S22 of -11.7 dB and -20.7 dB at 2.4 GHz and 5.2 GHz, which are shown in Fig. 6-7.The LNA exhibits 17.1 dB of gain, 3.6 dB of noise figure, -16.49dBm of P1 dB at 2.4 GHz; 8.5 dB of gain,3.3dB of noise figure, -9.53 dB of P1 dB at 5.2 GHz, which are shown in Fig.7-10.With a supply voltage of 1.2V , the power consumption of the receiver is 9.8 mW.As a summation, table1 shows all the simulation results.