Field Effect Transistors

Next to the bipolar device that has been studied thus far the Field Effect Transistor is very common in electronic circuitry, though different you will no doubt observe it is similar in many ways.

Features

Field Effect Transistors

FET;

Used;In Switching Circuits, Amplifiers, &

Digital circuits.Advantages;

1. High Input Impedance.2. Low Noise Figure3. Operates w/ a small voltage @ the

Gate, Controls 100% of the current from Source to Drain. 1 path of conduction.Field Effect;

Refers to the Depletion Field formed around the PN junction.

JFET & MOSFET

JFET; Junction Field Effect Transistor.1. Operates in the Depletion only mode.2. Conducts W/0v on the Gate.3. Reversed Biased for Operation.

MOSFET; Metal Oxide Semiconductor Field Effect Transistor.

1. Can operate in the Depletion or Enhancement Mode.2. Conducts W/0V on the Gate.3. Reversed or Forward Biased for

Operation.4. Also Known as an, IGFET.

MOSFET, [Enhancement only Mode];1. Operates in the enhanced only mode.2. Conducts w/a Positive on the Gate. A channel must be formed to allow for conduction. 3. Forward Biased for Operation. 4. Also Known as an, IGFET.

Field Effect Transistors

JFET & MOSFET;

Bi & Uni-Polar

Field Effect Transistors

Bipolar & Unipolar Comparison;

GATE

DRAIN

SOURCE

UNIPOLAR TRANSISTORUNIPOLAR TRANSISTOR

NN

PP

SS

GGDD

COLLECTOR

BASE

EMITTER

BIPOLAR TRANSISTORBIPOLAR TRANSISTOR

NNNN

PP

CC

EE

BB

JFET Operation

P P

N

N

-6 v P P

N

N

-2 v P P

N

N

0 vI

Field Effect Transistors

JFETJFET;Reverse Bias or Voltage @ the Gate .

Resistance , Current , E @ the Drain .

G

R

I

D

00

0 0

Schematic Symbol

Field Effect Transistors

JFETJFET; Schematic Symbols.

Source

GATE

Drain

N-Channel JFETN-Channel JFET

Source

GATE

Drain

ASYMETRICALASYMETRICAL

Wiring Integrity must be observed

Source

GATE

Drain

P-Channel JFETP-Channel JFET

DRAIN

GATE

SOURCE

SYMETRICALSYMETRICAL

Source & Drain Wiring can be

reversedIGFET

Construction

N

DRAIN

SOURCE

SUBSTRATESUBSTRATEINSULATORINSULATORMATERIALMATERIAL DRAIN

SOURCE

N

N

P

DRAIN

N

P

N

SOURCEMETAL GATEMETAL GATE

Field Effect Transistors

MOSFETMOSFET, [IGFETIGFET]; Construction.

Depletion Mode

DRAIN

SOURCE

N

P

N

+++++++++++++

N-materialChannel

Negative on the Gate

depletes the

channel of current carriers

-------

METAL GATEMETAL GATE

INSULATING MATERIAL

Field Effect Transistors

DEPLETION MODE MOSFETDEPLETION MODE MOSFET; Operation like the JFET.G

R

I

D

00

0 0

Enhanced Mode

N

P

N

DRAIN

SOURCE

+++++++

----------

N-materialChannel

Positive on the Gate adds

current carriers to the

channelMETAL GATEMETAL GATE

INSULATING MATERIAL

Field Effect Transistors

ENHANCEMENT MODE ENHANCEMENT MODE MOSFET;

G

R

I

D

++00

++00

Enhanced only Mode

MOFET SCH Symbols

TII Field Effect Transistors Field Effect Transistors

ENHANCEMENT ENHANCEMENT ONLYONLY MODE MOSFET

G

R

I

D

++00

++00

Creates Creates negative negative ion Fieldion Field

+++++++

----------

----------

----------

+++++++

+++++++

Creates Creates Electron Electron ChannelChannel

DRAIN

SOURCE

N

P

N

W/ 0V @ the Gate the device

is OFF, [ no Current flow].

