Lecture10 MOS Transistor Circuit Analysis.ppt
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Transcript of Lecture10 MOS Transistor Circuit Analysis.ppt
Field Effect Field Effect Transistors Transistors
Circuit Circuit AnalysisAnalysis
EE314
HP PA8000
Fujitsu 68903
Fairchild Clipper C100
Chapter 12: Field Effect Transistors
1.MOS Small Signal Equivalent
2.Transconductance3.Common-Source
Amplifiers4.Source Follower5.Logic gates
FET Circuit Analysis
Small-Signal Equivalent Circuit for FETsOutput signal from an amplifier using FET can be effectively modulated by small changes of input signal current. In this way it is possible to make small changes from the Q point. Symbols:The total quantities: iD(t), vGS(t)The dc point values: IDQ, VGSQ
The signal id(t), vgs(t) Characteristic
)()( tiIti dDQD
)()( tvVtv gsGSQGS
Small-Signal Equivalent Circuit -Transconductance
Schematic
20tGSD VvKi
20)()( tgsGSQdDQ VtvVKtiI
)(2)( 20
20 tKvtvVVKVVKtiI gsgstGSQtGSQdDQ
20tGSQDQ VVKI
0)( tGSQgs VVtv
Analysis… (a little bit of math)
We know that
Also we assume that
*
Small-Signal Equivalent Circuit -Transconductance
Schematic
)(2)( 20
20 tKvtvVVKVVKtiI gsgstGSQtGSQdDQ
20tGSQDQ VVKI
0)( tGSQgs VVtv
)(2)( 0 tvVVKti gstGSQd
We know that
Also we assume that
Drain current generated by signal
*
Small-Signal Equivalent Circuit -Transconductance
)(
)(
tv
tig
gs
dm
We define the transconductance as
or
so
)()( tvgti gsmd
02 tGSQm VVKg
Small-Signal Equivalent Circuit -Transconductance
20tGSD VvKi
Thus the transconductance
K
IVv DQtGS 0
so
DQtGSQm KIVVKg 22 0
Small-Signal Equivalent Circuit -Transconductance
The transistor has KP=50A/V2, Vto=2V, L=10m, and W=400m
Exercise
mSVVKg tGSQm 4)24(22 0
2/12
VmAKP
L
WK
Small-Signal Equivalent Circuit
0)( tigAlso we assume that
DQm KIg 2
Better performance is obtained with higher values of gm.Please notice that gm is proportional to the square root of the Q point drain current. Simply, we can increase gm by choosing a higher value of IDQ.
2
KP
L
WK
More Complex Equivalent Circuits
For more accurate analyses of FET transistor we have to add more components to an equivalent circuit.Small capacitance: for high response FET amplifiersDrain resistor: account for the effect of vDS on the drain current
ddsgsmd rvtvgti /)()(
Correction for id
Please read section: Transconductance and … pp.591Example 12.3
Drain Resistance Calculation
mSmS
V
mA
v
i
r DS
D
d
175.04
7.0
610
107.101
krd 7.5
so at vGS=4V
DS
D
d v
i
r
1
Common-Source Amplifier
Schematic
Equivalent circuit
GD
S
The dc supply voltage acts as a short circuit for the ac current.
Common-Source Amplifier
Schematic
Equivalent circuit
ac signal
Cs -bypass C
C1, C2 -coupling C
GD
S
The dc supply voltage acts as a short circuit for the ac current.
Common-Source Amplifier: Gain, Rin and Rout
Equivalent circuit (once more)
Voltage gain
LDdL RRrR
/1/1/1
1'
'0 Lgsm Rvgv gsin vv
'0Lm
inv Rgv
vA
21 RRRi
vR G
in
inin
Input resistance
From bias point analysis
Common-Source Amplifier: Gain, Rin and Rout
To find out the Rout we have to: disconnect the load, replace the signal source by short circuit – Thevenin equivalent resistance
No source connected to the input
if vgs=0 then gmvgs=0
dDout rRR
/1/1
1
Output resistance
Example 12.4
Source Follower
Small-Signal Equivalent Circuit –Source Follower
Notice that small signal IDS goes up. Why?
Voltage gain
LSdL RRrR
/1/1/1
1'
'0 Lgsm Rvgv '1 Lmgsogsin Rgvvvv
11 '
'0
Lm
Lm
inv Rg
Rg
v
vA
21 RRRi
vR G
in
inin
Input resistance
Small-Signal Equivalent Circuit –Source Follower
Since the output voltage is almost equal to the input – hence the name source follower
Logic gates – COMS Inverter
Vin V out
0 1
1 0
Logic truth table
Switch level equivalentcircuits
Logic gates – COMS NAND gate
A B V out
0 0 1
0 1 1
1 0 1
1 1 0
Logic truth table
Logic gates – COMS NOR gate
Exercise
Draw switch level circuits for different inputs and derive the truth table for this gate
A B V out
0 0 1
0 1 0
1 0 0
1 1 0
Logic truth table
Logic gates – COMS NOR gate