Electronic Troubleshooting
Chapter 4
Field Effect Transistors
Field Effect Transistors
• Characteristics
• Common type of transistor, just like the bipolor ones covered in the
previous section
• Use primarily where extremely high input impedance is required
• Most of today's transistors are "MOS-FETs", or Metal Oxide
Semiconductor Field Effect Transistors.
• Ref: http://www.pbs.org/transistor/science/info/transmodern.html#FET
• Types Covered
• JFET
• MOSFET
• VMOSFET
Field Effect Transistors
• JFET
• Theory of Doped Silicon
• Voltage across a lightly Doped silicon
• Small current flows
• Electrons enter the silicon through the Source
• Electrons exit through the Drain
• Bar of silicon acts as a resistor.
• Resistance dependent upon
» Amount of impurities (doping in the
silicon) in the silicon bar
» Length and cross sectional area if the
silicon bar
Field Effect Transistors
• JFET
• Theory of Doped Silicon w/Gate
• PN Junction on the side of the bar
• Called a Gate
• The area around the gat has few electrons and is
called the depletion region
» Acts as a good insulator
• Reverse Biasing the PN Junction
• Increases the size of the depletion region
• Increases the resistance of the silicon bar
• The more reverse biasing voltage the greater the
resistance
• Thus the current through the silicon bar can be
varied by varying the biasing voltage
Field Effect Transistors
• JFET
• Comparison parameter - Transconductance (gm)
• Ratio of change in drain current
to a change in gate-to-source voltage
gm 
• Units: mho
• Typically in µmho
• Example: A 2v change in VGS causes a 5mA change in ID
ID
 V GS
• Find: gm
gm
ID
5 mA


 2 . 5  10
 V GS
2V
• Circuit Symbols
3
 2500  mho
Field Effect Transistors
• JFET
• Circuit Symbols
• Arrows indicate direction of conventional current flow, thus the
polarity required to reverse bias the junction
• Characteristic curve for a typical N-channel JFET
• Above 4 V VDS and VGS =0V
• ID is at its max value
• ID is controlled by changes in VGS
• Sample Problem
• Use chart and determine gm
when changes from -2 to -3V
g
m

ID
( 4  2 ) mA

 2000  mho
 V GS
( 3  2 )V
Field Effect Transistors
• JFET Biasing
• A separate supply could be used to bias the JFET but self-biasing
by using a Source resistor is more economical
• Sizing the Source resistor (SR)
• Sample Problem:
• Given: Desired VGS = -3V and ID =2mA
• FIND: SR
• In Class exercises
• Page 83-4: 4-2, 4-5, 4-7, 4-9
RS 
VGS
3V

