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Common-Gate (Base) Amplifier and Cascode Circuits Dr. Paul Hasler Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Vout Vb Vin Vin Output Voltage Bias = 4.0V Common G: Resistive Load Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Vout Vb Vin Vin Output Voltage Bias = 4.0V Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Vout Vb Vin What is the bias current? Vin Iref = (1V) / R1 Output Voltage Bias = 4.0V Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Vout Vb Vin Output Voltage Bias = 4.0V Vin Iref = (1V) / R1 Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Vout Vb Vin BJT / Subthreshold VT Output Voltage Bias = 4.0V Vin Iref = (1V) / R1 Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Vout Vb Vin Vin BJT / Subthreshold VT (1V) / R1 = Ico e Vb-Vin/UT Vin = Vb - UT ln ( (1V) / R1 Ico ) Output Voltage Bias = 4.0V Iref = (1V) / R1 Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Output Voltage Bias = 4.0V Iref = (1V) / R1 Vout Vb Vin Vin BJT / Subthreshold VT (1V) / R1 = Ico e Vb-Vin/UT Vin = Vb - UT ln ( (1V) / R1 Ico ) Above Threshold (Vd > Vg - VT ) Common Gate: Resistive Load Vdd Vdd R1 R1 Vout Vb Output Voltage Bias = 4.0V Iref = (1V) / R1 Vout Vb Vin Vin BJT / Subthreshold VT (1V) / R1 = Ico e Vb-Vin/UT Vin = Vb - UT ln ( (1V) / R1 Ico ) Above Threshold (Vd > Vg - VT ) (1V) / R1 = (K/2) (Vb - Vin - VT )2 Vin = Vb - VT - sqrt((2V)/(K R1)) Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vb Vout Vb Vin BJT / Subthreshold VT Output Voltage Bias = 4.0V Vin Iref = (1V) / R1 Have Input Bias Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vb Vout Vb Vin BJT / Subthreshold VT gm = I / UT = (1V) / (R1 UT) Output Voltage Bias = 4.0V Vin Iref = (1V) / R1 Have Input Bias Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vb Output Voltage Bias = 4.0V Vout Iref = (1V) / R1 Have Input Bias Vb Vin BJT / Subthreshold VT gm = I / UT = (1V) / (R1 UT) Vin Above Threshold (Vd > Vg - VT ) Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vb Output Voltage Bias = 4.0V Vout Iref = (1V) / R1 Have Input Bias Vb Vin BJT / Subthreshold VT gm = I / UT = (1V) / (R1 UT) Vin Above Threshold (Vd > Vg - VT ) gm = 2I /(Vb - Vin -VT) = (2V) / (R1 (Vb - Vin -VT) ) Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vout Vb Vin rp + V - Vout R1 gmV Vin GND Iref = (1V) / R1 Have Input Bias gm = (1V) / (R1 UT) Vb Vin GND Output Voltage Bias = 4.0V or gm = (2V) / (R1(Vb- Vin-VT) ) Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vout Vb Vin rp + V - Vout R1 gmV Vin GND Iref = (1V) / R1 Have Input Bias gm = (1V) / (R1 UT) Vb Vin GND Output Voltage Bias = 4.0V or gm = (2V) / (R1(Vb- Vin-VT) ) Gain = gm R1 Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vout Vb Vin rp + V - Vout R1 gmV Vin GND Iref = (1V) / R1 Have Input Bias gm = (1V) / (R1 UT) Vb Vin GND Output Voltage Bias = 4.0V or gm = (2V) / (R1(Vb- Vin-VT) ) Gain = gm R1 Gain = (1V) / UT or Gain = (2V) / (Vb- Vin-VT) Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vout Vb Vin GND rp Vout R1 gmV Vin GND Iref = (1V) / R1 Have Input Bias gm = (1V) / (R1 UT) Vb Vin + V - Output Voltage Bias = 4.0V or gm = (2V) / (R1(Vb- Vin-VT) ) Gain = (1V) / UT or Gain = (2V) / (Vb- Vin-VT) Common Gate: Small-Signal Vdd Vdd R1 R1 Vout Vout Vb Vin GND rp Vout R1 gmV Vin GND Iref = (1V) / R1 Have Input Bias gm = (1V) / (R1 UT) Vb Vin + V - Output Voltage Bias = 4.0V or gm = (2V) / (R1(Vb- Vin-VT) ) Gain = (1V) / UT or Gain = (2V) / (Vb- Vin-VT) Output Resistance = R1 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 Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a transistor Vb V1 Vgate GND Cascode Circuits Vdrain Vbias V1 Vgate GND Cascode Circuits Vdrain Vbias V1 Vgate GND (kVbias -V1 )/UT Vdrain /VA Idrain = Io e e kVgate/UT V1 /VA = Io e e Cascode Circuits Vdrain Vbias V1 Vgate GND (kVbias -V1 )/UT Vdrain /VA Idrain = Io e e kVgate/UT V1 /VA = Io e e V1 ~ kVbias - kVgate + (UT/VA) Vdrain Cascode Circuits Vdrain Vbias V1 Vgate GND (kVbias -V1 )/UT Vdrain /VA Idrain = Io e e kVgate/UT V1 /VA = Io e e 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 Idrain = Io e GND (kVbias -V1 )/UT kVgate/UT e kVbias /VA e Vdrain / (Av VA ) Vdrain /VA Idrain = Io e e kVgate/UT V1 /VA = Io e e 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 Idrain = Io e GND (kVbias -V1 )/UT kVgate/UT e kVbias /VA e Vdrain / (Av VA ) Vdrain /VA Idrain = Io e e kVgate/UT V1 /VA = Io e e 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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