全CMOS噪声抵消和增益优化的宽带LNA

reduced harmonic distortion and to better power supply and substrate noise rejection.

In this paper we study a single-ended input LNA (Fig. 3), which combines the balun and LNA functionalities in order to obtain a simple and low cost LNA (trying to get the best of the two above described approaches).

We obtain a low noise figure LNA (NF < 3 dB), since the thermal noise of M1 is cancelled out. The noise produced by M1appears in phase at the two outputs, while the signals are in opposition. Thus, we double the gain and cancel the noise. The gain matching of the two stages is critical: we need the same Assuming a infinite transistor’s output impedance we can simplify (11) into,

AvLNA_c = + (12)

To achieve noise cancellation and balun operation (converting a single-ended input to a differential output) the CG and CS’s stages gain should be equal. Considering ro1(gmb1+gm1) >> 1 and for the same current and length (L) on M1 and M2, their output impedance (ro) are approximately equal, and making (gm1+gmb1) = gm2 = gm and R1=R2=RD, we gain to maximize the circuit performance.

Figure 3. Balun LNA with noise canceling [9].

1)Input Impedance

The LNA input impedance is the parallel of those of the CG and CS stages,

(7)

if it is assumed that the CS input impedance is very high,

and if the low frequency approximation is considered (2b),

(9) 2)Gain

Since the output signal is differential, and vthe CG and CS outputs, the differential gain is given by out1 and vout2 are

(10)

and if the low frequencies approximation is used (1b and 4b),

(11)

jy3

obtain from (11),

3)Noise Figure

Assuming the same approach for noise cancellation, the simplified noise factor is given by,

(14) 4)Dimensions and Biasing

The LNA is designed for 50 input impedance using

equation (2c) as a first approximation and imposing the transconductance of M1. Mresistors values are about 200 1 is biased with 2 mA.. The load to give a DC level at output that avoid signal limitation and to keep Msaturation region. The DC voltage V1 and M2 in the Mbias is used to adjust the DC current of 2 to the same value as that of M1. The dimensions are shown on table I.

TABLE I. LNA PARAMETERS

D m bias GS (mA)

( )

(mS)

(µm)

(µm)

(mv)

(mV)

M1 M2

5)Simulation Results

To validate the equations obtained previously for the

LNA’s performance parameters, and to find out the required level of approximation, a comparison is made with the simulation results.

The real part of the input impedance (Figs. 4 and 5) remains almost constant up to 10 GHz, and the imaginary part starts to be significant above 1 GHz, so the input matching must be

designed carefully for wideband applications. Equation (9) can be used for this purpose.

We confirm by simulations that equations (9) and (11) are accurate for our design, as shown in Figs. 4-6.