Biasing networks
• Biasing networks are needed to set appropriate
operating conditions for active devices
There are two types:
• Passive biasing (or self-biasing)
– resistive networks
– drawback: poor temperature stability
• Active biasing
– additional active components (thermally coupled)
– drawback: complexity, added power consumption
EEE 194RF
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Passive biasing
VCC
IB
I1
CB
IC
R2
R1
RFC
RFC
RFOUT
• Simple two element
biasing
• blocking capacitors CB
and RFCs to isolate RF
path
• Very sensitive to collector
current variations
CB
RFIN
EEE 194RF
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Passive biasing
VCC
R4
IC
R2
CB
RFC
IB
RFOUT
VX
IX
R3
RFC
CB
R1
RFIN
• Voltage divider to
stabilize VBE
• Freedom to choose
suitable voltage and
current settings (Vx, Ix)
• Higher component
count, more noise
susceptibility
IB~10 IX
EEE 194RF
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Active biasing
VCC
RC2
I1
IC2
RC1
RB1
RB2
VC1
RE1
EEE 194RF
IB1
RFC
CB
Q1
RFC
Q2
IC1
IB1
CB
RFIN
RFOUT
• Base current of RF
BJT (Q2) is provided
by low-frequency BJT
Q1
• Excellent temperature
stability (shared heat
sink)
• high component count,
more complex layout
4
Active biasing in common base
VCC
RC2
RFC
RC1
VCC
DC path
RC2
I1
RB1
RB2
Q2
RFC
CB
CB
Q1
IC2
RC1
RB1
RB2
VC1
IC1
IB1
RE1
RFC
RFC
RE1
IB1
RFOUT
Q2
RFC
CB
Q1
CB
RFIN
VCC
RFC
RC2
RF path
RC1
RB1
CB
RFC
RB2
RFOUT
Q2
RFC
CB
Q1
RFIN
RFC
RE1
EEE 194RF
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FET biasing
VG
VD
CB
Bi-polar power
supply
CB
VG<0 and VD>0
RFC
RFC
RFOUT
RFIN
VD
VD
Uni-polar power
supply
CB
CB
RFC
RFC
RFOUT
RFIN
RFC
CB
RFC
RFOUT
RFIN
RFC
CB
RS
VS
EEE 194RF
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Matching to Self-Biased BJT Amp
+V CC
• Design self-bias
circuit as usual
• Design input and
output matches to
S11 and S22
respectively
EEE 194RF_L22
RB1
RC
0.1 uF
0.1 uF
Lin_match
L out_match
Cout_match
VS
RL
RS
Cin_match
RB2
RE
CE
0.1uF
7
Equivalent RF Model of BJT Amp
• The equivalent RF model of the self-biased BJT
amp is shown. Note that bias resistors do not
affect RF performance
Cout_match
VS
Lout_match
RL
RS
Cin_match
L in_match
EEE 194RF_L22
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Matching to Self-Biased JFET Amp
+V CC
• Design self-bias
circuit as usual
• Design input and
output matches to
S11 and S22
respectively
EEE 194RF_L22
RD
0.1 uF
0.1 uF
Lin_match
Lout_match
Cout_match
VS
RL
RS
Cin_match
RG
1 M?
RS
CS
0.1uF
9
Equivalent RF Model of JFET Amp
• The equivalent RF model of the self-biased JFET
amp is shown. Note that bias resistors do not
affect RF performance
Cout_match
VS
Lout_match
RS
RL
Cin_match
Lin_match
EEE 194RF_L22
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Matching Networks for Amplifiers
• Conjugate matching must be used for
maximum power transfer
• Standard impedance matching using either
two element L-C, Pi- or Tee-type network,
or microstripline matching.
• Use Smith Charts with associated Node
Quality Factor Qn to determine network
EEE 194RF_L22
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Stub Tuner Matching for RF BJT Amp
• Can implement impedance matching
network with microstriplines
• Shown is single stub tuner with shorted stub
+V CC
RC
RB1
RB2
RFC
0.1 uF
Xmission Line
RFC
Xmission Line
VS
RL
RS
CS
0.1uF
Shorted Stub
Shorted Stub
RE
CE
0.1uF
C stub2
0.1uF
C stub1
0.1uF
EEE 194RF_L22
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