junction field effect transistor
TRANSCRIPT
Field Effect Transistor
FET
Classification of FET
JFET Schematic Symbols
N-channel JFET
Water Analog for JFET
Operation of n-channel JFET
Cont.
VGS = 0 V; VDS = Vp
VGS = -V; No voltage applied B/w the Drain & Source
VGS = -1V, VDS > 0 V
JFET Working
Operation of n-channel JFET Summary
Operation of n-channel JFET Summary
Drain Characteristics
Drain Characteristics of n channel JFET
Cont.
Important terms
1. Pinch-off Voltage (Vp)
2. Shorted-Gate Drain Current (IDSS) or Drain to Source current with gate shorted
3. Gate Source Cut-off Voltage (Turn off voltage)(VGS (off))
Drain Characteristics
• There are 4 important region:1.Ohmic Region2.Saturation Region3.Breakdown Region4.Cut-off Region
Transfer Characteristics of n-channel JFET
Transfer Characteristics
Amplification Factor (µ)
• It is the ratio of change in drain-source voltage (ΔVDS) to the change in gate-source voltage (ΔVGS) at constant drain current (ID)
•
Other Formulas
P channel JFET Working
P channel JFETTransfer & Drain Characteristics
Important Relationships (JFET versus BJT)
JFET as Voltage Variable Resistance
JFET as Voltage Variable Resistance
• The resistance of the device b/w drain & source for VDS < VP is a function of the applied voltage VGS
Merits & Demerits of JFET
Merits• Very High Input
Resistance• Better Thermal Stability• Produces less noise• Long Life & Small Size• Higher power gain• Relatively immune to
radiation
Demerits• Low Voltage gain
compared to transistor Amplifier
• Expensive• Low gain bandwidth
product
JFET BJTUnipolar device Bipolar deviceVoltage Controlled Device Current Controlled DeviceHigh Input Resistance (order of MΩ) Input Resistance Low Compared to JFET
Doesn’t Suffer from minority carrier storage effects (higher switching speeds & cut off frequencies)
Suffers from minority carrier storage effects (lower switching speeds & cut off frequencies)
Negative Temperature Coefficient at high current levels ( prevents from thermal breakdown). Better thermal Stability
Positive Temperature coefficient at high current levels (leads to thermal breakdown)
Less noisy than BJT or Vacuum Tube More Noisy than a JFETSimpler to Fabricate as an IC & occupies less space on IC chip than BJT
Comparatively difficult to Fabricate as an IC and occupies more space on IC chip
FET are expensive than BJT Cheaper to produce than FETsFET can tolerate a much higher level of radiation
Performance is degraded by neutron radiation
Poor Fanout & Low Power consumption Good Fanout & High Power Consumption
Fan-out Meaning
Application of JFET
• Used as a Buffer Amplifier, low noise Amplifier• Used as a Voltage Variable Resistor• Used in digital Circuits, in Computers• Used in Oscillator circuits• Used in low frequency amplifiers in hearing
aids & inductive transducer
Small Signal Low Frequency JFET Model
Small Signal High Frequency JFET Model
Reference
• Electronic Devices & Circuits by S Salivahanan• Electronic Devices & Circuit Theory by Robert
Boylestad and Louis Nashelsky