lecture 7 rf front-end design lna mixer - auburn university
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RFIC Design and Testing for Wireless Communications
A PragaTI (TI India Technical University) CourseJuly 18, 21, 22, 2008
Lecture 7: RF front-end design – LNA, mixer
BBy
Vishwani D. AgrawalFa Foster DaiFa Foster Dai
200 Broun Hall, Auburn University200 Broun Hall, Auburn UniversityAuburn, AL 36849-5201, USA
1
RFIC Design and Testing for Wireless Communications
TopicsMonday, July 21, 2008
9:00 – 10:30 Introduction – Semiconductor history, RF characteristics
11:00 – 12:30 Basic Concepts – Linearity, noise figure, dynamic range2:00 – 3:30 RF front-end design – LNA, mixer4:00 – 5:30 Frequency synthesizer design I (PLL)
T d J l 22 2008Tuesday, July 22, 2008
9:00 – 10:30 Frequency synthesizer design II (VCO)
11:00 – 12:30 RFIC design for wireless communications2:00 – 3:30 Analog and mixed signal testing
RF front-end design – LNA, mixer, FDAI, 2008 2
LNA Design Challenges(1) Amplify extremely low signals without adding much
noise.
(2) Amplify large signals without distortions.
(3) Variable gain to compensate large input signal variation.
(4) Input matching and flat gain over wide bandwidth for muti-mode transceivers.
(5) Input dynamic range of a WLAN LAN from -80dBm to -20dBm
Parameters for Microwave and RFIC DesignPeak-to peak voltage: Vpp
Root-mean-square voltage:
VV 2222
pprms
VV =
Power in Watt :
⎞⎛ WP tt ][
RV
RVPwatt pprms
8
2
==
Power in dBm : ⎟⎠⎞
⎜⎝⎛=
mWmWPwattPdBm 1
][log10 10
RZRefection coefficient :
Voltage standing wave ratio (VSWR):
Sin
Sin
RZRZ
S+−
==Γ 11 Γ−= log20lossReturn
Voltage standing wave ratio (VSWR):
Γ−
Γ+==
11
min
max
VV
VSWR
RF front-end design – LNA, mixer, FDAI, 2008 4
Basic Amplifiers• The common emitter amplifier is often used as a drive for an LNA.• The common-collector, with high input impedance and low output
impedance, makes an excellent buffer between stages or before the p , goutput driver.
• The common-base is often used as a cascode in combination with the common-emitter to form a LNA stage with gain to high frequency.g g g q y
VCC VCC
VCC
vout
vinvout
voutvin
vin
VEE VEEVEE
Common-Emitter(LNA Driver) Common-Collector
(Buffer)Common-Base
(Cascode)
RF front-end design – LNA, mixer, FDAI, 2008 5
Common-Emitter Amplifier
vi
ZL
vo
gmvπv Cπ
rb
r
Cμ
r Lgmvπvπ π rπ ro
• Voltage gain
e
LLm
bi
ovo r
ZZgrr
rvvA ≈
+−==
π
π
• Note that rπ=βre and gm=1/re. for low frequencies, the parasitic capacitances have been ignored and rb << rπ.
• Input impedance at low frequenciesInput impedance at low frequencies
πrrZ bin +=
RF front-end design – LNA, mixer, FDAI, 2008 6
Miller Capacitance in Common-Emitter Amplifier
Cμ is replaced with two equivalent capacitors CA and CB
μμπ
μ ZgCZgCvvCC LmLm
oA ≈+=⎟⎟
⎠
⎞⎜⎜⎝
⎛−= )1(1Cμvπ vo
μμπ
μ
π
CZg
CvvCC
LmoB ≈⎟⎟
⎠
⎞⎜⎜⎝
⎛+=⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
⎠⎝
111
⇓vπ vo • Two RC time constants or two poles: one consisting of CA+ Cπ, and the
CA CB
g A π,other consisting CB.
