RF Transceiver Module Design Chapter 5 Low Noise Amplifier

RF Transceiver Module Design
Chapter 5
Low Noise Amplifier
李健榮助理教授
Department of Electronic Engineering
National Taipei University of Technology

Outline
• Basic Amplifier Configurations
• Cascode Low Noise Amplifier (LNA)
• Feedback Topologies
• Classical Two-port Noise Theory
• Input Matching for an LNA
• Noise Figure and Bias Current
• Effect of the Cascode on Noise Figure
• Summary
Department of Electronic Engineering, NTUT2/26

Simple Transistor Amplifier (I)
• Common-emitter (CE) configuration
• Common-base (CB) configuration
• Common-collector (CC) configuration
CE (driver)
CCV
inV
outV
EEV
CB (cascode)
CCV
inV
outV
EEV
CC (buffer)
CCV
inV
outV
EEV

• Bipolar Transistor Amplifier
• MOSFET Transistor Amplifier
Simple Transistor Amplifier (II)
CE CB CC
Current Gain High (β) Low (~1) High (1+β)
Voltage Gain High High Low (~1)
Power Gain High Medium High
Zin Medium Low High
Zout Medium High Low
I/O Phasing 180o In-phase In-phase
CS CG CD
Voltage Gain High High Low (~1)
Power Gain High Medium High
Zin High Low High
Zout High High Low
I/O Phasing 180o In-phase In-phase

Common-Emitter Configuration
• Gain
• Input Impedance
o L
vo m L
i b e
v r Z
A g Z
v r r r
π
π
= = −
+
≃
er : B-E diode resistance as seen from emitter
er rπ β= 1m eg r=
in bZ r rπ= +
For low frequencies, the parasitic capacitances
have been ignored and rb has been assume to be
low compared to .rπ
CE (driver)
CCV
inV
outV
EEV
LZormg vπrπCπ
br
iv ov
Cµ
vπ
+
−
+
−
+
−
and

Miller Effect (I)
• Impedance that connects from input to output
fZ
LZ
inv outv
inZ outZ
vA
( ) 1
fin
in
in out f v
Zv
Z
v v Z A
= =
− −
( ) ( )
for 1
1 1
fout
out f v
out in f v
Zv
Z Z A
v v Z A
= = >>
− −
≃
fC
LZ
inV outV
inZ outZ
vA
( )
1 1
1 1
f
in
v f v
sC
Z
A sC A
= =
+ +
( ) ( )
1 1
1 1 1 1
f
out
v f v
sC
Z
A sC A
= =
+  + 
Like larger cap
Slightly larger

Miller Effect (II)
• At radio frequencies:
• Miller’s theorem
Cπ : Low impedance
Cµ : Provides feedback
( )1 1o
A m L m L
v
C C C g Z C g Z
v
µ µ µ
π
 
= − = + 
 
≃
1
1 1B
o m L
v
C C C C
v g Z
π
µ µ µ
   
= − = +   
   
≃
The dominant pole is usually the one formed by
andAC Cπ
( ) ( )1
1
2 ||
p
b s A
f
r r R C Cπ ππ
=
+ +  
sR : source resistance
Note that as ZL decreases, CA is reduced
and the dominant pole frequency is
increased.
Cµ
vπ ov
vπ ov
AC
BC

Simplified CE Small-signal Model
• Simplified model for transistor above the dominant pole:
Ignore and just use in transistor model with little error.
• Knowing the pole frequency, we can estimate the gain at
higher frequencies, assuming that there are no other poles
present, with
( )
1
1
vo
v
p
A
A f
f
j
f
=
+
目前無法顯示此圖像。
rπ
br
iv vπ Cπ
Cµ
mg vπ or LZ ovsv
sR
+
−
+
−
+
−
+
−

Common-Base Configuration
• CB amplifier is often combined with the CE amplifier to from
an LNA but it can be used by itself as well.
• Low Zin when driven from a current source, it can pass current
through it with near unity gain up to very high frequency.
Therefore, with an appropriate choice of impedance levels, it
can also provide voltage gain.
ini
br
vπ
Cµ
Cπ
mg vπrπ LZ
outi
+
−
Ignoring output impedance

Cascode LNA (I)
• CB + CE to form a cascode LNA.
• Since the CB amplifier has a current
gain of approximately 1, then,
ic1 ≈ ic2 = gm1vi .
• The gain is the same as for the CE
amplifier. However, the cascode
transistor reduces the feedback of ,
resulting in increased high-
frequency gain.
1Cµ
CR
CCV
CbiasV
outv
inv
EEV
Driver Q1
Cascode Q2
2ci
1ci
( ) ( )1
1 1
1
2 || 2
p
b s
f
r r R C Cπ π µπ
=
+ +  
21 mg≈

Cascode LNA (II)
• Advantages:
Improves frequency response.
Adding another transistor improves the isolation.
• Disadvantages:
Additional poles can become a problem for a large load resistance.
An additional bias voltage is required, and if this cascode bias node is
not properly decoupled, instability can occur.
Reduce signal swing at a given supply voltage, compared to the simple
CE amplifier.

