Consideration for Wi-MAX Switchable LNA

In general the conventional Wi-MAX LNAs are wideband LNA [1][2] for example 2-11GHz [1] and 2-6GHz [2]. However the frequency bands of Wi-MAX are different in different country. Thus there is no need to use the whole band of Wi-MAX. That is, different frequency band can use different LNA with narrow band so that we have advantages including

1. Reducing the input noise.

2. Restraining the interference from other bands.

It can reduce the input noise then raises the input SNR. Because if the LNA is whole band, all the noise in the whole band will be included in the system

as shown in Fig. 4.1. Hence the total noise is not decreased and all the noise including out of desired band will be amplified by the LNA. Hence we should use better filters to remove the noise at back-end. Another benefit is it can restrain

2 3 4 5 6

GHz Band

Noise

(a)

2 3 4 5 6

GHz Band

Noise

(b)

Figure 4.1: The input frequency is 3.5GHz, and noise is in two condition: (a) narrow band, (b) wideband.

the interference from other bands. If using the whole band LNA, all other signals will be received as shown in Fig. 4.2. But those signals are not in the desired band. They will interfere with the our desired signal. Also, the harmonics of those unwanted signals also distort our the desired signal. As discussed in Sec.

2.3, not only the 3-rd intermodulatoin but also 5-th and 7-th intermodulatoin can distort desired signal. Hence the switch band LNA is desired.

4.2 Wideband Input Matching

As discussed in Sec. 2.1, we need input matching circuit to get input matching.

However the switch bands is 4 bands (2.3GHz, 2.5GHz, 3.5GHz, 5.8GHz), thus using switchable input matching circuit is too complex. The switch can product

2 3 4 5 6

GHz Band

interference signal

(a)

2 3 4 5 6

GHz Band

interference signal

(b)

Figure 4.2: The input frequency is 3.5GHz, and the interferences are in two condition: (a) narrow band, (b) wideband.

the noise, because it is a noise source. The noise can influence our noise perfor-mance directly, hence we use the wideband input matching circuit as discussed in Sec. 3.2. The wideband matching circuit that we chose is shown in Fig. 4.3. But the design frequency is different, in this circuit the band we used is 2.3GHz to 5.8GHz. Hence we redesign the wideband matching circuit. In a word, according to Eq.(3.3) we first design the input impedance in 5.8GHz by adjusting Lg, Ls, CC, Cb and M1 in the inductive source degeneration circuit. Then according to Eq.(3.10) we design the input impedance in 2.3GHz by adjusting Ld. Finally the two band results in a wideband input matching.

4.3 Switch Circuit Topology

As discussed in Sec. 3.3, we already have a good switch band circuit. In this section we improve our switch band circuit again. From Fig. 3.11(c) , we remove

M

V

^L

L

v

R

L

C

C

Figure 4.3: The input matching for Wi-MAX LNA.

the capacitor that connects to V_dd, then connects the source of switch NMOS to ground as shown in Fig. 4.4, where C is the switched capacitor.

M

L

R V

Z

C V

Figure 4.4: The switch circuit of Wi-MAX LNA.

The benefit is this circuit saves a capacitor and reduces the size of loading capacitor, because now the loading capacitor is C when switch MOS is on. Hence the equivalent capacitance is C when Msw is on. However the conidtion is the same with Fig. 3.11(c) when Msw is off. Hence the equivalent capacitance is

Ceq = CgsCgd

C_gs+ C_gd, when Msw is off. That is

Ceq =

The resonant frequency is

ω = 1

pLCeq

.

The impedance of the band switch circuit shown in Fig.4.4 and Fig.3.11 are shown in Fig .4.5. The largest impedance at low frequency is the NMOS-based with large resistor connects to ground. This is because its ron is smallest, and the Q is largest at the low band. The low r_on is due to high mobility of NMOS and the size of switch. This implies that at the low band the gain of NMOS-based switch with large resistor connects to ground is the largest and that of the PMOS-based is the smallest. At the high band the gain of NMOS-based switch with large resistor connects to ground and the gain of NMOS-based switch with large resistor are both the smallest. This is because the condition is the same.

As discussed in Sec. 3.3 the PMOS-based switch is the largest. From Fig. 4.5 we can find the impedance of every switch is larger at high band than at low band. Hence the gain of low band is lower than high band. Thus in order to fit specification, the gain of low band is a more important issue. Hence we choose the NMOS-based with large resistor connects to ground as our switch band circuit to increase the gain of low band .

4.4 Output Buffer

As discussed in Sec. 3.4, we use buffer to achieve output matching. From Eg.(3.14), the output impedance is

Z_out = approach zero at the band that we want.

1G 2G 3G 4G 5G 6G 7G

NMOS with resistor

NMOS with resistor connects to ground impedance

NMOS with resistor

NMOS with resistor connects to ground impedance

(ohm)

(b)

Figure 4.5: The impedance of four types switch band circuit: (a) at low frequency, (b) at high frequency.