Common-Gate (Base) Amplifier and Cascode Circuits

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Common-Gate (Base) Amplifierand Cascode Circuits

• Dr. Paul Hasler

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Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Vout
Vout
Vb
Vb
Vin
Vin
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Common G Resistive Load
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Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Vout
Vout
Vb
Vb
Vin
Vin
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Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Vout
Vout
Vb
Vb
Vin
Vin
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Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
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Common Gate Resistive Load
Output Voltage Bias 4.0V
R1
R1
Iref (1V) / R1
Vout
Vout
Vb
Vb
Vin
Vin
BJT / Subthreshold VT
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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 )
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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 )
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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 )
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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
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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)
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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)
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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)
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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
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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
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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)
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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
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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
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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.
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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.
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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.
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Cascode Circuits
Vdrain
Vbias
V1
Vgate
GND
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Cascode Circuits
Vdrain
Vbias
V1
Vgate
GND
Idrain Io e (kVbias -V1 )/UT eVdrain /VA
Io e kVgate/UT eV1 /VA
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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
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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
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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
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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
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BJT - CMOS Cascode Circuits
Preserve High-gm/I
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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