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Common-Gate (Base) Amplifier and Cascode Circuits

Common-Gate (Base) Amplifier and Cascode Circuits. Dr. Paul Hasler. V dd. V dd. Common Gate: Resistive Load. Output Voltage Bias = 4.0V. R 1. R 1. V out. V out. V b. V b. V in. V in. Common G: Resistive Load. V dd. V dd. Common Gate: Resistive Load. Output Voltage Bias = 4.0V.

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Common-Gate (Base) Amplifier and Cascode Circuits

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

  2. Vdd Vdd Common Gate: Resistive Load Output Voltage Bias = 4.0V R1 R1 Vout Vout Vb Vb Vin Vin

  3. Common G: Resistive Load

  4. Vdd Vdd Common Gate: Resistive Load Output Voltage Bias = 4.0V R1 R1 Vout Vout Vb Vb Vin Vin

  5. Vdd Vdd What is the bias current? Iref = (1V) / R1 Common Gate: Resistive Load Output Voltage Bias = 4.0V R1 R1 Vout Vout Vb Vb Vin Vin

  6. Vdd Vdd Common Gate: Resistive Load Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Vb Vb Vin Vin

  7. Vdd Vdd Common Gate: Resistive Load Output Voltage Bias = 4.0V R1 R1 Iref = (1V) / R1 Vout Vout Vb Vb Vin Vin BJT / Subthreshold VT

  8. Vdd Vdd 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. Vdd Vdd 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 > Vg - VT ) (1V) / R1 = Ico eVb-Vin/UT Vin = Vb - UT ln ( (1V) / R1 Ico )

  10. Vdd Vdd 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 > Vg - VT ) (1V) / R1 = Ico eVb-Vin/UT (1V) / R1 = (K/2) (Vb - Vin - VT )2 Vin = Vb - VT - sqrt((2V)/(K R1)) Vin = Vb - UT ln ( (1V) / R1 Ico )

  11. Vdd Vdd 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. Vdd Vdd 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. Vdd Vdd 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 > Vg - VT ) gm = I / UT = (1V) / (R1 UT)

  14. Vdd Vdd 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 > Vg - VT ) gm = 2I /(Vb - Vin -VT) = (2V) / (R1 (Vb - Vin -VT) ) gm = I / UT = (1V) / (R1 UT)

  15. Vdd Vdd GND GND 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. Vdd Vdd GND GND 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. Vdd Vdd GND GND 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. Vdd Vdd GND GND 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. Vdd Vdd GND GND 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. 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.

  22. 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 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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