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Design of a Low Noise Amplifier and Mixer in 0.5um CMOS Process
Dr. Yao
RFIC Design
EE 5390
Miguel Hernandez IV
Rebecca Sullivan
April 19, 2004
Abstract
The performance of a single ended low noise amplifier (LNA) and a doubly
balanced Gilbert mixer fabricated in a 0.5um CMOS process are presented. The LNA
operates in the 940MHz to 980MHz frequency range, and has demonstrated a voltage
gain of 21dB, a noise figure of 1.25dB, and an input impedance of 48.43 at 960 MHz.
The P1dB was found to be –15.03dBm. The IIP3 was not measured due to a problem
with convergence. The system used a total current of 3.68mA. The mixer is operated
with an RF input of 960MHz and an LO input of 950MHz. The mixer has demonstrated
a conversion gain of 20dB, a DC offset of 0.39mV, LO leakage at the IF output of
8.4x10-17V, and LO leakage at the RF input of 0.0V.
Introduction
Many communication systems require that small signals be amplified while
preventing the addition of noise hence, preserving a signal-to-noise ratio at low power
levels. The role of an LNA in a receiver design is to do precisely this task. LNA’s are
also constructed to amplify large signals while avoiding distortion, thus eliminating
channel interference. Often the first stage of a receiver is an LNA and its main function
is to provide the gain needed to prevail over the noise of the stages that follow, such as
the noise created by mixers.
Communication systems also require baseband information signals to be shifted
to a frequency suitable for electromagnetic propagation to the desired destination. At
the destination, we reverse this process, shifting the received radiofrequency signal
back to baseband to allow the recovery of the information it contains. This frequency-
shifting function is known as mixing; the stages that perform it are mixers. Any device
that exhibits amplitude-nonlinear behavior can serve as a mixer, as nonlinear distortion
results in the production, from the signals present at the input of a device, of signals at
new frequencies. The RF port senses the signal to be converted and the LO port
senses the periodic waveform generated by the local oscillator. The IF port contains the
frequency translated signal.
Theory and Design
LNA. When designing a low noise amplifier, considerations must be taken to the
tradeoffs between specifications such as the desired gain, the noise figure, input
impedance, and power consumption. All of the LNA parameters are important but they
do not always work in each other’s favor. The tradeoffs include an increase in power
consumption when the width of the design NMOS transistor increases or if the dc
voltage increases. The noise figure increases if additional noise, such as thermal noise
from resistors is added. The gain of the amplifier changes when the dc voltage changes
or when the widths of the transistors are changed, unfortunately, the total current of the
design increases as well. Also the capacitances and inductances at the input and
output of the system affect the input impedance and the peak gain of the amplifier,
respectively.
Furthermore, two main design methods for low noise amplifiers are discussed.
They are the wideband design method and the narrowband design method. An
important difference between the two is their frequency response. For instance,
wideband low noise amplifiers require a wideband matching network that is needed to
provide a flat frequency response at the output, while narrowband low noise amplifiers
require a matching network at the input and output. For this project, the narrowband
LNA design method is chosen because it also differs from a wideband LNA in that it is
able to reduce the DC power substantially in addition to providing good quality low-noise
performance. The specifications for the LNA designed are shown in Table 1 and the
design itself shown in Figure 1.
Table 1: LNA Design Specifications
Frequency range :
Input Impedance, |Zin|
940 MHz ~ 980 MHz
50 Ohms ±10%
Voltage gain, Av
> 20 dB
Noise Figure, NF
< 3 dB
P1dB
> -20 dBm
IIP3
> -10 dBm
Total current
< 5 mA
Figure 1: Single-ended Low Noise Amplifier
When simulating the s-parameters, the component values were adjusted in order to
provide better results. The component values used for the design are shown in Table 2.
Table 2: Component Values
Vdd
2.7 V
R1
2 KΩ
R2
15 KΩ
Ld
7 nH
Lg
7 nH
Ls
1.5 nH
Cb
10 pF
CL
3.6 pF
Zin
50 Ω
Zout
1.5 kΩ
M1
900 μm
M2
700 μm
M3
150 μm
Mixer. Mixers can be divided into four categories: Passive, active, singlebalanced, and double-balanced. Passive mixers do not provide any gain, and typically
achieve higher linearity. Active mixers, on the other hand, provide gain and help reduce
the effect of noise contributed by subsequent stages. Single-balanced mixers accept a
differential LO signal and a single-ended RF signal. A drawback of this topology is LOIF leakage. A double-balanced mixer accepts both differential LO and RF signals. It
has an advantage over signal-balanced mixers in that LO-IF feedthrough is cancelled.
Because of the advantages that active mixers and double-balanced topologies provide,
an active mixer with a double-balanced topology, also known as a Gilbert mixer, was
selected, and is shown in Figure 2.
The proposed mixer was designed with the following design specifications shown
in Table 2 in mind.
Table 2. Mixer Design Specifications
RF Input
LO Input (positive node)
LO Input (negative node)
IF Output
Frequency [MHz]
960
950
950
10
Amplitude [V]
0.001
0.2
0.2
n/a
DC Level [V]
0
2
2
n/a
One of the most important design parameters of a mixer is the conversion gain.