RF front-end design – LNA, mixer, FDAI, 2008 7
Miller Capacitance in Common-Emitter Amplifier• The dominant pole is one formed by CA and Cπ:
[ ]f =1
( )[ ] [ ]AsbP CCRrr
f+⋅+⋅
=πππ21
• Recall Ft calculation, the output is loaded with a short circuit removes Miller multiplication, 3db current gain bandwidth:
1βf =
)(2 μππβ π CCr
f+⋅
The unity current gain frequency:The unity current gain frequency:
)(2 μππ CCgf m
T +⋅=
RF front-end design – LNA, mixer, FDAI, 2008 8
)( μπ
Common-Base Amplifierrb
Cμ
Cπ r g v Z
iout
vππ rπ gmvπ ZL
iin
Current gain (ignoring Cu and ro)T
ein
out
jrCjii
ωωω π +
≈+
≈1
11
1
Tω
At l f i th t i 1
( )CSLeS
Lv CCRjCrjR
RA++
⋅+
⋅≈μπ ωω
α1
11
10
At low frequencies, the current gain = 1.The pole in this equation is usually at much higher frequency than the one in the common-emitter amplifier Sbe Rrr +<
Cascode LNA R
VCC
Cascode LNA
Cmio Rgvv −≈/ Cascode Q2
vout
VCbias
RC
vi
vc1
Driver Q1
1Common emitter dominant pole
VEE
• Current ic1 through Q1 is about the same as the current ic2 through Q2
2/1 me gr ≈( )[ ] [ ]usbP CCRrr
f22
1
111 +⋅+⋅=
πππ
• Current ic1 through Q1 is about the same as the current ic2 through Q2current gain ~1 Gain is the same as for the common-emitter amplifier.
• The cascode transistor reduces the feedback of Cu1 (why?) increased high frequency gain.
• Cascode has good isolation with reduced S12.• Disadvantage: cascode transistor uses voltage headroom reduced
linearity; add another pole to the amp -12dB/oct roll-off; add little extra noise (to the 1st order approximation, cascode NF = common emitter NF);
RF front-end design – LNA, mixer, FDAI, 2008 10
( pp , );common base may cause noise and oscillation.
Bipolar Transistor Noise Model
C
rb
icn
cb’
ibn
vbb
4kTrb
Cμ
Cπ gmvb’erπcn
e
ibf
2qIB
ro
2qIC
e1/f noise Base Collector eBase
Shot Noise
CShot Noise
bbn kTrv 42 =
gmVπ1
Baserb
Zπ
vbn Collector
222 rπ Cπro icnibn
Emitter
ccn qIi 22 =Bbn qIi 22 =
RF front-end design – LNA, mixer, FDAI, 2008 11
Noise Figure versus Bias Current
2 40)
Lmbrno RgkTrvb
⋅≈ 4,
Output signal Lmsiso Rgvv ⋅≈
Base thermal noise
2.20
2.40
e Fi
gure
(dB
)
LCIno RqIvc
2, ≈
C RgRqIv 2≈
Collector shot noise
Base shot noise
1.80
2.00
nim
um N
oise
base th l
collector shot noise
base shot
LmsIno RgRvB β, ≈Base shot noise
1.001.60
Collector Current (mA)
Min
2.0 3.0 4.0 5.0 6.0
thermal noise
shot noise (correlated)
shot noise
2222111
ββSmSmbeq RgRg
RgRr
RR
NF ++++=+=
RF front-end design – LNA, mixer, FDAI, 2008 12
0 222 ββSmSS RgRR
Noise Figure of A Two Stage LNA
Ω11rGHzft 5=
Ω=11br
800 =β
GHzfin 1=
5=β
RgRgRR
rNF SmSmb
2211 2++++=
β
11.0 −Ω=mg
NF dominated
dB
RgR SmS
621551111
222 20
=++++=
ββ NF increases at high frequency
NF dominated by base resistance
RF front-end design – LNA, mixer, FDAI, 2008 13
dB62.1501601050
1 ++++
Minimum Noise Figure of Common Emitter LNA
SS
SmSm
SmS
b RbR
aRgRg
RgRr
NF ⋅++=++++=11
22211 2
0 ββ
012 =+−= b
Ra
dRdNF
SS
m
bTbm
moptS g
rffrg
gbaR
211
211, ≈
+
+==
RS=50 Ohm choose bias (gm) and emitter length to
hi imm gfg2
0
+ββ
⎞⎛
achieve noise matching
( ) ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛+++=+= 2
0min
1121121ββbmrgabNF
RF front-end design – LNA, mixer, FDAI, 2008 14
More on LNA Noise Figure2
( ) ( )2,2
2
min2
,min 2 optSSST
moptSS
S
RRRf
gfNFRR
RbNFNF −+≈−+=
m
bToptS g
rffR ,
2
2 frequency,High
β
≈ min
2
21 ωπ
m
b
gr
CNF +≈
m
boptS g
rR 0
,2
frequency, Lowβ
≈0
min2
1β
bmrgNF +≈
• For a given technology, NFmin is a strong function of bias current.• For low operation frequency, NFmin can be reduced by increasing emitter
length.• For high operation frequency, NFmin is a weak function of emitter length.
