Broadband LNA andSummary of a paper about gm-Boosted
Common-Gate LNA
Xiaoyoung Li, Member, IEEE,Sudip Shekhar, Student Member, IEEE,
David J. Allstot, Fellow, IEEE&
Summarized by Taeyoung Chung
2
Outline
• Broadband LNA Techniques
• Common-Gate LNA
• gm-Boosted Common-Gate LNA
– Principle of Operation
– Capacitor Cross-Coupled gm-Boosted CGLNA
– Transformer-Coupled gm-Boosted CGLNA
3
Broadband LNA Techniques
• To obtain broadband input matching, the input impedance of
the amplifier should be resistive and equal to 50Ω over the en-
tire bandwidth.
• Broadband input matching is difficult for common source type
amplifiers due to capacitive nature of the transistor input
• Types of broadband LNA
– Resistive Feedback
– LC Ladder Matching Network
– Common Gate Input Stage
– Distributed Amplifiers
4
Common Gate
• Gain and Input Matching
– The input matching is achieved by making the effective input re-
sistance(1/gm) equal to RS(50Ω)
1
m Lv
L
o
g ZA
Z
r
11 L
inm o
ZR
g r
5
Common Gate
(1 )
in gs m gs S
m S gs
V V g V R
g R V
,
1
d m gs
mg
m S
m eff g
i g V
gV
g R
g V
1
1gs gm S
V Vg R
, 1m
m effm S
gg
g R
Small signalequivalent circuit
• Noise Factor
–
– Source Degeneration
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( ) ( )
( ) ( ) ( )
/ /
/ ( ) /
1
i i source i i source o totali
o o o total i o total i source
o total o source o added o added
o source o source o source
S N S N NSNRF
SNR S N S G N G N
N N N NF
N N N
6
Common Gate
• Noise Factor2
222
2 22
1
1 11 1
1
ndm S nd
m SnSm SnS
m S
ig R i
Fg Rig R
ig R
204nd di kT g f
2 14nS Si kTR f
2
01
02
1
4 11
4
1 1m
m S
d
S m S
d
g RS
kT g fF
kTR f g R
g
g R
• ins : the current noise of the source resistance
• ind : the drain current noise of the MOSFET
• Induced gate noise is usually negligible.
7
gm-Boosted CGLNA
• Noise Factor
2
01
02 2
(1 ) 1
4 11
4 (1 )
1 1(1 ) (1 )m
m S
d
S m S
d
S A g R
kT g fF
kTR f A g R
g
A g R A
(1 )m mG A g
=> Enhanced NF by the fraction (1+A)
8
Capacitor Cross-Coupled gm-Boosted CGLNA
1c
c gs
CA
C C
(1 ) 1
1(1 )
12
m SA g R
c gs
c gs
FA
C C
C C
In general, Cc>>Cgs
12
F
9
Capacitor Cross-Coupled gm-Boosted CGLNA
• Two drawbacks of Capacitor Cross-Coupled CGLNA
① It consumes twice the bias current and silicon area as its sin-
gle-ended counterpart
② A is always less than one due to the capacitor divider between
Cgs and coupling capacitance Cc.
1c
c gs
CA
C C
10
Transformer-Coupledgm-Boosted CGLNA
(a) Single-ended (b) Fully differential
(c) Small signal model
S xx m gs S
P P
Mi vi g v i
L sL ⓐ
g PS
S S
v Mii
sL L ⓑ
Sgs
gs
iv
sCⓒ
S xP
P P
Mi vi
L sL ⓓ
g x gsv v v ⓔ
/ P Sk M L L
/S Pn L L
From the small signal equivalent circuit,
coupling coefficient
turns ratio
11
Transformer-Coupledgm-Boosted CGLNA
• Solving (a)-(e) in the previous slide,
2 2
2 2 2 2
1 / 1 / 11 1
1 1 1 1m S S gs m S S gsx
inx P SS gs S gs
k n g sL k n s L C g sL nk s L CiY
v sL sLk s L C k s L C
11 (1 )S S
in m gs m gsP P P
L Lk kY g sC g nk sC
sL n L n L
211 (1 2 )in m gs
P
Y nk g nk n sCsL
2 21 1S gsk s L C
2 /S Pn L L
1(1 )
Fnk
(when induced gate noise is not considered)
12
Transformer-Coupledgm-Boosted CGLNA
• when induced gate noise is considered,
• To minimize F, choose k as a maximized value. (k=1)
• Setting results in
22 2
3
1 211
1 5 1T
nk nwF
nk w nk
2
11 1
1 5 T
wF n
n w
channel noise∝1/(1+n)
induced gate noise∝1+n
/ 0F n
2
51T
opt
wn
w
1 25opt
T
wF
w
(optimum noise factor)
13
Transformer-Coupledgm-Boosted CGLNA
• Result