The conversion gain of a mixer is defined as the ratio of the IF voltage amplitude and
the RF voltage amplitude. The equation that describes the conversion gain for the
proposed Gilbert mixer is shown below:
Gc 
4

g m1 xRL 
(1)
where,
gm1 = transconductance of transistor M1
RL = load resistance.
With the design specifications and conversion gain in mind, the Gilbert mixer in
Figure 2 was designed and simulated in Advanced Design System. Table 3 shows the
values of all components used in the Gilbert Mixer.
Figure 2. Gilbert Mixer
Table 3: Component Values
Component Value Unit
R1
500 Ohms
R2
500 Ohms
Vdd
5
V
Iss
10
mA
W_M1
500
Um
L_M1
0.5
Um
W_M2
500
Um
L_M2
0.5
Um
W_M3
500
Um
L_M3
0.5
Um
W_M4
500
Um
L_M4
0.5
Um
W_M5
500
Um
L_M5
0.5
Um
Results
LNA. The following results were all measured by using the Advanced Design
System. Figure 3 shows a plot of the input impedance of the amplifier. Figure 4 shows
the amplifier gain. Figure 5 shows the noise figure of the amplifier. Table 4 shows the
results measured for the design at 940MHz, 960MHz, and 980MHz.
Figure 3: Input impedance of LNA
Figure 4: Voltage Gain of LNA
m2
22
m1
freq= 960.0MHz
S(1,1)=0.611 / -92.036
impedance = Z0 * (0.442 - j0.862)
S(1,1)
dB(S(2,1))
21
20
m2
freq= 963.0MHz
dB(S(2,1))=21.527
19
18
m1
17
940
945
950
freq (940.0MHz to 980.0MHz)
Voltage Gain,
Av (dB)
Noise Figure,
NF (dB)
Input
impedance
(Ω)
nf(2)
1.26
m3
1.25
m3
freq=960.0MHz
nf(2)=1.250
1.23
945
950
955
960
965
freq, MHz
965
970
975
970
975
940
MHz
960
MHz
980
MHz
17.854
21.406
19.413
1.269
1.25
1.231
63.49
48.43
59.33
980
P1db (dBm)
-15.03
Mixer. Figures 6 and 7 below show the IF voltage output at 10MHz and the RF
voltage input at 960MHz respectively. The conversion gain, defined as the ratio of the
IF voltage output to the RF voltage input, DC offset at IF, and LO leakage at IF and RF
are shown in Table 5.
980
Table 4: Design Metrics
1.27
940
960
freq, MHz
Figure 5: Noise Figure of LNA
1.24
955
Figure 6: IF Voltage Output
Figure 7: RF Voltage Input
Vif
Vrf
0.0020
m1
0.012
0.0018
0.0016
mag(HB.VrfP)
mag(HB.Vo)
0.010
0.008
m1
freq= 10.00MHz
mag(HB.Vo)=0.010
0.006
0.004
0.0014
0.0010
0.0008
0.0006
0.0004
0.002
m2
0.0012
0.0002
m2
freq= 960.0MHz
mag(HB.VrfP)=0.001
0.0000
0.000
-10
0
10
20
30
940
960
freq, MHz
freq, MHz
Table 5: Metrics
Metric
Conversion Gain
Conversion Gain
Amplitude of LO leakage at IF
Output
Amplitude of LO leakage at RF
Input
DC Offset at IF Output
Unit
[V]
[dB]
Value
10.4
20.4
[V]
8.40E-17
[mV]
[V]
0
0.39
Conclusion
The performance of a single ended low noise amplifier (LNA) and a doubly
balanced Gilbert mixer fabricated in a 0.5um CMOS process were presented. The LNA
demonstrated a voltage gain of 21 dB, a noise figure of 1.25 dB, and an input
impedance of 48.43  at 960 MHz. The P1dB was found to be –15.03 dBm, the IIP3
was not measured due to a problem with convergence, and the system used a total
current of 3.68 mA. The mixer demonstrated a conversion gain of 20dB, a DC offset of
980
0.39mV, LO leakage at the IF output of 8.4x10-17V, and LO leakage at the RF input of
0.0V.
References
T. Lee, “The Design of CMOS Radio Frequency Integrated Circuits,” pp. 272- 305,
1998
B. Leung, “VLSI for Wireless Communication”, pp.74- 114, 2002
J. Lucek and R. Damen, “Designing an LNA for a CDMA Front End”,
www.rfdesign.com, February 1999
V. Geffroy, G. De Astis, E. Bergeault, “RF Mixers Using Standard Digital CMOS
0.35um Process,” IEEE MTT-S Digest, 2001
B. Leung, “VLSI for Wireless Communication,” pp. 118-216, 2002
T. Lee, “The Design of CMOS Radio Frequency Integrated Circuits,” pp. 134144, 178-216, 308-337, 1998
B. Razavi, “RF Microelectronics,” pp.180-204, 1998
P.J. Sullivan, B.A Xavier, W.H. Ku, “Low Voltage Performace of a Microwave
CMOS Gilbert Cell Mixer,” IEEE Journal of Solid-State Circuits, Vol. 32,
No. 7, July 1997