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

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V

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Common G: Resistive Load

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V What is the bias current?I ref = (1V) / R 1

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T (1V) / R 1 = I co e Vb-Vin/U T V in = V b - U T ln ( (1V) / R 1 I co )

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T Above Threshold (V d > V g - V T ) (1V) / R 1 = I co e Vb-Vin/U T V in = V b - U T ln ( (1V) / R 1 I co )

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Common Gate: Resistive Load V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T Above Threshold (V d > V g - V T ) (1V) / R 1 = (K/2) (V b - V in - V T ) 2 (1V) / R 1 = I co e Vb-Vin/U T V in = V b - U T ln ( (1V) / R 1 I co ) V in = V b - V T - sqrt((2V)/(K R 1 ))

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T Have Input Bias

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T Have Input Bias g m = I / U T = (1V) / (R 1 U T )

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T Above Threshold (V d > V g - V T ) Have Input Bias g m = I / U T = (1V) / (R 1 U T )

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 BJT / Subthreshold V T Above Threshold (V d > V g - V T ) Have Input Bias g m = I / U T = (1V) / (R 1 U T ) g m = 2I /(V b - V in -V T ) = (2V) / (R 1 (V b - V in -V T ) )

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 Have Input Bias g m = (1V) / (R 1 U T ) g m = (2V) / (R 1 (V b - V in -V T ) ) or gmVgmV r GND V out R1R1 +V-+V- V in

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 Have Input Bias g m = (1V) / (R 1 U T ) g m = (2V) / (R 1 (V b - V in -V T ) ) or gmVgmV r GND V out R1R1 +V-+V- V in Gain = g m R 1

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 Have Input Bias g m = (1V) / (R 1 U T ) g m = (2V) / (R 1 (V b - V in -V T ) ) or gmVgmV r GND V out R1R1 +V-+V- V in Gain = g m R 1 Gain = (1V) / U T Gain = (2V) / (V b - V in -V T ) or

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 Have Input Bias g m = (1V) / (R 1 U T ) g m = (2V) / (R 1 (V b - V in -V T ) ) or gmVgmV r GND V out R1R1 +V-+V- V in Gain = (1V) / U T or Gain = (2V) / (V b - V in -V T )

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Common Gate: Small-Signal V dd V out V in VbVb R1R1 V dd V out V in VbVb R1R1 Output Voltage Bias = 4.0V I ref = (1V) / R 1 Have Input Bias g m = (1V) / (R 1 U T ) g m = (2V) / (R 1 (V b - V in -V T ) ) or gmVgmV r GND V out R1R1 +V-+V- V in Gain = (1V) / U T or Gain = (2V) / (V b - V in -V T ) Output Resistance = R 1

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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. Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a 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. Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a transistor V drain VbVb GND V1V1 V gate

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Cascode Circuits V drain V bias GND V1V1 V gate

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Cascode Circuits V drain V bias GND V1V1 V gate I drain = I o e ( V bias -V 1 )/U T e V drain /V A = I o e V gate /U T e V 1 /V A

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Cascode Circuits V drain V bias GND V1V1 V gate I drain = I o e ( V bias -V 1 )/U T e V drain /V A = I o e V gate /U T e V 1 /V A V 1 ~ V bias - V gate + (U T /V A ) V drain

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Cascode Circuits V drain V bias GND V1V1 V gate Fixes the voltage at V 1 or isolates V 1 from the output I drain = I o e ( V bias -V 1 )/U T e V drain /V A = I o e V gate /U T e V 1 /V A V 1 ~ V bias - V gate + (U T /V A ) V drain Drain is fixed

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Cascode Circuits V drain V bias GND V1V1 V gate Fixes the voltage at V 1 or isolates V 1 from the output I drain = I o e ( V bias -V 1 )/U T e V drain /V A = I o e V gate /U T e V 1 /V A V 1 ~ V bias - V gate + (U T /V A ) V drain Drain is fixed I drain = I o e V gate /U T e V bias /V A e V drain / (A v V A )

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Cascode Circuits V drain V bias GND V1V1 V gate Fixes the voltage at V 1 or isolates V 1 from the output GND V gate V drain I drain = I o e ( V bias -V 1 )/U T e V drain /V A = I o e V gate /U T e V 1 /V A V 1 ~ V bias - V gate + (U T /V A ) V drain Drain is fixed I drain = I o e V gate /U T e V bias /V A e V drain / (A v V A )

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BJT - CMOS Cascode Circuits Preserve High-g m /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

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