MEMS1082
Chapter 3-2 Semiconductor devices Transistors and Amplifiers JFET
Department of Mechanical Engineering Transistors
Collector base
Emitter
Department of Mechanical Engineering JFET Transistor
General property of a linear amplifier
Vo AV = Vi
Io Ai = Ii
Po Vo Io AP = = = AV AI Pi Vi Ii
Input impedance Zi
Output impedance Zo
Department of Mechanical Engineering JFET Transistor
Junction field-effect transistor Physical construction Device symbol
Department of Mechanical Engineering JFET Transistor
Terminal Properties of a JFET
Transconductance amplifier: The output current is controlled by an input voltage in JFET
Department of Mechanical Engineering JFET Transistor
Recall depletion region of a diode
Department of Mechanical Engineering JFET Transistor
Pinch off in JFET as VDS increases
Department of Mechanical Engineering JFET Transistor
The Terminal characteristics of A n-channel JFET
Department of Mechanical Engineering Junction Field Effect Transistor (FET)
The terminal characteristics of a p-channel JFET
Active region Department of Mechanical Engineering JFET Transistor and Amplifier
A simple JFET amplifier: Circuit diagram
Department of Mechanical Engineering Signal wave form and the load line
Load line
VDS,max −VDS,min AV = 2Vs
Department of Mechanical Engineering JFET Amplifier
Distortion caused by large input signals
Department of Mechanical Engineering dc analysis of a typical JFE amplifier
Operating point
Department of Mechanical Engineering dc analysis of a typical JFE amplifier
Thévenin equivalent circuit
Department of Mechanical Engineering dc analysis of a typical JFE amplifier
Rs tends to stabilize against the variations in JFET parameters
Department of Mechanical Engineering JFET Amplifier
DC and small AC analysis
Department of Mechanical Engineering JFET Amplifier
DC and small AC analysis
Department of Mechanical Engineering JFET Amplifier
Small AC analysis
Department of Mechanical Engineering JFET Amplifier
Small signal analysis
Department of Mechanical Engineering JFET Amplifier
Small signal analysis
Department of Mechanical Engineering JFET Amplifier
Small signal analysis
Department of Mechanical Engineering JFET Amplifier
Small signal analysis
Department of Mechanical Engineering JFET Amplifier
Small signal analysis
Department of Mechanical Engineering JFET Amplifier
Small signal analysis of the common-source JFET Amplifier
Department of Mechanical Engineering JFET Amplifier
For ac analysis, set dc sources to zero and replace the capacitors with a short circuit (approximation)
Department of Mechanical Engineering JFET Amplifier
Further simplified to
Department of Mechanical Engineering JFET Amplifier
Small signal analysis
Department of Mechanical Engineering Example
Department of Mechanical Engineering Simplified small-signal equivalent circuit of a JFET
Department of Mechanical Engineering Effect of the bypass capacitor Cs
If no bypass capacitor Cs, the simplified JFET amplifier circuit will be
Department of Mechanical Engineering Effect of the bypass capacitor Cs
Effect of the bypass capacitor Cs ∆vGS = 0
Department of Mechanical Engineering Example: JFET amplifier with Cs removed
y fs (RD RL ) Zi A = − = −0.097 Ai = AV = −9.7 V R 1+ Rs y fs L Z = R =1kΩ Zi = RG =100kΩ o D Department of Mechanical Engineering Frequency response of the JFET Amplifier
Department of Mechanical Engineering Frequency response of the JFET Amplifier
CDS is caused by other less predominant For 2N3819 JFET factors, such as stray CGD =CGS=4pF, capacitance CDS=0
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 Department of Mechanical Engineering Frequency response of the JFET Amplifier
Convert the controlled current source to controlled voltage source:
Note:
∆vi = ∆vGS Department of Mechanical Engineering Frequency response of the JFET Amplifier
Note:
∆vi = ∆vGS KVL about the right loop
Department of Mechanical Engineering Frequency response of the JFET Amplifier
Equivalent circuit
Department of Mechanical Engineering Example: High frequency JFET amplifier
Department of Mechanical Engineering Example: High frequency JFET amplifier
Department of Mechanical Engineering