Lecture 16
MOSFET (cont’d)
MOSFET 1-1 Sunday 3/12/2017
Outline Continue Enhancement-type MOSFET Characteristics DC Biasing Circuits and Examples
Introduction to BJT-FET Combination Circuits Combination of BJT and FET devices in a circuit
MOSFET 1-2
Enhancement-Type MOSFET (Quick Review)
MOSFET 1-3
MOSFET is also known as insulated gate FET (IGFET)
Enhancement-Type MOSFET Symbol
MOSFET 1-4
n-channel E-Type MOSFET
p-channel E-Type MOSFET
Enhancement-Type MOSFET Characteristic
MOSFET 1-5
TGSTGSD VVVVkI for 2
2)(
)(
TonGS
onD
VV
Ik
Enhancement-Type MOSFET Characteristic
MOSFET 1-6
2
22
)(
)(mA/V 278.0
28
10
m
VV
Ik
TonGS
onD
22278.0 GSD VmI
Enhancement-Type MOSFET Transfer Curve
MOSFET 1-7
Plotting all the VGS(on) from the characteristic curve, the transfer curve can be obtained:
22278.0 GSD VmI
Enhancement-Type MOSFET Important Relationships
MOSFET 1-8
2
T)on(GS
)on(D
TGS
2
TGSD
SD
G
VV
Ik
VVfor VVkI
II
0I
Enhancement-Type MOSFET DC Biasing
Fixed-bias, self-bias and many more bias configuration can be applied to enhancement-type MOSFET
Two most popular MOSFET biasing configurations Feedback-bias configuration
Voltage-divider bias configuration
MOSFET 1-9
Feedback-Bias Configuration
MOSFET 1-10
As the situation IG = 0 still applied, the resistor RG will be ignored resulting in the drain and gate terminal to have the same voltage (VG = VD)
Example (1)
Determine VGSQ and IDQ
for the E-Type MOSFET shown in the figure
MOSFET 1-11
Example (1) – Solution
MOSFET 1-12
For the enhancement-type MOSFET’s equation, the value of k have to be obtained first:
2
2 2
T)on(GS
)on(DmA/V 24.0
38
m6
VV
Ik
the ID equation for the device:
3for 324.02
GSGSD VVmI
12 2G D DV V I
Since IG equals zero, then
Inserting the VGS equation into the device equation:
12 2 0 12 2GS G S D DV V V I I
Example (1) – Solution Substituting the ID equation for the MOSFET
MOSFET 1-13
2 2 2
2 2
2
0.24 3 0.24 12 2 3 0.24 9 2
0.24 81 36 4 19.44 8.64 960
960 9.64 19.44 0
D GS D D
D D D D
D D
I m V m I m I
m I I m I I
I I m
Solving the equation, we get:
Enhancement-type MOSFET doesn’t have limitation for saturation current (IDSS), the true value of ID is the smaller one
mA 2.79 andmA 25.7
)960(2
)44.19)(960(4)64.9(64.9
2
422
m
a
acbbID
2.79 mA 12 2 6.42 VQD GS DI and V I
(since VGS should be greater than VT)
Example (1) – Solution
MOSFET 1-14
For graphical approach, several plot points have to be obtained first:
3for 324.02
GSGSD VVmI
VGS ID
3 V 0 mA
4 V 0.24 mA
5 V 0.96 mA
6 V 2.16 mA
7 V 3.84 mA
8 V 6 mA
For the bias line, only two plot points are required:
12 2GS DV I
VGS ID
12 V 0 mA
0 V 6 mA
Example (1) – Solution
MOSFET 1-15
Plots all the device transfer curve and device representation points:
VT
Voltage-Divider Bias Configuration
MOSFET 1-16
Basically, the configuration is the same as in depletion-type MOSFET, JFET or BJT except the change of device to the enhancement-type MOSFET
All the calculation would be the same except for the transfer curve of enhancement-type MOSFET is different from those depletion-type MOSFET and JFET
Determine IDQ and VGSQ
Example (2)
MOSFET 1-17
Example (2) – Solution
MOSFET 1-18
Determining VG:
For VS:
So, for VGS:
V 18M22M18
M18 *40VG
DS IV 31082.0
DSGGS IVVV 31082.018
2
22
)(
)(mA/V 12.0
510
3
m
VV
Ik
TonGS
onD
Inserting the circuit representation equation into the device equation:
5for 51012.023
GSGSD VVI
2 23 3
2 3
0.12 10 5 0.12 10 18 0.82 5
80.69 3.56 20.28 10 0
D GS D
D D
I V I
I I
Example (2) – Solution
MOSFET 1-19
Solving the equation, we get:
mA 6.72 andmA 4.37
)69.80(2
)m28.20)(69.80(4)56.3(56.3
a2
ac4bbI
22
D
3
18 0.82
For 6.72 mA, 18 0.82 10 (6.72 ) 12.49 V
GS D
D GS
V I
I V m
We take the smaller value: mA 6.72DI
V 49.12
mA 72.6
Q
Q
GS
D
V
I
The Q-point for MOSFET is defined by
Example (2) – Solution
MOSFET 1-20
For graphical approach, several plot points have to be obtained first:
For the bias line, only two plot points are required:
VGS ID
5 V 0 mA
10 V 3 mA
15 V 12 mA
20 V 27 mA
25 V 48 mA
30 V 75 mA
VGS ID
18 V 0 mA
0 V 21.95 mA
5for 512.02
GSGSD VVmI DGS kIV 82.018
Example (2) – Solution
MOSFET 1-21
Plots all the device transfer curve and device representation points:
p-Channel Enhancement-Type MOSFET
MOSFET 1-22
It is the complement to n-channel enhancement-type MOSFET
All the current flow will be in the opposite direction
Although the current direction is reverse, however the current equation are still the same (just like in JFET and depletion-type MOSFET)
Construction Transfer Curve Characteristics
Comparisons between MOSFETs and BJTs
FET Small AC Signal Model 1-23
MOSFETs BJTs
Pros Cons
High input impedance Low input impedance
Minimal drive power, no DC current required at gate
Large drive power, continuous DC current required at base
Simple drive circuits Complex drive circuits as large +ve and –ve currents are involved
Devices can be easily paralleled Devices cannot be easily paralleled
Max. operating temp. ~ 200 oC , less temp. sensitive
Max. operating temp. ~ 150 oC , more sensitive to temp
Very low switching losses Medium to high switching losses (depends on trade-off with conduction losses)
High switching speed Lower switching speed
Cons Pros
High on-resistance Low on-resistance
Low transconductance High transconductance
BJT-FET Combination Circuits
Combination of BJT and FET device in a circuit Innovative circuits that take some advantages of FETs,
such as the high-input-impedance and low input power operation, and some merits of BJTs, such as high output current-driving capability
How to analyze such circuits Firstly, recognize both of the devices and their current
flows
To make the calculation simple and easier to view, transform the circuit into the equivalent form to avoid complexity
List down all the important relationships that involve for both of the devices
Start with approaching the device that is closer to the ground (bottom device)
MOSFET 1-24
Example (3)
MOSFET 1-25
Determine VD and VC
Example (3) – Solution
MOSFET 1-26
We know that for the JFET device, IG = 0 making the resistor RG = 1 MΩ useless and can be remove from the circuit
By analyzing the circuit, we notice that the configuration is a voltage-divider bias for both the JFET and BJT device
Due to involvement of BJT, we have to check βRE ≥ 10R2 to use the approximate analysis
As for βRE = (180)(1.6k) = 288k and 10R2 = 10(24k) = 240k, situation βRE ≥ 10R2 is satisfied and we can use approximate analysis for this configuration
Obtaining the ETH:
V 62.38224
24*16ETH
Example (3) – Solution
MOSFET 1-27
ETH = 3.62 V
Transforming the circuit into its equivalent form:
ETH = 3.62 V
Example (3) – Solution
MOSFET 1-28
By approaching BJT (bottom device) first, we know VBE = 0.7 active operating mode
From earlier calculation, we got ETH = VB = 3.62 V
Obtaining VE:
Obtaining IB from VBE = 0.7:
1
181 1.6 289.6
E E E B E
B B
V I R I R
I I
A 08.10I
kI6.28962.37.0
7.0VVV
B
B
EBBE
Example (3) – Solution
MOSFET 1-29
From the circuit, IB is not really important but IC is very important because
IC = IS = ID
As for that, obtain IC:
Knowing the value of ID, VD can be obtained:
mA 81.1)08.10)(180( BC II
V 11.11107.216 3 DD IV
ETH = 3.62 V
Example (3) – Solution
MOSFET 1-30
From the configuration, we notice that VS = VC
By obtaining VGS for the JFET, the value of VS can be achieved:
V 29.7
6
62.311281.1
1
62.362.3
2
2
C
C
P
GSDSSD
CSSGGS
V
Vmm
V
VII
VVVVV
ETH = 3.62 V
Conclusion: FET Advantages
FETs provide:
Excellent voltage gain
High input impedance
Low-power consumption
Good frequency range
MOSFET 1-31
MOSFET 1-32
Lecture Summary
Covered material
Continue Enhancement-type MOSFET Characteristics Biasing Circuits and Examples
Introduction to BJT-FET Combination Circuits
Material to be covered next lecture
Small AC Signal Analysis for FETs