Contents MOSFET small signal equivalent circuit BJT small signal equivalent circuit From signal point of view MOSFET behaves as voltagecontrolled current source Accepts between gate and source ID: 722852
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Slide1
ENEE 303 3rd DiscussionSlide2
ContentsMOSFET small signal equivalent circuit
BJT small signal equivalent circuitSlide3
From signal point of view,
MOSFET behaves as voltage-controlled current source.
Accepts
between gate and source
Provides current at drainInput resistance is highOutput resistance is high
Small-signal
models for the MOSFET:
(a) neglecting the dependence of iD on vDS insaturation (the channel-lengthmodulation effect) and (b) including the effect of channel length modulation
MOSFET: Small-Signal Equivalent Circuit
ENEE 303 Fall 2017
Y
admittance matrixSlide4
p
- Model
T
- Model
Transconductance
Output Resistance
Same formula for PMOS except use |VOV |, |Vt|, |l|
MOSFET: Small Signal Equivalent Circuit Models
ENEE 303 Fall 2017Slide5
MOSFET: Small-Signal Equivalent Circuit
ENEE
303
Fall
2017
Summary
DC bias
AC signal
The small signal approximation:
DC bias:
Set the signal to zero to obtain the quiescent point.
AC signal: Set the DC bias voltages and currents and analyze the response of the circuit to the signal. Use the small-signal equivalent circuit.Slide6
Given:
V
t
= 1.5 V,
k
n = 0.25 mA/V2 , VA = 50 V
Find: small-signal voltage gain, input resistance, largest allowable input
signal(remain in
saturation region)Example : MOSFET Amplifier
ENEE 303 Fall
2017Slide7
Given:
V
t
= 1.5 V,
kn = 0.25 mA/V2 , VA = 50 V
Find: small-signal voltage gain, input resistance, largest allowable input signal
ENEE 303 Fall 2017
Example : MOSFET AmplifierSlide8
Given:
V
t
= 1.5 V,
k
n
= 0.25 mA/V2 , VA = 50 V
Find: small-signal voltage gain, input resistance, largest allowable input signal
ENEE 303 Fall 2017
Example : MOSFET AmplifierSlide9
Given:
V
t
= 1.5 V,
k
n
= 0.25 mA/V2 , VA
= 50 V
Find: small-signal voltage gain, input resistance, largest allowable input signalThe largest allowable input signal is constrained by,
ENEE 303 Fall 2017
Example :
MOSFET AmplifierSlide10
ENEE
303
Fall
2017
Small Signal Operation and Models: BJT
Define the
BJT
transconductance
as,
The BJT
transconductance
is the slope of the input characteristic
Slide11
ENEE
303
Fall
2017
Small Signal Operation and Models: BJT
The total base current,
The signal current at the base is then,
The small signal input resistance
between the base and emitter, looking into the base, is defined as,
Slide12
From signal point of view,
BJT behaves as voltage-controlled current source.
Accepts
between base and emitter
Provides current at the collectorInput resistance is high but finiteOutput resistance is high
Small-signal
models for the
BJT: (a) neglecting the dependence of iC on vCE in saturation (the Early effect) and (b) including the Early effect
BJT: Hybrid- Equivalent Circuit
ENEE 303 Fall 2017
Slide13
BJT: Hybrid-
Equivalent Circuit
ENEE
303
Fall
2017
Summary
DC bias
AC signal
The small signal approximation:DC bias: Set the signal to zero to obtain the quiescent point.AC signal:
Set the DC bias voltages and currents and analyze the response of the circuit to the signal. Use the small-signal equivalent circuit.
Slide14
BJT: Hybrid-
Equivalent Circuit
ENEE
303
Fall
2017Slide15
BJT: Hybrid-
Equivalent Circuit
ENEE
303
Fall
2017Slide16
The Three Transistor Amplifier Configurations
ENEE
303
Fall
2017Slide17
The input resistance is,
The output voltage is,
The output resistance is,
The overall voltage gain is,
The Common-Source (CS) Amplifier Configuration
ENEE
303
Fall
2017Slide18
The CS Amplifier with Source Resistance
ENEE
303
Fall
2017
Input Resistance
Output Resistance
Open-Circuit Voltage Gain
Amplifier and Overall Voltage Gain
reduces the amplifier gain