Increase device size does reduce rb, yet capacitance also increase.• For high operation frequency, NFmin degrades as frequency increases.
RF front-end design – LNA, mixer, FDAI, 2008 15
Input Matching of LNA Noise
Power matching:Q1RFin
Lb
• Input impedance (assuming Cuand rπ is not significant):
ππ
ωω
ωC
LgLj
CjrLjZ em
ebbin ++−+=Le
and rπ is not significant):
• Power matching:Real part=Rs Sb
em RrC
Lg=+
π T
S
m
bSe
Rg
CrRL
ωπ ≈
−=
)(
Imaginarypart=0
πω CLL be 2
1=+
( )T
bS
m
T
m
bSb
rRgg
CrRC
Lωω
ωω
π
π
−−≈
−−= 22
1
• If Cu is considered, Cπreplaced by Cπ + CA, and therefore a larger inductor is required for matching.
LmLmA ZgCZgCC μμ ≈+= )1(
RF front-end design – LNA, mixer, FDAI, 2008 16
LmLmA gg μμ )(
LNA Design StepsL Q1
Le
RFinLb(1) Noise matching: sizing the transistor (emitter length)
and adjusting bias current to achieve minimum NF
Ω=+
= 50211 bmrgR Ω=+
= 50112
0
,
ββm
optS gR
( )⎟⎟
⎠
⎞
⎜⎜
⎝
⎛+++= 2
0min
11211ββbmrgNF
(2) Power matching: adjusting Le such that the real part of the LNA input impedance equals to 50 Ohm. For 5AM, Le is about 0.2nH Use multiple downbonds to reduce the package effect
TT
Se
RLωω50
==
to reduce the package effect.
(3) Power matching: adding Lb such that the imaginary part of the LNA input impedance equals to zero 2ω
ω
m
Tb g
L ≈
(4) G i /b d idth Ch i l d t k t t i d
(5) Using SPICE sim to fine tune the component values, it ti t t d ff th i t t d
(4) Gain/bandwidth: Choosing load tank to meet gain and bandwidth requirements.
RF front-end design – LNA, mixer, FDAI, 2008 17
iterations to trade off the various parameters are expected.
Mixing with Nonlinearity
• Mixer is to convert a signal from one frequency to another intrinsically needs a nonlinear transfer function. A diode or a transistor can be used as a nonlinear device.ca be used as a o ea de ce
• Two inputs at ω1 and ω2 , which are passed through a nonlinearity multiplier will produce mixing terms at ω1±ω2 with other terms (harmonics, feed-through, intermodulation) that need to be filtered out.
• Mixers (multiplier) can be made from an amplifier with a controlled• Mixers (multiplier) can be made from an amplifier with a controlled switch.
R R
VCCVCC
Rc Rc
vout +
v2
vout -
switch
v1 i(v1)
RF front-end design – LNA, mixer, FDAI, 2008 18
Controlled Transconductance Mixer
Th t i l t d t th i t lt b th t d t• The current is related to the input voltage v2 by the transconductance of the input transistors Q1 and Q2. The transconductance is controlled by the current I0, which in turn is controlled by the input voltage v1.