Common-Collector Configuration (I)
• The CC amplifier (emitter follower) is a very useful general-
purpose amplifier.
• Voltage gain is close to 1 (buffer).
• High input impedance and low output impedance (good
buffer/output stage).
ER
CCV
iv
EEV
ov
iv
B s bR R r= +
vπ rπ Cπ
ER
mg vπ
Cµ
+
−

Common-Collector Configuration (II)
• Miller effect is not a problem, since the collector is grounded.
• Since is typically much less than , it can be left out of the
analysis with little impact on the gain.
• The input impedance:
• The output impedance:
CµCπ
iv
B s bR R r= +
vπ rπ Cπ
ER
mg vπ
Cµ
+
−
( )1A E mZ Z R g Zπ π= + +
1
1
B B
out e
m m
r R sC r R
Z r
g r sC r g
π π π
π π π
+ +
= ≈ ≈
+ +

CE with Series Feedback (I)
• CE with Series Feedback (Emitter Degeneration)
Cascode:
Higher frequencies, superior
reverse isolation, but suffers
from reduced linearity.
Most CE and cascode LNAs:
Employing the degeneration
transforms the impedance real
part looking into the base to a
higher impedance for
matching. De-generation also
trades gain for linearity.
outRF
CCV
1L 1C LR
inRF 1Q
eL
CE tuned LNA
CCV
1L 1C LR
2Q
1Q
eL
inRF
biasV
outRF
Cascode tuned LNA

CE with Series Feedback (II)
• As the degeneration becomes larger, the gain becomes solely
dependent on the ratio of the two impedances.
• If ZE is inductive, then it will become a real resistance when
reflected to base (raise Zin, useful for matching purposes).
• Conversely, if ZE is capacitive, it will tend to reduce Zin and
can even make it negative.
1
out m L L
in EE
m E
v g R R
v ZZ
g Z
Zπ
−
= ≈ −
 
+ + 
 
sR rπ Cπ mg vπ
EL EREC
xiinv
vπ
Zπ
EZ
+
−
( )1in E mZ Z Z g Zπ π= + +

CE with Shunt Feedback (I)
• Matching over a broad bandwidth while having minimal
impact on the noise figure.
• Rf forms the feedback and Cf allow for independent biasing.
• Can be modified to become a cascode amplifier.
• Ignoring the Miller effect and assuming Cf is a short circuit
(1/ωCf << Rf ), the gain is given by
1 1
L
m L
o m LF
v
L Li
f f
R
g R
v g RR
A
R Rv
R R
−
−
= = ≈
+ +
The gain without feedback (−gmRL) is reduced
by the presence of feedback.
sR
fC
fR LR
ov
sv
inZ
outZ

CE with Shunt Feedback (II)
• Input impedance
The last term, which is usually dominant, shows that the input impedance is equal
to Rf +RL divided by the open loop gain. Input impedance for the shunt feedback
amplifier has less variation over frequency and process than open-loop amplifier.
• Output impedance
• Feedback results in the reduction of the role the transistor
plays in determining the gain and therefore improves linearity,
but the presence of Rf may degrade the noise depending on the
choice of value for this resistor.
( )
( )
|| ||
1
f L f L f L
in f
f L m L m L m L
Z R R R R R R
Z R Z
R R Z g R g R g R
π
π
π
+ + +
= ≈ ≈
+ + +
( ) ( )1 || ||1
1 || ||
f f
out
m s f
s f m
f
R R
Z
g R R Z
R R Z g
R
π
π
= ≈
  +
+ −  
 

Example
2.5 pF
2 V
3 V
LR
sR
fR
12-GHz fT transistors
currents about 5 mA
ov
sv
Input matching
Sample plots using shunt feedback
22
20
18
16
14
12
10
100 300 500 700 900 1100 1300 1500
Gain
Noise figure
OIP3
IIP3
2
0
2−
4−
6−
8−
10−
3
2.5
2
1.5
1.0
0.5
0
IIP3
(dBm)
NF
(dB)
Rf
Gain(dB),OIP3(dBm)

CE w/ Shunt Feedback and CC Output Buffer
• CE with an output tends to make for a better match.
• With an output buffer, the voltage gain is
no longer affected by the feedback, so it is approximately that
of a CE amplifier given by [RL /(RE + 1/gm )] minus the loss in
the buffer.
fC fR
LR
CCV CCV
biasI
ER
inV
outVC