Field Effect Transistors

MOSFET Schematic SymbolsSchematic Symbols;

MOSFET Depletion & Enhanceme

nt N-channel

MOSFET MOSFET Enhancement Enhancement

onlyonly. N-channel. N-channel

MOSFET MOSFET Enhancement Enhancement

onlyonly. P-. P-channelchannel

JFET Ckt Comp

VDD

RD

RS

RG

VS

ID

Field Effect Transistors

JFET Circuit; Support components;

JFET AMP

Troubleshooting procedure

Field Effect Transistors

JFET Amplifier; Support Component Identification.

R1R1 & 22; AC Voltage Divider

R3R3; Gate-Source Bias, Ensures a very short TC so C1 does not fully charge & 0V on the Gate.

R4R4; Drain load, Develops the Ckt Gain.

R5R5; Source - Gate Bias.

C3C3; Prevents signal degeneration.

C1C1 & 22; Input & Output Coupling Caps.

R6R6; Adjust output Amplitude.

++

R3R3 2.2 M 2.2 M R2R2 7.5 K7.5 K

R1R1 100 K100 K

R5R5 1 K1 K

E

O

T

CC22

C3C3

Q12N5484

R4R4 3.9 K3.9 K

23

4

56

7R6R6

50 K50 K

C1C1 .05 .05 ff

Challenge

Field Effect Transistors

JFET Amplifier; Troubleshooting procedure.

+

R3 2.2 M R2 7.5 K

R1 100 K

R5 1 K

E

O

T

R4

R6

C1

C2

C3

Q12N5484

3.9 K

23

4

56

7

50 K

.05 f

9.28 9.28 VDCVDC

0 0 VDCVDC

.672 .672 VDCVDC

2nd

1st

3rd3rd

5th5th

4th4th

Challenge

Field Effect Transistors

JFET Amplifier; Troubleshooting Challenge #1.

+

R3 2.2 M R2 7.5 K

R1 100 K

R5 1 K

E

O

T

R4

R6

C1

C2

C3

Q12N5484

3.9 K

23

4

56

7

50 K

.05 f

9.28 9.28 VDCVDC

0 0 VDCVDC

.672 .672 VDCVDC

0 VDC0 VDC

0 VDC0 VDC

0 VDC0 VDC

Challenge

Field Effect Transistors

JFET Amplifier; Troubleshooting Challenge #2.

+

R3 2.2 M R2 7.5 K

R1 100 K

R5 1 K

E

O

T

R4

R6

C1

C2

C3

Q12N5484

3.9 K

23

4

56

7

50 K

.05 f

9.28 9.28 VDCVDC

0 0 VDCVDC

.672 .672 VDCVDC

2.54 V2.54 V

0 VDC0 VDC

2.54 V2.54 V

Review ?

Field Effect Transistors

JFET Amplifier; Troubleshooting Challenge #3.

+

R3 2.2 M R2 7.5 K

R1 100 K

R5 1 K

E

O

T

R4

R6

C1

C2

C3

Q12N5484

3.9 K

23

4

56

7

50 K

.05 f

9.28 9.28 VDCVDC

0 0 VDCVDC

.672 .672 VDCVDC

2.48 V2.48 V

2.48 V2.48 V

Field Effect Transistors

Review Questions;

1. The most significant advantage of the field effect transistor is that no ______________________ flows from ________________ to ________________ .

2. The above advantage allows the FET to display the important characteristic of ______________ input __________________ .

3. All JFETs operate in the ________________ mode.

4. What will happen to the current in an N Channel MOSFET when a positive voltage is applied to the Gate ? __________________________________

5. On PC 49 card, what is the phase relationship of Q1 Gate to Drain? ________

6. What condition must be present for a JEFET to operate normally?

_________________________________ _________________________________

7. What direction does current flow in an FET? ____________________________

Answers

Field Effect Transistors

Review Questions;

8. Which type of FET must have a voltage applied to the Gate to create a channel ? ___________________ ___________________

9. The FET is a ________ ________ operated transistor.operated transistor.

10. What value would be measured @ the source if R5 were to open? __________

11. A transistor for which only one Majority current carrier need be considered during normal operation is said to be ____polar____polar.

12. What are the FET terminals called? _________ _________ _________________ _________ ________

Answers