 1500 
ID
2mA
VGS
RS 
ID
Field Effect Transistors
• JFET as an AC Amplifier
• Example Circuit
• With the circuit shown and the desire to prevent signal clipping
• Assume ID = 2mA THEN VD = VDD - ID RD = 10V
• The input signal VS is coupled through CC
gm 
then
i
ID
 d
 V GS
v gs
v o  g m v gs R D
and
vo
Av 
 g m RD
v gs
then
g m v gs  i d
and
v o  id R D
Field Effect Transistors
• JFET as an AC Amplifier
• Example Circuit
• With the previous circuit shown and gm = 3000µmho
vo
Av 
 g m RD  3000 10 6  5k  15
v gs
• Add a load Resistor and coupling Cap
• The Gain changes to AV = gm rL
• RL and RD are in parallel for AC signals
• Input resistance
• Since the Gate-to-Source
resistance is so high the Amp
resistance matches the
resistor just before the gate
Field Effect Transistors
• JFET as an AC Amplifier
• In-class Exercises
• Page 84. Problems: 4-10 and 4-12
• Metal Oxide Semiconductor Field Effect
Transistor (MOSFET)
• Current through the device is controlled by the Gate voltage
• Construction is similar to IC construction in that the various layers
are individually deposited
Field Effect Transistors
• MOSFET
• Types
• Depletion Mode
• Enhanced Mode
• Most Common
• Covered in this course
• Layers (continued)
• N+ layers are heavily doped
• N- areas are lightly doped
• Operation
• W/ 0volts on the gate the isn’t enough electrons in the space
between the source and drain for any significant current to flow
Field Effect Transistors
• MOSFET
• Operation
• W/ + voltage on the gate
electrons from the substrata
are drawn into the channel
between the Drain and
Source
• This enhancement makes the
channel conductive
• Current flows between the
Source and Drain
• Enhanced Mode Device
• No current flows through the
gate
• The more positive the Gate
voltage the higher the current
MOSFET Symbols
Field Effect Transistors
• MOSFET As Small Signal
Amplifier
• Practically no drain current
flows w/ 0V on gate
• VO depends upon:
• How high the gate voltage goes
• Value of RD
• How low of resistance that the
MOSFET appears to have
» RD and the MOSFET act as
a voltage divider
Field Effect Transistors
• MOSFET As Small Signal Amplifier
• Practically no drain current flows w/ 0V on gate
• Typical
• With a reasonable
digital input of 5V
» VO would = almost
of
zero
• With a reasonable input
of 0V
» VO would = almost 15V
• The range is much
better than for a bipolar
transistor
Field Effect Transistors
• Power MOSFET
• Typical non-power MOSFET’s develop only a narrow
channel between Source and Drain
• Typical only useful in low power applications due to the
resistance of the channel
• Several Different types are used that have much less
resistance Source to Drain
• Types:
• Lateral Double Defused MOSFET (LDMOSFET)
» Shorter Channel between Source
and Drain – Less Resistance
» Thus higher currents
Field Effect Transistors
• Power MOSFET
• Several Different types (continued)
• Types:
• Vertical MOSFET or V-Channel MOSFET
(VMOSFET)
» HAS a shorter/wider channel
» Lower resistance
» Thus more current flows
» Has two Source and a Gate connection on top
» Drain on the bottom
» Lower Capacatance
• TMOSFET
» Similar to the VMOSFET
» No V Channel
Field Effect Transistors
• Power MOSFET
• Several Different types (continued)
• Types:
• TMOSFET
» Gate is embedded in the Silicon
Dioxide Layer
» The contact from the gate is over a wider
area than for the non-power MOSFETs
» Has smaller physical size than VMOSFETs
• Key Characteristics
• Lower Source to Drain resistance
• Higher currents
• The text covers VMOSFET in more detail
Field Effect Transistors
•
Vertical MOSFET
• Becomes conductive
with a + charge on the
Gate
• Channel resistance
increases with
temperature, thus
VMOSFETs don’t succumb to thermal runaway
• Much higher current capacity than traditional MOSFETs
•
Example: IRF-100 N-Channel MOSFET – ID Max = 16A and typical
transconductance of 3 mhos
VMOSFET Package & Curves
IRF-100
Field Effect Transistors
Vertical MOSFET
•
•
Symbol
•
•
Same as for a plain MOSFET
Like other MOSFETs these are susceptible to static
discharges
•
Many VMOSFETS have a Zener diode built in to protect the
against static discharge
•
Zeners act as a like an
open circuit when
reversed biased with an
appropriate voltage
• If the gate becomes
negative, the zener
will conduct
» Best to have a 1kΩ on gate
Field Effect Transistors
Vertical MOSFET
•
•
Typical MOSFET Power Interface
•
Figure 4-16 on page 81
•
•
•
•
Gate signals can come from a digital circuit, e.g., microcontrollers,
microprocessors, TTL circuits
L1 is a solenoid coil
D1 and R1 are used to prevent coil kickback voltage from growing
high enough to damage Q1 or other components
» Normally reversed biased when solenoid is energized
» Kickback voltage can forward bias D1
Operation
•
•
W/gate at a logic low (approx 0 VDC) Q1 is off and L1 is off
W/gate at a logic level 1 (+5 VDC) Q1 is on and L1 is energized
Field Effect Transistors
Vertical MOSFET
•
•
Troubleshooting Tips
•
If a bad MOSFET is found, check before replacing the MOSFET
and applying power
Typical FET Circuits
•
•
Q9 on Next Slide and Figure 4-17 on page 82
•
•
Used as a Noise amplifier
Biasing
•
•
•
Current flow through Source resister R142
Gate voltage held at 0 VDC by R58
Input signal coupling
•
Through C112 and coil L20
» Higher impedance for higher frequeny
Courier Cruiser CB Circuit
Typical FET Circuits
•
•
Q9 on Next Slide and Figure 4-17 on page 82
•
Output drives Q10 (looks like Q1)
Field Effect Transistors
Typical FET Circuits
•
•
Q1 on Next Slide and Figure 4-18 on page 82
•
•
Dual Gate MOSFET used as a amplifier
Biasing
•
•
•
Positive voltage on G2 provides biasing
» Feed by AGC through R49
» When Input amplitude increases AGC voltage on G2 decreases
» When Input amplitude decreases AGC voltage on G2 increases
Biasing controls Q1 gain
» Q1 Gain decreases when voltage on G2 decreases
» Q1 Gain increases when voltage on G2 increases
Output signal stays constant with changes in Input
Courier Cruiser CB Circuit