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```					EE 359 Electronic Circuits

Field Effect Transistors (FET)

Examples
Frequency Response

Sep. 21, 2007                           0
EE 359 Electronic Circuits

Homework
• Chapter 4, FET Transistors due Sep. 24
– 4.12,4.46,4.75,4.96
– Quiz 1 Sep 28 (next Friday)
– Quiz 2   Oct 30
Project abstracts due October 9 (Tuesday following
Monday schedule)
1 page long, your name, project title, a paragraph
summary of problem you want to analyze

Sep. 21, 2007                                             1
EE 359 Electronic Circuits
Review of FET operation

• In general, we will use FET in saturation
region.    0 V V VGS   t    GD

• Design ckt,
ID=0.4mA VD=1V, Vt=2V
 nCox  20 A / V 2 , L  10 m, W  400 m

Sep. 21, 2007                                    2
EE 359 Electronic Circuits
DC Analysis

• VD=1 V , saturation region

1         W
I D   n Cox (VGS  Vt ) 2
2          L
3 400
0.4  0.5 x 20 x10 x     (VGS  Vt ) 2
10
 VGS  2  1  VGS  3V

Sep. 21, 2007                              3
EE 359 Electronic Circuits
Biasing Issues
VS  VSS
• VGS=3V, VGS>VT          RS 
ID
 3   5
RS 
0.4
RS  5k
VDD  VD
• To establish +1V at drain           RD 
ID
5  1
RS 
0.4
Sep. 21, 2007
RS  10k       4
EE 359 Electronic Circuits
Biasing (PMOS)

• Design ckt st. +9.9V at the
source. Effective resistance
ro ? (Note PMOS)
• VGS=0, VGD=-0.1V < |Vt|
• Triode region operation:
                       
I D  10  (1) x0.1  x0.01  0.1mA
1
                 2     

Sep. 21, 2007                                     5
EE 359 Electronic Circuits
Output resistance
9.9V
• Select:     RD 
0.1mA
 99 k  100 k

• Find rDS        rDS 
VDS

0.1V
 1k
I D 0.1mA

Sep. 21, 2007                                   6
EE 359 Electronic Circuits
Amplifier Circuit

• Input signal vi
• Output signal vo
DC bias currents
1
ID    x0.25(VGS  1.5) 2
2
VD  15  RD I D  15  10 I D
I D  1.06mA
VD  4.4V

Sep. 21, 2007                            7
EE 359 Electronic Circuits
Small Signal Parameters

• Find conductance
W
g m  k 'n   (VGS  Vt )
L
g m  0.25 (4.4  1.5)  0.725 mA / V

• Output resistance
VA   50
ro             47 k
I D 1.06

Sep. 21, 2007                                      8
EE 359 Electronic Circuits
Small Signal Gain

• Draw small signal equiv.
• RG very large > 10M, neglect
• Calculate gain: v   g v ( R || R
o            m   gs   D     L   || ro )
vo
  g m ( RD || RL || ro )  3.3
vi
• Input resistance:
i1  (vi  vo ) / RG

i1 
vi
1  (3.3)  4.3 vi
RG                    RG
Rin  RG / 4.3  10M / 4.3  2.2 M

Sep. 21, 2007                                                           9
EE 359 Electronic Circuits
Frequency response
Internal and external
capacitances
High-frequency equivalent
circuit model for the
MOSFET (a)

High-frequency equivalent
circuit model for the
MOSFET when
Source is connected to Body
(b)
Sep. 21, 2007                        10
EE 359 Electronic Circuits
Simplified Capacitance Model

• Notice we omit
capacitance to
Body!

Sep. 21, 2007                                   11
EE 359 Electronic Circuits
MOSFET High Freq. Model
W              W        I
g m   nCox     VOV  2 nCox I D  2 D
L              L       VOV
2
C gs  WLCox  WLovCox                 C gd  WL ov Cox
3

Sep. 21, 2007                                                     12
EE 359 Electronic Circuits
Freq.
Response CS

• AM = ?
•BW=?

Sep. 21, 2007                13
EE 359 Electronic Circuits
Small-signal equivalent circuit
•    (a) equivalent
circuit;

• (b) the circuit
of (a)
simplified at
the input and
the output;

Sep. 21, 2007                            14
EE 359 Electronic Circuits
Equivalent circuit
• In the simplified
model, we combine
the capacitances to
ground.

Sep. 21, 2007                         15
EE 359 Electronic Circuits
Low Frequency Resp.

Sep. 21, 2007                16
EE 359 Electronic Circuits
Pspice Simulation

Sep. 21, 2007                       17
EE 359 Electronic Circuits
Frequency Respons

Sep. 21, 2007                       18

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