Bài giảng Electronic circuits - Chapter 3: FET - Trịnh Xuân Dũng
Dr. Dung Trinh
HCMUT / 2017
Electronic Circuits
Chapter 3: FET
Dr. Dung Trinh
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Dr. Dung Trinh
HCMUT / 2017
Content
MOSFET
Current Voltage Characteristics
MOSFET Circuits at DC
Small Signal Operation and Models
Basic MOSFET Amplifier Configurations
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Dr. Dung Trinh
HCMUT / 2017
MOSFET
MOSFET: Metal-Oxide-Semiconductor Field-Effect Transistor.
According to the physics of the device, we can classify transistors into two main classes:
Field Effect Transistors (FET): Conduction is controlled by electric field which is
produced by voltage applied to the control terminals. So, the control draws no current
and FET is a voltage- controlled device.
Bipolar Junction Transistors (BJT): Diode-based device which is usually blocked unless
the control terminals are forward- biased. So, the control is a current, and BJT is a
current amplifier by nature.
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Dr. Dung Trinh
HCMUT / 2017
MOSFET
MOSFET is a four-terminal device: gate (G), source (S), drain (D) and body (B).
Two kinds of MOSFETs: n-channel (NMOS) and p-channel (PMOS) devices.
The device structure is basically symmetric in terms of drain and source.
Source and drain terminals are specified by the operation voltage.
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HCMUT / 2017
MOSFET
Gate voltage exceeds a threshold
voltage 풗푮푺 > 푽풕: electrons start
to accumulate on the substrate
surface. 퐕 = ퟎ. ퟑ ÷ ퟏ (V)
퐭
The positive 풗푮푺 > 푽풕 is used to
induce the channel and it is called
n-channel enhancement type
MOSFET.
The induced n region forms a channel
for current flow from drain to source.
The field controls the amount of charge in the channel and determines the
channel conductivity.
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HCMUT / 2017
MOSFET – Small 푣퐷푆
Small 풗푫푺 is applied: free electrons
travel from source to drain through the
induced n-channel.
The resulting current 푖퐷 flows from drain
to source (opposite to the direction of
the flow of negative charge).
The resulting current 푖퐷 flows from drain
to source (opposite to the direction of
the flow of negative charge).
The channel is controlled by the effective
voltage or overdrive voltage:
푽푶푽 = 푽푮푺 − 푽풕
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HCMUT / 2017
MOSFET – Increasing 푣퐷푆
As 푣퐷푆 is increased, the channel becomes more
tapered and its resistance increases correspondingly.
At the point 풗푫푺풔풂풕 = 풗푮푺 − 푽풕 , the channel is
pinched off at the drain side.
Triode region:
풗푫푺 < 풗푫푺풔풂풕
Saturation region: 풗푫푺 ≥ 풗푫푺풔풂풕
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Dr. Dung Trinh
HCMUT / 2017
MOSFET – IV Relationship
Triode region:
풊푫 = 풌풏 풗푮푺 − 푽풕 풗푫푺 − ퟐퟏ 풗ퟐ푫푺
Saturation region: 풊푫풔풂풕 = ퟏퟐ 풌풏 풗푮푺 − 푽풕
ퟐ
ퟏ
Channel resistance: 풓푫푺 = 풌 풗 −푽
풏
푮푺
풕
푾
푾
푳
풌풏 = 흁풏푪풐풙 = 풌′풏
푳
Transconductance parameter:
where:
푪풐풙: oxide capacitance per unit area.
흁풏: mobility of electron in the inversion layer.
푾 and 푳: channel width and length.
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HCMUT / 2017
MOSFET – Circuit Symbols
n-channel enhancement-mode MOSFET:
p-channel enhancement-mode MOSFET:
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NMOS – Current Voltage Characteristics
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MOSFET - Current Voltage Characteristics
Example 1: Consider an NMOS transistor fabricated in a L = 0.18휇푚 process with L =
0.18휇푚 and W = 2휇푚. The process technology is specified to have 퐶표푥 = 8.6 fF/μ푚2,
휇푛 = 450 푐푚2/푉푠 and 푉 = 0.5푉.
푡푛
a. Find 푉퐺푆 and 푉퐷푆 that result in the MOSFET operating at the edge of saturation with
퐼퐷 = 100휇퐴.
b. If 푉퐺푆 is kept constant, find 푉퐷푆 that results in 퐼퐷 = 50휇퐴.
c. To investigate the use of the MOSFET as a linear amplifier, let it be operating in
saturation with 푉퐷푆 = 0.3푉. Find the change in 푖퐷 resulting from 푉퐺푆 changing from
0.7푉 by +0.01푉 and by −0.01푉.
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MOSFET – Finite Output Resistance
In practice, increasing 푣퐷푆 beyond 푣푂푉 does
affect the channel somewhat.