i1 i2 2Io
io
v2
Q1 Q2+
i0=i1-i2o
TvvO
eI
i /1 21+= −
2Io
-
-2Io
v2
v1 TvvO
eI
i /2 21+=
RF front-end design – LNA, mixer, FDAI, 2008 19
Controlled Transconductance Mixer
Tovv
Ovv
Ooo v
vIeI
eI
IiiiTT 2
tanh11
2//21 22
=⎟⎠
⎞⎜⎝
⎛+
−+
=−= −
• If v2<<vT
Too v
vIi2
2≈
• Current source is modulated by small v1 Io is replaced with Io+gmcv1, where gmc is transconductance of the current source:
44 344 21434212
tanh2
tanh2
tanh)( 21
221 v
vvgvvI
vvvgIi
Tmc
To
Tmcoo +=+=
)(2 mixingtionmultiplicahfeedthrougV
not appear in differential output
RF front-end design – LNA, mixer, FDAI, 2008 20
not appear in differential output voltage double balanced mixer
Double-balanced MixerVCC
• Use switching quad to eliminate the v2 feedthrough
vo1 vo2
RC1 RC2
i3 i4 i5 i6
Q3 Q4
v1 -v12I 2I
v2
Q5 Q6
vv
v1 -v12Io 2Io
Tmc
Too v
vvgvvIiii
2tanh
2tanh 2
12
56' −=−=• 2nd pair current:
• Total differential current:T
mcooob vvvgiii
2tanh2 2
1' =−=
RF front-end design – LNA, mixer, FDAI, 2008 21
Double-balanced Mixer⎞⎛⎞⎛ A i( ) ( ) ( )
EeTT Rrv
vvii
vviiii
+⎟⎟⎠
⎞⎜⎜⎝
⎛=−⎟⎟
⎠
⎞⎜⎜⎝
⎛=+−+ 12
212
6453 2tanh
2tanh
Assuming small signal for V1
RC1 RC2
VCC
TT vvvv
iiii
eii
eii
22
/1
4/1
3 22 11 −
==
+=
+=
Q3 Q4 Q5 Q6
i3 i4i5 i6
v2
vo1 vo2
Eeo Rr
vIi+
+≈1
21
1
TT vvvv ei
ei /6/5 22 11 −+
=+
=
Q1 Q2
i1 i2
RE RE
Eo
Ee
RrvIi
and
+−=
121
2
v1
2Io
RF front-end design – LNA, mixer, FDAI, 2008 22
Ee Rr +2VEE
⎞⎛
Double-balanced Mixer• Output differential voltage
CEeT
o RRr
vvvv
+⎟⎟⎠
⎞⎜⎜⎝
⎛−= 12
2tanh
• Conversion gain relative to v1Co
RrR
vv
vv
+⎟⎟⎠
⎞⎜⎜⎝
⎛−=
2tanh 2
EeT Rrvv +⎟⎠
⎜⎝ 21
• With RE=0, a general large-signal expression for the output:
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛−=
TToCo v
vvvIRv
2tanh
2tanh2 21
RF front-end design – LNA, mixer, FDAI, 2008 23
LO Level at Upper Quad TransistorsTh diff i l i d i l i f b 4 f h i• The differential pair needs an input voltage swing of about 4 to 5 vT for the transistors to be hard-switched one way or the other.
• LO input to the mixer should be at least 100mV peak for complete switching. At 50Ω, 100mV peak is -10 dBm.
• -10 to 0 dBm (100~300 mVpp = 200~600mVpp diff) is a reasonable compromise-10 to 0 dBm (100 300 mVpp = 200 600mVpp diff) is a reasonable compromise between noise figure, gain and required LO power. This is also the reasonable level for all switching circuits
• If the LO voltage is too large, large current has to be moved into and out of the bases of the transistors during transition lead to spikes in the signals and reduce the switching speed cause an increase in LO feed throughswitching speed cause an increase in LO feed-through.
• Large LO also pushes switching transistor into saturation loose switching speed and inject mixer noise into substrate.
Va) b)Va) b)
Rc Rc
Vc
c
VLO+ VLO-
Vin+ Vin-
a) b)
Rc Rc
Vc
c
VLO+ VLO-
Vin+ Vin-
a) b)
Ibias
LOQ3 Q4 VdVd
Ibias
LOQ3 Q4 VdVd
RF front-end design – LNA, mixer, FDAI, 2008 24
w1
Slide 24
w1 weishen, 7/16/2003
Noise Contributions
total
Mixer Noise10
8
6
4
top transistors
bottom t i t
total
Noi
se P
ower
ve v
alue
s)
sourceNN
NF tot
11)(0
log10 0=
4
2
0-0.1 0 0.1
transistors
source resistance
VLO (instantaneous)
Out
put N
(Rel
ativdB65.8
5.111log10 ==
• Top transistor contributes significant noise during transition and contributes ignorable noise when fully switched, either in cutoff or saturation (without gain).
