Transistors:
Field-effect transistors
Eugeniy E. Mikhailov
The College of William & Mary
Week 8
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Electronics 1
Week 8
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Field-effect transistor (FET)
Junction-FET
NJFET
PJFET
Drain
Gate
Drain
Source
Gate
Source
Metal-Oxide-Semiconductor FET (MOSFET)
gate is truly isolated
there is a forth terminal usually connected to source
NMOSFET
PMOSFET
Drain
Drain
Gate
Gate
Source
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Source
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Week 8
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FET vs BJT
FET pros
very high input impedance
FET cons
have large parameter spread
JFET - impedance 1012 Ω
MOSFET - impedance
1014 Ω
thus very small current into the
base (pA range)
can operate bidirectionally
as result very little power
consumption for the biasing
network
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
3 / 14
Let’s focus on NJFET
Drain
Gate
Source
General notes
N-channel usually faster due to higher mobility of electrons vs
holes
current goes from drain to source (thus the names)
drain and source are nearly identical
thus sometimes gate is centered on some diagrams
G
D
S
can be used backwards with almost the same performance
arrow indicates direction of the PN junction
D
G
S
thus normal operation (low current into the gate) when VG < VS
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
4 / 14
NJFET gate voltage rules
Vp : pinch-off voltage (intrinsic parameter), Vp < 0 for NJFETs
it is called Vth :threshold voltage for MOSFETs
Vgs < Vp
ID = 0
Vgs > 0.6 V
device fails, remember about gate diode
D
G
S
Vp < Vgs < 0.6 V
normal operation
Id (Vgs , Vds )
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
5 / 14
Id vs VDS
Linear region
VDS < VGS − Vp
2
Id(VDS ) = k (2(VGS − Vp )VDS − VDS
)
Saturation region
VDS > VGS − Vp
Id(VDS ) = k (VGS − Vp )2
k is a constant
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
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Real Id vs VDS linear region
7
6
ID (mA)
5
4
3
2
Id(VDS)=k(2 (VGS-Vp) VDS - VDS2)
1
Id(VDS)=k(VGS-Vp)2
0
0
2
4
Eugeniy Mikhailov (W&M)
6
8
10 12
VDS (mV)
14
16
Electronics 1
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20
Week 8
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Real Id vs VDS linear region
7
6
ID (mA)
5
4
3
2
1
Id(VDS)=k(2 (VGS-Vp) VDS - VDS2)
0
0
0.5
1
VDS (mV)
1.5
2
VDS < VGS − Vp
2
Id(VDS ) = k (2(VGS − Vp )VDS − VDS
) ≈ 2k (VGS − Vp )VDS
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
7 / 14
Real Id vs VDS saturation region
7
6
ID (mA)
5
4
3
2
Id(VDS)=k(2 (VGS-Vp) VDS - VDS2)
1
Id(VDS)=k(VGS-Vp)2
0
0
2
4
6
8
10 12
VDS (mV)
14
16
18
20
IDSS saturation drain current, depends on VGS and Vp
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
8 / 14
Real Id vs VDS saturation region
7
6
ID (mA)
5
4
3
2
1
Id(VDS)=k(VGS-Vp)2=IDSS
0
8
10
12
14
VDS (mV)
16
18
20
VDS > VGS − Vp
Id(VDS ) = k (VGS − Vp )2
IDSS saturation drain current, depends on VGS and Vp
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
8 / 14
Reading the specifications
VGS(off ) is the same as Vp
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Electronics 1
Week 8
9 / 14
NJFET voltage controlled divider
Vin
Rd
Vout
Id
Vctrl
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Electronics 1
Week 8
10 / 14
NJFET compensated voltage controlled divider
Vin
Rd
Vout
R
R
Id
Vctrl
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Electronics 1
Week 8
11 / 14
NJFET constant current source
Vdd
RL
Id
Rs
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Electronics 1
Week 8
12 / 14
FET source follower
Vdd
Vout
Vin
Rs
vout =
Rs
vin
Rs + 1/gm
Vout > Vin
Eugeniy Mikhailov (W&M)
Electronics 1
Week 8
13 / 14
FET source follower improved
Vdd
Vin
Rs
vout = vin
Vout = Vin
Vout
Rs
Vss
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Electronics 1
Week 8
14 / 14