# Common-Gate (Base) Amplifier and Cascode Circuits

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Common-Gate (Base) Amplifier and Cascode Circuits
Dr. Paul Hasler

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Vout Vout Vb Vb Vin Vin

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Vout Vout Vb Vb Vin Vin

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Vout Vout Vb Vb Vin Vin What is the bias current? Iref = (1V) / R1

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Vb Vb Vin Vin

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Vb Vb Vin Vin BJT / Subthreshold VT

Vdd Vdd 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 )

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Vb Vb Vin Vin BJT / Subthreshold VT Above Threshold (Vd > Vg - VT ) (1V) / R1 = Ico eVb-Vin/UT Vin = Vb - UT ln ( (1V) / R1 Ico )

Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Vb Vb Vin Vin BJT / Subthreshold VT Above Threshold (Vd > Vg - VT ) (1V) / R1 = Ico eVb-Vin/UT (1V) / R1 = (K/2) (Vb - Vin - VT )2 Vin = Vb - UT ln ( (1V) / R1 Ico ) Vin = Vb - VT - sqrt((2V)/(K R1))

Common Gate: Small-Signal
Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Have Input Bias Vb Vb Vin Vin BJT / Subthreshold VT

Common Gate: Small-Signal
Vdd Vdd 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)

Common Gate: Small-Signal
Vdd Vdd 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 > Vg - VT ) gm = I / UT = (1V) / (R1 UT)

Common Gate: Small-Signal
Vdd Vdd 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 > Vg - VT ) gm = 2I /(Vb - Vin -VT) = (2V) / (R1 (Vb - Vin -VT) ) gm = I / UT = (1V) / (R1 UT)

Common Gate: Small-Signal
Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Have Input Bias Vb Vb gm = (1V) / (R1 UT) or Vin Vin gm = (2V) / (R1(Vb- Vin-VT) ) Vout GND + V - R1 rp gmV Vin GND

Common Gate: Small-Signal
Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Have Input Bias Vb Vb gm = (1V) / (R1 UT) or Vin Vin gm = (2V) / (R1(Vb- Vin-VT) ) Vout Gain = gm R1 GND + V - R1 rp gmV Vin GND

Common Gate: Small-Signal
Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Have Input Bias Vb Vb gm = (1V) / (R1 UT) or Vin Vin gm = (2V) / (R1(Vb- Vin-VT) ) Vout Gain = gm R1 GND + V - R1 rp Gain = (1V) / UT gmV or Vin GND Gain = (2V) / (Vb- Vin-VT)

Common Gate: Small-Signal
Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Have Input Bias Vb Vb gm = (1V) / (R1 UT) or Vin Vin gm = (2V) / (R1(Vb- Vin-VT) ) Vout Gain = (1V) / UT or Gain = (2V) / (Vb- Vin-VT) GND + V - R1 rp gmV Vin GND

Common Gate: Small-Signal
Vdd Vdd Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Have Input Bias Vb Vb gm = (1V) / (R1 UT) or Vin Vin gm = (2V) / (R1(Vb- Vin-VT) ) Vout Gain = (1V) / UT or Gain = (2V) / (Vb- Vin-VT) GND + V - R1 rp gmV Vin Output Resistance = R1 GND

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.

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. Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a transistor

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. Vdrain Vb GND V1 Vgate Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a transistor

Cascode Circuits Vdrain Vbias V1 Vgate GND

Cascode Circuits Idrain = Io e (kVbias -V1 )/UT eVdrain /VA
Vgate GND Idrain = Io e (kVbias -V1 )/UT eVdrain /VA = Io e kVgate/UT eV1 /VA

Cascode Circuits Idrain = Io e (kVbias -V1 )/UT eVdrain /VA
Vgate GND Idrain = Io e (kVbias -V1 )/UT eVdrain /VA = Io e kVgate/UT eV1 /VA V1 ~ kVbias - kVgate + (UT/VA) Vdrain

Cascode Circuits Idrain = Io e (kVbias -V1 )/UT eVdrain /VA
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

Cascode Circuits Vdrain Vbias V1 Vgate GND Idrain = Io e kVgate/UT e kVbias /VA eVdrain / (Av VA ) 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

Cascode Circuits Vdrain Vdrain Vbias Vgate GND V1 Vgate GND Idrain = Io e kVgate/UT e kVbias /VA eVdrain / (Av VA ) 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

BJT - CMOS Cascode Circuits
Preserve High-gm/I

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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