Classical Two-port Noise Theory (I)
• Use these equivalences, the expression for noise factor can be
written purely in terms of impedances and admittances:
Noisy
Two-portsYsi sYsi
ne
ni
Noiseless
Two-port
22
2
s n s n
s
i i Y e
F
i
+ +
= n c ui i i= + c c ni Y e=
( )
2 22 2 2
2 2
1
s u c s n u c s n
s s
i i Y Y e i Y Y e
F
i i
+ + + + +
= = +
where
2
4
n
n
e
R
kTB
≡
2
4
u
u
i
G
kTB
≡
2
4
s
s
i
G
kTB
≡
( ) ( )
2 2
2
1 1
u c s c s nu c s n
s s
G G G B B RG Y Y R
F
G G
 + + + ++ +  = + = +
, ,and

Classical Two-port Noise Theory (II)
• Optimum source admittance:
s c optB B B= − = 2u
s c opt
n
G
G G G
R
= + =and
2
min 1 2 1 2 u
n opt c n c c
n
G
F R G G R G G
R
 
 = + + = + + +  
 
( ) ( )
2 2
min
n
s opt s opt
s
R
F F G G B B
G
 = + − + −  
GA circles
NF circles
Input
matching
Output
matching
Amplifier
sΓ LΓ
0Z
0Z
inΓ outΓ
outZinZ
Department of Electronic Engineering, NTUT
Min. noise figure, min ,, s optNF Γ
Max. available power gain, s in
∗
Γ = Γ
21/26

Input Matching of LNAs for Low Noise
• Many methods for matching the input using passive circuit
elements are with varying bandwidth and complexity.
• Use two inductors to provide the power and noise match for
the LNA, the input impedance is (assume Miller effect is not important
and that rπ is not significant at the frequency of interest)
• To be matched:
, therefore
If Miller effect is considered, the capacitance
will be larger than Cπ , and a larger inductor
will be required to perform the match. Also, the
imaginary part of the input impedance must
equal zero. Therefore,
inRF
bL
1Q
eLC
m e
s
g L
R
Cπ
= s
e
m
R C
L
g
π
=
2
1 s
b
m
R C
L
C g
π
πω
= −
m e
in e b
g Lj
Z j L j L
C Cπ π
ω ω
ω
−
= + + +

NF and Bias Current (I)
• Noise due to the base resistance is in series with the input
voltage, so it sees the full amplifier gain. The output noise due
to base resistance is given by
Note that this noise voltage is proportional to the collector current, as is the signal,
so the SNR is independent of bias current.
• Collector shot noise is in parallel with collector signal current
and is directly sent to the output load resistor:
Note that this output voltage is proportional to the square root of the collector
current, and therefore, to improve the noise figure due to collector shot noise, we
increase the current.
, 14bno r b m Lv kTr g R≈ ⋅
, 2Cno I C Lv qI R≈ ⋅

NF and Bias Current (II)
• Base shot noise can be converted to input voltage. If Zeq is the
impedance on the base (formed by a combination of matching,
base resistance, source resistance, and transistor input
impedance), then
Note that this output voltage is proportional to the collector current raised to the
power of 3/2. Therefore, to improve the noise figure due to base shot noise, we
decrease the current.
• At low currents, collector shot noise will dominate and noise
figure will improve with increasing current. However, the
effect of base shot noise also increases and will eventually
dominate. Thus, there will be some optimum level to which
the collector current can be increased, beyond which the noise
figure will start to degrade again.
,
2
B
C
no I eq m L
qI
v Z g R
β
≈ ⋅

Effect of the Cascode on NF
• The cascode transistor is a CB
amplifier with current gain close to 1.
The cascode transistor is forced to pass
the current of the driver on to the
output. This includes signal and noise
current. Thus, to a first order, the
cascode can have no effect on the noise
figure of the amplifier. In reality it will
add some noise, the cascode LNA can
never be as low noise as a CE amplifier.
CCV
EEV
1br
iv
1cv
2ei
2ci outv
2br
CR
cbiasv

Summary
• For three transistor amplifier configurations, the CE amplifier
has higher gain but poor frequency response than CB and CC
amplifiers due to miller effects.
• Cascode configuration of CE and CB has the advantages of
improving frequency response and a little impact on noise
figure.
• Feedback topologies are usually used to improve linearity with
sacrificing some power gain and noise performance.
• Using two inductors (one at emitter and the other at base) to
provide the power and noise match is a common and
convenient matching strategy for the LNA design.

RF Transceiver Module Design Chapter 5 Low Noise Amplifier

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RF Transceiver Module Design Chapter 5 Low Noise Amplifier