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Department of Mechanical Engineering
MEMS1082
Chapter 3-2 Semiconductor devices Transistors and Amplifiers JFET
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Transistors
base
Emitter
Collector
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JFET Transistor
General property of a linear amplifier
i
oV V
VA =
i
oi I
IA =
IVii
oo
i
oP AA
IVIV
PPA ===
Input impedance Zi
Output impedance Zo
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JFET Transistor Junction field-effect transistor Physical construction Device symbol
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JFET Transistor Terminal Properties of a JFET
Transconductance amplifier: The output current is controlled by an input voltage in JFET
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JFET Transistor Recall depletion region of a diode
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JFET Transistor Pinch off in JFET as VDS increases
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JFET Transistor The Terminal characteristics of A n-channel JFET
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Junction Field Effect Transistor (FET)
The terminal characteristics of a p-channel JFET
Active region
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JFET Transistor and Amplifier A simple JFET amplifier: Circuit diagram
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Signal wave form and the load line
Load line
s
DSDSV V
VVA
2min,max, −
=
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JFET Amplifier Distortion caused by
large input signals
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dc analysis of a typical JFE amplifier
Operating point
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Théveninequivalent circuit
dc analysis of a typical JFE amplifier
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dc analysis of a typical JFE amplifier Rs tends to stabilize
against the variations in JFET parameters
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JFET Amplifier DC and small AC analysis
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JFET Amplifier DC and small AC analysis
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JFET Amplifier Small AC analysis
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Small signal analysis
JFET Amplifier
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Small signal analysis
JFET Amplifier
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Small signal analysis
JFET Amplifier
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Small signal analysis
JFET Amplifier
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Small signal analysis
JFET Amplifier
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Small signal analysis of the common-source JFET Amplifier
JFET Amplifier
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For ac analysis, set dc sources to zero and replace the capacitors with a short circuit (approximation)
JFET Amplifier
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Further simplified to
JFET Amplifier
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Small signal analysis
JFET Amplifier
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Example
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Simplified small-signal equivalent circuit of a JFET
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Effect of the bypass capacitor Cs If no bypass
capacitor Cs, the simplified JFET amplifier circuit will be
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Effect of the bypass capacitor Cs Effect of the bypass capacitor Cs
0=∆ GSv
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Example: JFET amplifier with Cs removed
( )097.0
1−=
+−=
fss
LDfsV yR
RRyA
Ω== kRZ Gi 100
7.9−==L
iVi R
ZAA
Ω== kRZ Do 1
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Frequency response of the JFET Amplifier
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Frequency response of the JFET Amplifier
Gate-source and gate-drain pn junctions are reverse biased.The depletion layers are a separation of mobile charge carriers, thus at high f, we have CGD and CGS
CDS is caused by other less predominant factors, such as stray capacitance
For 2N3819 JFETCGD =CGS=4pF, CDS=0
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Frequency response of the JFET Amplifier
GSi vv ∆=∆Note:
Convert the controlled current source to controlled voltage source:
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Frequency response of the JFET Amplifier
KVL about the right loop
GSi vv ∆=∆Note:
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Frequency response of the JFET Amplifier
Equivalent circuit
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Example: High frequency JFET amplifier
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Example: High frequency JFET amplifier
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