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Common-Gate (Base) Amplifier and Cascode Circuits

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Vb. R1. Output Voltage. Bias = 4.0V. Common G: Resistive ... (1V) / R1 = (K/2) (Vb - Vin - VT )2 (1V) / R1 = Ico eVb-Vin/UT. Vin = Vb - UT ln ( (1V) / R1 Ico ) ... – PowerPoint PPT presentation

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Title: Common-Gate (Base) Amplifier and Cascode Circuits


1
Common-Gate (Base) Amplifierand Cascode Circuits
  • Dr. Paul Hasler

2
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Vout
Vout
Vb
Vb
Vin
Vin
3
Common G Resistive Load
4
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Vout
Vout
Vb
Vb
Vin
Vin
5
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Vout
Vout
Vb
Vb
Vin
Vin
6
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
7
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
8
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
(1V) / R1 Ico eVb-Vin/UT
Vin Vb - UT ln ( (1V) / R1 Ico )
9
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
Above Threshold (Vd gt Vg - VT )
(1V) / R1 Ico eVb-Vin/UT
Vin Vb - UT ln ( (1V) / R1 Ico )
10
Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
Above Threshold (Vd gt Vg - VT )
(1V) / R1 (K/2) (Vb - Vin - VT )2
(1V) / R1 Ico eVb-Vin/UT
Vin Vb - VT - sqrt((2V)/(K R1))
Vin Vb - UT ln ( (1V) / R1 Ico )
11
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
12
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
gm I / UT (1V) / (R1 UT)
13
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
Above Threshold (Vd gt Vg - VT )
gm I / UT (1V) / (R1 UT)
14
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
Above Threshold (Vd gt Vg - VT )
gm 2I /(Vb - Vin -VT) (2V) / (R1 (Vb -
Vin -VT) )
gm I / UT (1V) / (R1 UT)
15
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
gm (1V) / (R1 UT)
Vb
or
Vin
Vin
gm (2V) / (R1(Vb- Vin-VT) )
Vout
V -
R1
rp
gmV
Vin
16
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
gm (1V) / (R1 UT)
Vb
or
Vin
Vin
gm (2V) / (R1(Vb- Vin-VT) )
Vout
Gain gm R1
V -
R1
rp
gmV
Vin
17
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
gm (1V) / (R1 UT)
Vb
or
Vin
Vin
gm (2V) / (R1(Vb- Vin-VT) )
Vout
Gain gm R1
V -
R1
rp
Gain (1V) / UT
gmV
or
Vin
Gain (2V) / (Vb- Vin-VT)
18
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
gm (1V) / (R1 UT)
Vb
or
Vin
Vin
gm (2V) / (R1(Vb- Vin-VT) )
Vout
Gain (1V) / UT or Gain (2V) / (Vb- Vin-VT)
V -
R1
rp
gmV
Vin
19
Common Gate Small-Signal
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Have Input Bias
Vb
gm (1V) / (R1 UT)
Vb
or
Vin
Vin
gm (2V) / (R1(Vb- Vin-VT) )
Vout
Gain (1V) / UT or Gain (2V) / (Vb- Vin-VT)
V -
R1
rp
gmV
Vin
Output Resistance R1
20
Cascode Circuits
Use a common-gate/base transistor to 1. Improve
the output resistance of another transistor. 2.
Reduce the Gate-to-Drain capacitance effect
of another transistor.
21
Cascode Circuits
Use a common-gate/base transistor to 1. Improve
the output resistance of another transistor. 2.
Reduce the Gate-to-Drain capacitance effect
of another transistor.
22
Cascode Circuits
Use a common-gate/base transistor to 1. Improve
the output resistance of another transistor. 2.
Reduce the Gate-to-Drain capacitance effect
of another transistor.
23
Cascode Circuits
Vdrain
Vbias
V1
Vgate
GND
24
Cascode Circuits
Vdrain
Vbias
V1
Vgate
GND
Idrain Io e (kVbias -V1 )/UT eVdrain /VA
Io e kVgate/UT eV1 /VA
25
Cascode Circuits
Vdrain
Vbias
V1
Vgate
GND
Idrain Io e (kVbias -V1 )/UT eVdrain /VA
Io e kVgate/UT eV1 /VA
V1 kVbias - kVgate (UT/VA) Vdrain
26
Cascode Circuits
Vdrain
Vbias
V1
Vgate
GND
Idrain Io e (kVbias -V1 )/UT eVdrain /VA
Io e kVgate/UT eV1 /VA
V1 kVbias - kVgate (UT/VA) Vdrain
Drain is fixed
Fixes the voltage at V1 or isolates V1 from the
output
27
Cascode Circuits
Vdrain
Vbias
V1
Vgate
Idrain Io e kVgate/UT e kVbias /VA eVdrain /
(Av VA )
GND
Idrain Io e (kVbias -V1 )/UT eVdrain /VA
Io e kVgate/UT eV1 /VA
V1 kVbias - kVgate (UT/VA) Vdrain
Drain is fixed
Fixes the voltage at V1 or isolates V1 from the
output
28
Cascode Circuits
Vdrain
Vdrain
Vbias
Vgate
GND
V1
Vgate
Idrain Io e kVgate/UT e kVbias /VA eVdrain /
(Av VA )
GND
Idrain Io e (kVbias -V1 )/UT eVdrain /VA
Io e kVgate/UT eV1 /VA
V1 kVbias - kVgate (UT/VA) Vdrain
Drain is fixed
Fixes the voltage at V1 or isolates V1 from the
output
29
BJT - CMOS Cascode Circuits
Preserve High-gm/I
30
Summary
  • Large signal model of Common-Gate (Base)
    Amplifier
  • Small signal model of Common-Gate (Base)
    Amplifier
  • Cascode Circuits --- makes a node insensitive
    to voltage changes
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