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DE4401

FIELD EFFECT TRANSISTOR

Intro

• BJT-Current controlled in controls out

• FET is Voltage controlled in controls out

• FET has major advantage - very high , so sensitive to small input voltages, but can be damaged by static electricity

• FET more efficient than BJT because (main output current path) is low, so less power dissipation

• Two types – JFET (junction Field Effect) vs MOSFET (Metal Oxide Silicon FET = ‘Insulated’ Gate)

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The JFET

• JFET has no PN junctions, has a narrow CHANNEL of N or P silicon between Drain and Source, surrounded by a gate of the opposite type, which creates an Electric Field to ‘pinch off’ the main output current flow.

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JFET construction and symbol

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N channel JFET Depletion Layer Biasing

• Small Negative controls width of main channel (a ‘Voltage-controlled resistor’) and hence size of note( must never be positive)

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Typical JFET characteristic I-V curves

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Biasing in the Active (amplifier) Region

• =0 when = . is biased between and 0

• Transconductance gain (compares to BJT β )

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3 MODES – CS compares to CE for BJT

• CS most common- High and good Voltage Amplification

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JFET Amplifier- class A

• Bias with / divider to set approx /4

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MOSFET – Insulated Gate

• Extremely high Input Resistance (MΩ) – very senstive

• Vulnerable to damage if high static charge builds on gate

• PMOS and NMOS each available in 2 types

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MOSFET structure and Symbol

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MOSFET construction and bias

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Depletion-mode MOSFET (Normally Closed )

• Less common since is ON (conducting) with no gate bias• N-channel, Neg will ‘deplete’ channel electrons, turning it off

• P-channel, Pos will ‘deplete’ channel holes, turning it off

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Enhancement-mode MOSFET (normally off)

• only flows when >

• Pos on N type attracts more electrons to gate, enhancing thickness of channel causing more to flow

• N chan Enh-MOSFET: Pos turns it ON, 0 or – turns OFF

• P chan Enh-MOSFET: Pos turns it OFF, 0 or – turns ON

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Enhancement-mode MOSFET characteristic

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Enhancement-mode MOSFET classA Amplifier

• Inverting amplifier

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MOSFET Summary

• Efficient Low Power consumption (CMOS logic gates)

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MOSFET as SWITCH

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MOSFET switch Bias

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MOSFET Switch Example

• The faster we turn FET on/off, the less power ‘loss’

• Simple resistive load ok, but Inductive or Capacitive loads require protection (Flywheel Diodes)

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MOSFET Switches Summary

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MOSFET switch example

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Simple Motor Control (On/Off)

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P channel Switch

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Complementary CMOS Motor Control

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FET summary

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FET chart

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FET compared to BJT

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