Table of contents | |
The Field Effect Transistor (FET) | |
Schematic Symbols | |
Common Nomenclature (n-channel FET example) | |
Common Specifications | |
Common Circuit Applications |
In this Document we will focus on JFETs.
IDSS is the drain current in the active region for VGS = 0. (ID source shorted to gate)
VGS,off is the minimum VGS where ID = 0. VGS,off is negative for n-channel and positive
for p-channel.
gm is the transconductance, the change in ID with VGS and constant VDS.
Voltage Controlled Switch. For the on state the gate voltage VGS = 0 and for the off
state |VGS| > |VGS,off| (of great magnitude than VGS,off and with the same sign). The sign of
the voltage depends on the type of FET, negative for n-channel and positive for p-channel.
Current Source: The drain current is set by RS such that VGS = IDRS. Any value of
current can be chosen between zero and IDSS (see the ID vs VGS graph for the JFET).
Source Follower: The simple source follower is shown below. The improved version is shown at the right. The lower JFET forms a current source. The result is that VGS is held constant, removing the defects of the simple circuit.
Voltage-Controlled Resistor: VGS must be between zero and VGS,off.
JFET Diode: The JET pn gate junction can be used as a diode by connecting the source and the drain terminals. This is done if very low reverse leakage currents are required. The leakage current is very low because the reverse leakage current scales with the gate area. Small gate areas are designed into JFETs because it decreases the gate-source and the gate-drain capacitances.
23 videos|89 docs|42 tests
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1. What is the schematic symbol for a field effect transistor (FET)? |
2. What is the common nomenclature used for an n-channel FET? |
3. What are some common specifications of FETs? |
4. What are some common circuit applications of FETs? |
5. What are some frequently asked questions about the differences between BJT and FET? |
23 videos|89 docs|42 tests
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