View
216
Download
1
Category
Preview:
Citation preview
FET AND ITS APPLICATIONS
UNIT - III
04/19/23 www.noteshit.com 1
Field Effect Transistor Family
04/19/23 www.noteshit.com 2
Construction of an N-channel JFET and the schematic symbol.
04/19/23 www.noteshit.com 3
Circuit that can be used to measure N-channel JFET characteristics.
04/19/23 www.noteshit.com 4
Drain characteristics for an ideal representative N-channel JFET.
04/19/23 www.noteshit.com 5
MOSFETs
• Depletion-Mode MOSFET
• Enhancement-Mode MOSFET
04/19/23 www.noteshit.com 6
MOSFETs
Construction of an N-channel depletion-mode MOSFET with external connection for substrate and the schematic symbol.
04/19/23 www.noteshit.com 7
MOSFETs
Construction of an N-channel enhancement-mode MOSFET with external connection for substrate and the schematic symbol.
04/19/23 www.noteshit.com 8
MOSFETs
Drain characteristics for an ideal representative N-channel enhancement-mode MOSFET.
04/19/23 www.noteshit.com 9
MOSFETs
FET Switches
• JFET Resistance in Ohmic Region
• N-Channel JFET Switch
• N-Channel MOSFET Switch
04/19/23 www.noteshit.com 10
N-channel JFET switch.
04/19/23 www.noteshit.com 11
Switch using an N-channel enhancement-mode MOSFET.
04/19/23 www.noteshit.com 12
FET small signal model
FETs provide
• excellent voltage gain
• high input impedance
• low-power consumption
• good frequency range
• small in size and weight
04/19/23 www.noteshit.com 13
FET small signal model
• Transconductance
The relationship of VGS (input) to ID (output) is called transconductance.
The transconductance is denoted gm.
GS
Dm
V
Ig
04/19/23 www.noteshit.com 14
FET small signal model
Graphical Determination of gm
04/19/23 www.noteshit.com 15
FET small signal model
Mathematical Definition of gm
GS
Dm
V
Ig
P
GS
P
DSSm
V
V1
V
2Ig
P
DSSm0
V
2Ig
gm for VGS =0V:
P
GSm0m
V
V1gg
04/19/23 www.noteshit.com 16
FET small signal model
FET AC Equivalent Circuit
04/19/23 www.noteshit.com 17
Cascade Connection
• The output of one amplifier is the input to the next amplifier.
• The overall gain:
Av total : Av1 * Av2
• Note the DC bias circuits are isolated from each other by the coupling capacitors.
• The DC calculations are independent of the cascading. • The AC calculations for gain and impedance are
interdependent.
04/19/23 www.noteshit.com 18
FET Cascade Amplifier
04/19/23 www.noteshit.com 19
Darlington Connection
This combination provides large current gain, typically a few thousand.It has a voltage gain of near 1, a low output impedance and a high input impedance.
04/19/23 www.noteshit.com 20
Frequency response
• Internal and external capacitances
• High-frequency equivalent circuit model for the MOSFET (a)
• High-frequency equivalent circuit model for the MOSFET when
• Source is connected to Body (b)
04/19/23 www.noteshit.com 21
Simplified Capacitance Model
04/19/23 www.noteshit.com 22
Common source Amplifier
common-source amplifier is one of three basic single-
stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or Tran conductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal enters, and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit. The analogous bipolar junction transistor circuit is the common-emitter amplifier.
04/19/23 www.noteshit.com 23
Common source Amplifier
04/19/23 www.noteshit.com 24
COMMON DRAIN AMPLIFIER
Common-drain amplifier, also known as a source follower, is one of three basic single-stage field effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit the gate terminal of the transistor serves as the input, the source is the output, and the drain is common to both (input and output), hence its name. The analogous bipolar junction transistor circuit is the common-collector amplifier.
04/19/23 www.noteshit.com 25
COMMON DRAIN AMPLIFIER
04/19/23 www.noteshit.com 26
Freq. Response CS amplifier
04/19/23 www.noteshit.com 27
Freq. Response CS amplifier
04/19/23 www.noteshit.com 28
Summary of CS, CG, and CD Amplifiers
04/19/23 www.noteshit.com 29
Summary of CS, CG, and CD Amplifiers
04/19/23 www.noteshit.com 30
THANK U
04/19/23 www.noteshit.com 31
Recommended