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A. Kruger 1 Radio Frequency Electronics The University of Iowa
Radio Frequency Electronics
Image from Wikipedia
• Born in 1791 in Massachusetts
• Fairly accomplished painter
• After witnessing various electrical experiments,
got intrigued by electricity
• Designed the first single-wire telegraph
• Invented the concept “relay” what we now call
repeaters
• Created Morse Code (digital communications?)
• Held several patents related to the telegraph
• Dies in 1872
Active Components IV
Samuel Morse
A. Kruger 2 Radio Frequency Electronics The University of Iowa
CE Amplifier (CS is Similar)
Inverting amplifier
High-gain because of CE
)]([2
1
eqSB
HCRRr
f
Use time constant technique:
p
Hf2
1
eqC
A. Kruger 3 Radio Frequency Electronics The University of Iowa
SPICE Results for Common Emitter
A. Kruger 4 Radio Frequency Electronics The University of Iowa
Common Base Amplifier
Notice where the input signal goes
A. Kruger 5 Radio Frequency Electronics The University of Iowa
Common Base Amplifier
Remember, it is 𝑣𝐵𝐸 that controls collector current in BJTs
𝐼𝐶 = 𝐼𝑆𝑒𝑉𝐵𝐸𝑉𝑇
Common Emitter Common Collector Common Base
𝑣𝑏𝑒 𝑣𝑏𝑒 𝑣𝑏𝑒
A. Kruger 6 Radio Frequency Electronics The University of Iowa
Designing a Common-Base Amplifer
Design a CE amplifier Ground the base
𝑅1, 𝑅2, 𝐼𝐶 , 𝑅𝐸 , 𝑅𝐶 are determined to
achieve a desired operating point
𝐼𝐶
𝐴𝑣 = −𝑔𝑚𝑅𝐶
𝐴𝑣 = − 40𝐼𝐶 𝑅𝐶 = ? ?
𝑅𝑖
𝑅𝑖 = 𝑟𝜋 =𝛽
𝑔𝑚=
100
40𝐼𝐶= ? ?
Feed signal into emitter
𝐴𝑣 = Same as for CE
𝑅𝑖 =𝑟𝜋𝛽= =
1
𝑔𝑚= ? ?
𝛽 = 100 𝛽 = 100
𝑅𝑖
𝐼𝐶
A. Kruger 7 Radio Frequency Electronics The University of Iowa
CB Amplifier
This is not an inverting amplifier
Thus, Miller no multiplication effect.
A. Kruger 8 Radio Frequency Electronics The University of Iowa
CB Amplifier (CG is Similar)
These are NOT inverting
amplifiers.
Thus, Miller no multiplication
effect.
A. Kruger 9 Radio Frequency Electronics The University of Iowa
CB Amplifier
CRR
rf
SE
H
12
1
CRR
fLC
H)(2
1
Equivalent input circuit
Equivalent output circuit
Either one could
determine bandwidth
(normally Cμ)
Regardless, higher bandwidth than CE
A. Kruger 10 Radio Frequency Electronics The University of Iowa
Cascode Circuit
1 𝑔𝑚
Cascode amplifiers are composite amplifiers where
a CE amplifier feeds a CB amplifier.
One can view the CB amplifier as the load of the CE
amplifier.
The CB has input impedance 1 𝑔𝑚 .
If we were to “glue” the CB to the CE, the CE
would see a load of 𝑅𝐶 = 1 𝑔𝑚 .
A. Kruger 11 Radio Frequency Electronics The University of Iowa
Cascode Circuit
1 𝑔𝑚
Cascode amplifiers are composite amplifiers where
a CE amplifier feeds a CB amplifier.
One can view the CB amplifier as the load of the CE
amplifier.
The CB has input impedance 1 𝑔𝑚 .
If we were to “glue” the CB to the CE, the CE
would see a load of 𝑅𝐶 = 1 𝑔𝑚 . 1 𝑔𝑚
The gain of the CE would be −𝑔𝑚𝑅𝑐 = −1
This means small Miller Effect
The gain of the CB stage is 𝑔𝑚𝑅𝑐 and there is no
Miller effect
𝑅𝐶
A. Kruger 12 Radio Frequency Electronics The University of Iowa
BJT Cascode Amplifier
36 dB Voltage Gain at 6 MHz
12 dB Voltage Gain at 70 MHz
2N2222 BJTs Common Base
Common
Emitter
A. Kruger 13 Radio Frequency Electronics The University of Iowa
BJT Cascode Circuit
CE is an inverting amplifier => Miller effect present
CE voltage gain ~ 1 => low Miller effect
A. Kruger 14 Radio Frequency Electronics The University of Iowa
BJT Cascode Circuit
)(2
1
1111 MBS
HCCrRR
f
2)(2
1
CRR
fLC
H
Either one could
determine bandwidth
(normally Cμ) Wide bandwidth
A. Kruger 15 Radio Frequency Electronics The University of Iowa
SPICE Results for Cascode
A. Kruger 16 Radio Frequency Electronics The University of Iowa
FET Cascode
Common Gate
Common Source
A. Kruger 17 Radio Frequency Electronics The University of Iowa
FET Cascode
1 𝑔𝑚
A. Kruger 18 Radio Frequency Electronics The University of Iowa
Dual Gate FETs
The cascode amplifier is very popular in RF and the “stacked” configuration has other
useful applications.