• Gain from RF input is maximum when top transistors are fully switched (cascode)
VLO (instantaneous)
• Gain from RF input is maximum when top transistors are fully switched (cascode).• need sharp transition buffer for LO large LO such that minimal time is spent around 0V.• Mixer noise figure can be approximately analyzed using a lowly swept dc voltage at the LO
input or with an actual LO signal.• With a slowly swept dc voltage, the mixer becomes equivalent to a cascode amplifier and the
LO input served as a gain-controlling signal.• For mixer, any noise (or signal) is mixed to two output frequencies, thus reducing the output
level mixer having less gain than the equivalent differential pair.
RF front-end design – LNA, mixer, FDAI, 2008 25
• However, both RF and image is mixed to IF doubling noise power at output.
Mixer with Simultaneous Noise and Power MatchUse inductor degeneration and inductor input achieving simultaneous noise and• Use inductor degeneration and inductor input achieving simultaneous noise and power matching similar to that of a typical LNA.
ZLO j t
VCC
R R
TE f
ZLπ2
0=
LO
IF- IF+
LO rejectRL RL
LO
RF
VLE LE
Q1 Q2
V T
m
fZgIIP
πω2
3 0≈VBB VBB
Tf
Single-to-differentialFilter harmonics
• Noise matching: sizing LE, and RF transistor, and operating the RF transistors at the current required for minimum NF.
• The quad switching transistors are sized for maximum fT, (typically about five to
RF front-end design – LNA, mixer, FDAI, 2008 26
q g fT, ( yp yten times smaller than the RF transistors.)
Mixer Design IssuesSizing Transistors
• The RF differential pair is basically an LNA stage, and the transistors and associated passives can be optimized using the LNA design techniques.
• The switching quad transistors are sized so that they operate close to their
Sizing Transistors
g q y ppeak fT at the bias current that is optimal for the differential pair transistors are biased at their minimum noise current, then the switching transistors end up being about one-eighth the size.
Increasing Gain
R2
Increasing Gain
• Voltage gain without matching and assuming full switching of the upper quad:
inEe
Co v
RrRv+
=π2
• To increase the gain increase the load resistance RC, to reduce degeneration resistance RE, or to increase the bias current IB.
• Make sure that increasing output voltage swing will not cause the switching transistors to become saturated. Enough headroom.
RF front-end design – LNA, mixer, FDAI, 2008 27
g
Increasing IP3Mixer Design Issues
• Identify which part of the circuit is compressing. Compression can be due to overdriving of the lower differential pair, clipping at the output, or the LO bias voltage being too low, causing clipping at the collectors of the bottom differential pair. Adjust the bias and voltage swing to avoid clipping.p j g g pp g
• 1. If the compression is due to the bottom differential pair (RF input), then linearity can be improved by increasing RE or by increasing bias current.
• 2. Compression caused by clipping at the output is typically due to the quad transistors going into saturation. Saturation can be avoided by reducing the g g y gload resistance or adjust the quad transistor bias. Too large LO will also cause saturation.
• 3. If compression is caused by clipping at the collector of the RF input differential pair, then increasing the LO bias voltage will improve linearity; h hi l i li i hhowever, this may result in clipping at the output.
Improving Noise Figure• NF will be largely determined by the choice of topology• NF will be largely determined by the choice of topology.• Use the simultaneous matched design technique. • To minimize noise, the emitter degeneration resistor should be kept as small
as possible. Use inductor as degeneration to achieve low noise.
RF front-end design – LNA, mixer, FDAI, 2008 28
• Make top transistors switching fast.
M t hi Bi R i t d G i
Mixer Design Issues
Matching, Bias Resistors, and Gain
• Use resistive matching to achieve broad band. For a resistively degenerated mixer, the RF input impedance will be fairly high; for example, with RE=100 Ω, Z b f th d f Kil Oh i f LNA t t t t d i thZin can be of the order of a Kilo Ohm easier for LNA output stage to drive the mixer.
• At the output, if matched, the load resistor Ro is equal to the collector resistor Rc. Furthermore, to convert from voltage gain Av to power gain Po/Pi, one must consider the output resistance Ri and load resistance Ro=Rc as follows:
ov2
2
C
i
E
C
o
iv
o
i
i
o
i
i
o
i
o
RR
RR
RR
ARR
vv
RvR
PP
22
2
2
2
2/2⎟⎟⎠
⎞⎜⎜⎝
⎛≈===
π
RF front-end design – LNA, mixer, FDAI, 2008 29