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Common-Gate (Base) Amplifierand Cascode Circuits
Dr. Paul Hasler
Common Gate: Resistive Load
Vdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Common G: Resistive Load
Common Gate: Resistive Load
Vdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Common Gate: Resistive Load
Vdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
What is the bias current? Iref = (1V) / R1
Common Gate: Resistive LoadVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
Common Gate: Resistive LoadVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT
Common Gate: Resistive LoadVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT
(1V) / R1 = Ico eVb-Vin/UT
Vin = Vb - UT ln ( (1V) / R1 Ico )
Common Gate: Resistive LoadVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT Above Threshold (Vd > Vg - VT )
(1V) / R1 = Ico eVb-Vin/UT
Vin = Vb - UT ln ( (1V) / R1 Ico )
Common Gate: Resistive LoadVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT Above Threshold (Vd > Vg - VT )
(1V) / R1 = (K/2) (Vb - Vin - VT )2 (1V) / R1 = Ico eVb-Vin/UT
Vin = Vb - UT ln ( (1V) / R1 Ico ) Vin = Vb - VT - sqrt((2V)/(K R1))
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT
Have Input Bias
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT
Have Input Bias
gm = I / UT = (1V) / (R1 UT)
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT Above Threshold (Vd > Vg - VT )
Have Input Bias
gm = I / UT = (1V) / (R1 UT)
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
BJT / Subthreshold VT Above Threshold (Vd > Vg - VT )
Have Input Bias
gm = I / UT = (1V) / (R1 UT)
gm = 2I /(Vb - Vin -VT) = (2V) / (R1 (Vb - Vin -VT) )
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
Have Input Bias
gm = (1V) / (R1 UT)
gm = (2V) / (R1(Vb- Vin-VT) )
or
gmVr
GND
GND
Vout
R1
+V-
Vin
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
Have Input Bias
gm = (1V) / (R1 UT)
gm = (2V) / (R1(Vb- Vin-VT) )
or
gmVr
GND
GND
Vout
R1
+V-
Vin
Gain = gm R1
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
Have Input Bias
gm = (1V) / (R1 UT)
gm = (2V) / (R1(Vb- Vin-VT) )
or
gmVr
GND
GND
Vout
R1
+V-
Vin
Gain = gm R1
Gain = (1V) / UT
Gain = (2V) / (Vb- Vin-VT)
or
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
Have Input Bias
gm = (1V) / (R1 UT)
gm = (2V) / (R1(Vb- Vin-VT) )
or
gmVr
GND
GND
Vout
R1
+V-
Vin
Gain = (1V) / UT
orGain = (2V) / (Vb- Vin-VT)
Common Gate: Small-SignalVdd
Vout
Vin
Vb
R1
Vdd
Vout
Vin
Vb
R1
Output VoltageBias = 4.0V
Iref = (1V) / R1
Have Input Bias
gm = (1V) / (R1 UT)
gm = (2V) / (R1(Vb- Vin-VT) )
or
gmVr
GND
GND
Vout
R1
+V-
Vin
Gain = (1V) / UT
orGain = (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.
Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a transistor
Vdrain
Vb
GND
V1
Vgate
Cascode CircuitsVdrain
Vbias
GND
V1
Vgate
Cascode CircuitsVdrain
Vbias
GND
V1
Vgate
Idrain = Io e (Vbias
-V1 )/UT e
Vdrain /VA
= Io e Vgate/UT e
V1 /VA
Cascode CircuitsVdrain
Vbias
GND
V1
Vgate
Idrain = Io e (Vbias
-V1 )/UT e
Vdrain /VA
= Io e Vgate/UT e
V1 /VA
V1 ~ Vbias - Vgate + (UT/VA) Vdrain
Cascode CircuitsVdrain
Vbias
GND
V1
Vgate
Fixes the voltage at V1 or isolates V1 from the output
Idrain = Io e (Vbias
-V1 )/UT e
Vdrain /VA
= Io e Vgate/UT e
V1 /VA
V1 ~ Vbias - Vgate + (UT/VA) Vdrain
Drain is fixed
Cascode CircuitsVdrain
Vbias
GND
V1
Vgate
Fixes the voltage at V1 or isolates V1 from the output
Idrain = Io e (Vbias
-V1 )/UT e
Vdrain /VA
= Io e Vgate/UT e
V1 /VA
V1 ~ Vbias - Vgate + (UT/VA) Vdrain
Drain is fixed
Idrain = Io e Vgate/UT e
Vbias /VA eVdrain / (Av VA )
Cascode CircuitsVdrain
Vbias
GND
V1
Vgate
Fixes the voltage at V1 or isolates V1 from the output
GND
Vgate
Vdrain
Idrain = Io e (Vbias
-V1 )/UT e
Vdrain /VA
= Io e Vgate/UT e
V1 /VA
V1 ~ Vbias - Vgate + (UT/VA) Vdrain
Drain is fixed
Idrain = Io e Vgate/UT e
Vbias /VA eVdrain / (Av VA )
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