This effect can be accounted for 푖퐷 in the
expression for by including a factor
ퟏ + 흀풗푫푺 :
ퟏ
ퟐ
풊푫풔풂풕 = 풌풏 풗푮푺 − 푽풕 ퟏ + 흀풗푫푺
ퟐ
Defining the output resistance 푟 as
표
흏풗푫푺
흏풊푫
푽푨
푰푫
풓풐 ≡
풗
=
푮푺=풄풐풏풔풕
ퟏ
푽푨 =
흀
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Dr. Dung Trinh
HCMUT / 2017
MOSFET Circuits at DC
DC analysis for MOSFET circuits:
• Assume the operation mode and solve the dc bias utilizing the corresponding
current equation.
• Verify the assumption with terminal voltages (cutoff, triode and saturation).
• If the solution is invalid, change the assumption of operation mode and
analyze again.
Example 2: The NMOS transistor in the following
circuit has 푉 = 0.7푉, 휇푛퐶표푥 = 100μA/푉2, 퐿 = 1휇푚
푡
and 푊 = 32휇푚 . Design the circuit so that the
transistor operates at 퐼퐷 = 0.4푚퐴 and 푉퐷 = 0.5푉.
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Dr. Dung Trinh
HCMUT / 2017
MOSFET Circuits at DC
Example 3: The NMOS transistor in the following circuit has 푉 = 0.6푉, 휇푛퐶표푥 =
푡
200μA/푉2, 퐿 = 0.8휇푚 and 푊 = 4휇푚. Design the circuit so that the transistor
operates at 퐼퐷 = 80퐴. Find the DC voltage 푉퐷.
Example 4: Design the circuit so that the transistor operates at 푉퐷 = 0.1푉. Let
푉 = 1푉, 푘푛 = 1mA/푉2. Find the effective resistance between drain and source at
푡
this operating point.
Example 3
Example 4
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MOSFET Circuits at DC
Example 5: Determine the voltage and the current of all nodes and branches? 푉푡 =
1푉, 푘푛 = 1푚퐴/푉2
.
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Dr. Dung Trinh
HCMUT / 2017
MOSFET Circuits at DC
Example 6: Design the following circuit so that the transistor operates in saturation
with 퐼퐷 = 0.5푚퐴 and 푉퐷 = 3푉. Let the enhancement-type PMOS transistor have
푊
푉 = −1푉 and 푘푝′ = 1푚퐴/푉2. Assume 휆 = 0. What is the largest value that 푅퐷
푡푝
퐿
can have while maintaining saturation-region operation?
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Dr. Dung Trinh
HCMUT / 2017
MOSFET Circuits at DC
Example 7: The NMOS and PMOS transistors in the following circuit are matched,
푊
푊
with 푘푛′ = 푘푝′ = 1푚퐴/푉2 and 푉 = −푉 = 1푉 . Assume 휆 = 0 for both
푡푛
푡푝
devices, f퐿ind the퐿drain currents 푖퐷푁 and 푖퐷푃 as well as the voltage 푣표 for 푣퐼 =
0푉, 2.5푉 푎푛푑 − 2.5푉.
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Dr. Dung Trinh
HCMUT / 2017
MOSFET in Amplifier Design
ퟏ
ퟐ
풊푫 = 풌풏 풗푮푺 − 푽풕
ퟐ
ퟐ풌풏푹푫푽푫푫 + ퟏ − ퟏ
풌풏푹푫
푽푮푺|푩 = 푽풕 +
풗푫푺 = 푽푫푫 − 풊푫푹푫
Input voltage increase drain
current increase output voltage
decrease.
ퟏ
ퟐ
풗푫푺 = 푽푫푫 − 풌풏푹푫 풗푮푺 − 푽풕
ퟐ
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Dr. Dung Trinh
HCMUT / 2017
MOSFET in Amplifier Design
DC operating point
ퟏ
ퟐ
풗푫푺 = 푽푫푫 − 풌풏푹푫 풗푮푺 − 푽풕
ퟐ
The signal to be amplified, 푣푔푠(푡), a function of time t, is superimposed on the bias
voltage. Thus the total instantaneous value of becomes:
풗푮푺 풕 = 푽푮푺 + 풗품풔(풕)
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Dr. Dung Trinh
HCMUT / 2017
MOSFET in Amplifier Design
The signal to be amplified, 푣푔푠(푡), a
function of time t, is superimposed
on the bias voltage. Thus the total
instantaneous value of becomes:
풗푮푺 풕 = 푽푮푺 + 풗품풔(풕)
훛풗푫푺
훛풗푮푺
푨풗 ≡
|
= −풌풏 푽푮푺 − 푽풕 푹푫
푮푺
풗 =푽
푮푺
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