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RF Amplifiers
Biasing of Transistors: The Base Emitter junction should be
Forward biased and Base Collector junction should be
reverse biased.
VCC
R2
Rc
Cc
Cb
RL
Rs
Vs
R1
RE
Equivalent Circuit:
Rs
gmvbe
RB
rp
V
Vs
Small signal gain = gm (Rc II Rl)
Cp
Rc
Rl
Ccb
CL
Extending Bandwidth in RF Amplifiers
Inductive load:
L
L
C
gmvbe
R
R
C
Inductive load to enhance bandwidth
Load impedance:
Z(s) = (sL +R) II 1/sC = R[sL/R+1]/[S2 LC+ sRC + 1]
If we define m=RC/[L/R], t = L/R
Z(s) = R. [ts+ 1]/[s2t2m + stm +1]
Gain with inductive load/gain wihout indutcive load
= |Z(jw)|/R = [
Band width with inductive load/Bandwidth without inductive load=
Condition
m=R2C/L Bandwidth boost factor
Maximum bandwidth 1.41
1.85
|Z|=R
2
1.8
Best Magnitude Flatness 2.41
1.72
Beat delay flatness
3.1
1.6
No Shunt Peaking
Infinite
1
Normalized Peak Freq.res
1.19
1.03
1
1
1
10V
Design Shunt Inductor
Peaking amplifier
5nH
CB1, coupling
capacitor,
Should offer
Low resistance,
les parasitics.
9k
Rc=100 ohms
Rs=
50ohms
Vs
Vout
RB2, Bias resistor
BFP193
1V
100pf
Lm2
100pf
Lm1
.12V
RB1
1k
IE=10ma
RE=12
ohm.
Cm1
Current through bias resistors 10 times base current. Base
current is =Emitter current/beta = 0.1mA.
RL=50 ohms
1.5pF
Selection of Transistor
BFP 193 RF transistor, ft, unity gain frequency = 8 GHz
HFE = 125 (typical). All the transistor parameters have to be entered in
the model.
Package equivalent circuit.
Package Equivalent Circuit:
B LBO=
0.65 nH
CCB= 19fF
LBI =
0.84 nH
B Transistor
Chip
LCI =
0.07nH
C
LCO =
0.42nH
E
LEI =
0.31nH
CBE =
145fF
LEO =
0.14 nH
E
CCE=281fF
C
Design of Feedback Amplifier
Let us design the amplifier for a power gain of 10 dB.
This corresponds to a voltage gain of 3.2.
10 log Pout/Pin = 10 log Vout2/vin2 = 20 log Vout/Vin = 10db.
Vout/Vin = (10) 0.5 = 3.3.
Av= Vout/Vin = RC/RE=- 3.3
Rin =Rout =50 ohm.
Rin = RF/ 1-Av = RF/1+3.3
RF = 50(4.3)= 215 ohm . You can select 210 ohm or 240 ohm as the RF.
Select gm.
Gain = gm. Ro= 3.3 = gm.50
gm = 3.3/50= 3300/50= 66 ms
RE=1/gm= 1/ 66ms = 50/3.3= 15 ohm. Preferable value is about 12 ohm or 10
ohms.
Gm=Ic/vt ,
Ic= 66.25= 1.5mA. We keep Ic about 10 mA so that we get enough gain.
RL=500 ohms, so that VCB=5V to reduce Base to Collector capacitance.
10V
CB1, reactance 10 times less
than RB2
CB1, coupling
capacitor,
Should offer
Low resistance,
les parasitics.
RF,
feedback
resistor
.2k
3.3k
RB2, Bias resistor
Vs
.12V
RB1
1k
5V
BFP193
.9V
Rs=
50ohms
Rc=500 ohms to get
adequate reverse bias to
reduce Cbc
IE=10ma
RE=12
ohm.
Current through bias resistors 10 times base current. Base
current is =Emitter current/beta = 0.1mA.
RL=50 ohms
Matching Network
At frequency wo,
The impedance of the network = jwoLs+Rs = jwoLp|| Rp
= = [(woLp)2 Rp + jwoLpRp2]/Rp2+(woLp)2
Ls
C
Rs
Lp
C
Rp
Rp= Rs(Q2+1), Lp=Ls(Q2+1)/Q2 = Ls if Q>>1
Cp= Cs(Q2)/(Q2+1)
L match Circuit
Ls
Rp= Rs(Q 2 + 1)
Rs
C
Rp
= Rs Q 2
= Rs(1/(woRsC)2
= (1/Rs) (Ls/C)
Downward impedance transformer
RsRp= Ls/C = Zo2
Ls
Rp
C
Upward impedance transformer
Rs
Tuned Amplifiers
Gain x bandwidth = constant
If we reduce the bandwidth, gain can be high.
G (BW) = gmR.(1/RC) = gm/C
10V
5nH
CB1, coupling
capacitor,
Should offer
Low resistance,
les parasitics.
9k
Rs=
50ohms
Vs
Vout
RB2, Bias resistor
BFP193
1V
100pf
Lm2
100pf
Lm1
.12V
RB1
1k
IE=10ma
RE=15
ohm.
Cm1
Current through bias resistors 10 times base current. Base
current is =Emitter current/beta = 0.1mA.
RL=50 ohms
1.5pF
Strange Impedance Behaviors and Stability
Circuit Model for Base Impedance Effect:
ib
The impedance seen at base of
the transistor,
Cbe
bib
Zb
Zb= 1/jwCbe + Z(b+1)
= 1/jwCbe + Z(-jwT /w +1)
Z
b = ic/ib = gm vbe/ib
= gm/sCbe =-j wT /w
b goes to 1 at w =wT
If Z= R, resistor Zb sees it as a capacitor
If Z is due to inductor, it appears as a
resistance.
If Z is a capacitor, it appears as –ve resistance
and may cause oscillations.
1 = gm/wT.Cbe, wT = gm/Cbe
Impedance Looking into the Emitter Terminal:
Ze= 1/jwCbe + Z/(b+1)
where Z is the impedance in the base side
= 1/jwCbe + Z/ (-j wT/w +1)
= 1/jwCbe + jZ(w/wT)
If Z=jwL,
Ze= = 1/jwCbe - (w2/wT) L
Inductance at base appears as a negative
resistance at emitter.