Consequently, semiconductor companies make dual-gate FETs where two gates
“squeeze” the same channel
Cascode amplifier with 2 separate FETs
Cascode amplifier with dual gate FET
A. Kruger 19 Radio Frequency Electronics The University of Iowa
Dual Gate FETs
𝑫 𝑫
𝑺 𝑺
𝑮𝟏
𝑮𝟐
𝑮𝟏
𝑮𝟐
Construction Schematic Symbols
n-Channel p-Channel
𝑮𝟏
𝑮𝟐
𝑫
𝑺
A. Kruger 20 Radio Frequency Electronics The University of Iowa
Dual Gate FETs
Note the two gates
These form back-to-back Zeners that
protect the FET against damage from
static electricity
A. Kruger 21 Radio Frequency Electronics The University of Iowa
Emitter-Follower Circuit (Source-Follower is Similar)
A. Kruger 22 Radio Frequency Electronics The University of Iowa
'
'
1)1(2
1
Lm
LmBS
H
Rg
CCrRgRR
f
'
'
1)1(
Lm
LmBSpRg
CCrRgRR
Wide bandwidth
'
''
'
11
1
1L
LL
b R
gmR
sC
gmRr
Z
A. Kruger 23 Radio Frequency Electronics The University of Iowa
SPICE Results for Emitter Follower
A. Kruger 24 Radio Frequency Electronics The University of Iowa
Single-Tuned Amplifier
Tuned amplifier using a depletion-mode MOSFET
Circuit for bias calculations
The equivalent ac circuit The small-signal model
A. Kruger 25 Radio Frequency Electronics The University of Iowa
𝑣𝑜 𝑠 − 𝑣𝑖(𝑠 )𝑠𝐶𝐺𝐷 + 𝑔𝑚𝑣 + 𝑣𝑜 𝑠1
𝑟𝑜+
1
𝑠𝐿+ 𝑠𝐶 +
1
𝑅𝐷 𝑅3 = 0
The small-signal model
Let 𝑅𝑝 = 1 𝐺𝑝 where 𝐺𝑝 = 1 𝑟𝑜 + 1 𝑅𝐷 + 1 𝑅3 (i.e., the parallel combination of the resistances
au the output. Solving for the voltage transfer function 𝑣𝑜(𝑠) 𝑣𝑖(𝑠) yeilds
𝐴𝑣 𝑠 = 𝑣𝑜 𝑠
𝑣𝑖(𝑠)= 𝑠𝐶𝐺𝐷 − 𝑔𝑚 𝑅𝑝
𝑠𝑅𝑝 𝐶 + 𝐶𝐺𝐷
𝑠2 +𝑠
𝑅𝑝 𝐶 + 𝐶𝐺𝐷+
1𝐿 𝐶 + 𝐶𝐺𝐷
𝐴𝑣 𝑠 ≅ 𝐴𝑚𝑖𝑑
𝑠𝜔𝑄
𝑠2 + 𝑠𝜔𝑄+ 𝜔0
2, 𝜔0 =
1
𝐿(𝐶 + 𝐶𝐺𝐷), 𝑄 = 𝜔0𝑅𝑝 𝐶 + 𝐶𝐺𝐷 , 𝐴𝑚𝑖𝑑 = −𝑔𝑚𝑅𝑃
Neglecting the right-half-plane zero ( “𝑠𝐶𝐺𝐷” in 𝑠𝐶𝐺𝐷 − 𝑔𝑚 ) then
Further, 𝑄 = 𝑅𝑝 𝜔𝐿 and 𝐵𝑊 = 𝜔𝑜 𝑄
KCL @ output
A. Kruger 26 Radio Frequency Electronics The University of Iowa
𝐴𝑣 𝑠 ≅ 𝐴𝑚𝑖𝑑
𝑠𝜔𝑄
𝑠2 + 𝑠𝜔𝑄+ 𝜔0
2,
𝜔0 =1
𝐿(𝐶 + 𝐶𝐺𝐷,
𝑄 = 𝜔0𝑅𝑝 𝐶 + 𝐶𝐺𝐷
𝐴𝑚𝑖𝑑 = −𝑔𝑚𝑅𝑃
𝐶𝐺𝐷
𝜔0 = 1 𝐿 𝐶 + 𝐶𝐺𝐷 = 1 10 × 10−6 120 × 10−12 = 28.6 × 106 rad s
𝑓0 = 𝜔0 2𝜋 = 4.59 MHz
𝑟𝑜 = 1 𝜆𝐼𝐷 = 1 0.02 3.2 × 10−3 = 15.6K
𝐶 + 𝐶𝐺𝐷 = 100 + 20 = 120 pF 𝑅𝑝 = 𝑟𝑜 100K 100K=11.9K
𝑔𝑚 = 2 𝐾𝑛𝐼𝐷 = 2 2.5 × 10−3 3.2 × 10−3 = 5.66 mA V2
𝐴𝑚𝑖𝑑 = −𝑔𝑚𝑅𝑝 = − 5.66 × 10−3 11.9 × 103 = −67
𝑄 = 𝜔0𝑅𝑝 𝐶 + 𝐶𝐺𝐷 = − 28.6 × 106 11.9 × 103 120 × 10−12 = 40.8
𝐵𝑊 = 𝑓0 𝑄 = 4.59 × 106 40.8 = 112 kHz
Assume 𝜆 = 0.02 𝑉−1, 𝐼𝐷 = 3.2 mA, 𝐶𝐺𝐷 = 20 pF
A. Kruger 27 Radio Frequency Electronics The University of Iowa