Download Self-Bias Configuration

SMALL SIGNAL FET (Field–
Effect Transistors) AMPLIFIER
1.
2.
3.
4.
5.
6.
7.
Introduction/Basic
FET Small-Signal Model
Fixed-Bias Configuration
Self-Bias Configuration
Voltage-Divider Configuration
Common-Drain Configuration
Common-Gate Configuration
1. INTRODUCTION /
BASIC
Field–Effect Transistors (FET)
FET’s (Field – Effect Transistors) are much like BJT’s (Bipolar Junction Transistors). FET is
usually called a unipolar transistor being that the current carrier is wholly of a single type
either electron or holes only. While the bipolar transistor is a dual current carrier system
having both electrons and holes together.
Similarities:
• Amplifiers
• Switching devices
• Impedance matching circuits
Differences:
• FET’s are voltage controlled devices whereas BJT’s are current controlled
devices.
• FET’s also have a higher input impedance, but BJT’s have higher gains.
• FET’s are less sensitive to temperature variations and because of there
construction they are more easily integrated on IC’s.
• FET’s are also generally more static sensitive than BJT’s.
FET Types
•
JFET ~ Junction Field-Effect Transistor
• MOSFET ~ Metal-Oxide Field-Effect Transistor
- D-MOSFET ~ Depletion MOSFET
- E-MOSFET ~ Enhancement MOSFET
Bipolar Junction Transistors
Transistor Construction
There are two types of transistors: pnp and npn-type.
Note: the labeling of the transistor:
E - Emitter
B - Base
C - Collector
JFET Construction
There are two types of JFET’s: n-channel and p-channel.
The n-channel is more widely used.
n
There are three terminals: Drain (D) and Source (S)
Gate (G)
Basic Operation of JFET
JFET operation can be compared to a water spigot:
The source of water pressure – accumulated electrons at the negative pole of the applied
voltage from Drain to Source
The drain of water – electron deficiency (or holes) at the positive pole of the applied
voltage from Drain to Source.
The control of flow of water – Gate voltage that controls the width of the n-channel,
which in turn controls the flow of electrons in the
n-channel from source to drain.
JFET Operating Characteristics
There are three basic operating conditions for a JFET:
A.
VGS = 0, VDS increasing to some positive value
B.
VGS < 0, VDS at some positive value
C.
Voltage-Controlled Resistor
JFET Symbols
n-channel
p-channel
Common JFET Biasing Circuits
General Relationships
The general relationships that can be applied to dc analysis of all FET amplifiers are:
IG  0A
ID  IS
Shockley’s equation is applied to relate the input and output quantities:
Pinch-off
ID  IDSS(1
VGS 2
)
VP
Fixed-Bias Configuration
Fixed-bias configuration
VGS = - VGG
VDS = VDD – IDRD
VS = 0
VD = VDS
VG = VGS
Network for dc analysis
Plotting the Transfer Curve
From Shockley’s equation, IDSS and Vp (VGS(off)), the Transfer Curve can be plotted using
these 3 steps:
Step 1:
ID  IDSS(1
Solving for VGS = 0V:
ID  IDSS
Step 2:
VGS  0V
ID  IDSS(1 
Solving for VGS = Vp (VGS(off)):
ID  0 A
VGS 2
)
VP
VGS 2
)
VP
VGS  VP
Step 3:
Solving for VGS = 0V to Vp:
ID  IDSS(1
VGS 2
)
VP
Fixed-Bias Transfer Curve
Finding the solution for the fixed-bias
configuration
Plotting shockley’s equation
Shorthand Method
VGS versus ID using Shockley’s equation
VGS
ID
0
IDSS
0.3 VP
IDSS/2
0.5 VP
IDSS/4
VP
0 mA
ID  IDSS(1 
VGS
VGS 2
)
VP

ID
 VP 1 
I DSS





Example 1: Determine : VGSQ, IDQ, VDS, VD, VG and VS
Graphical solution for the network
Self-Bias Configuration
JFET self-bias configuration
DC analysis of the self-bias configuration
Self-Bias Calculations
For the indicated loop:
VGS   I D RS
• To solve this equation select an ID < IDSS and use the component value for
RS.
• Plot this point: ID and VGS and draw a line from the origin of the axis to this
point.
• Next plot the transfer curve using IDSS and VP (VP = VGSoff in specification
sheets) and a few points such as ID = IDSS/4 and ID = IDSS/2 etc.
Where the first line intersects the transfer curve is the Q-point.
Use the value of ID at the Q-point (IDQ) to solve for the other voltages:
VDS  VDD  I D ( RS  RD )
VS  I D RS
VG  0V
VD  VDS  VS  VDD  VRD
Self-Bias Transfer Curve
Defining a point on the self-bias line.
Self-Bias Transfer Curve
Sketching the self-bias line.
Example 6.2
Sketching the self-bias line for the network
Sketching the device characteristics for the JFET
Determining the Q-point for the network
Determining the quiescent point of operation for the network of Example 2.
Example 3
Example 4
Sketching the dc equivalent of the network
Voltage-Divider Bias
IG = 0A in FETs. Unlike BJTs, where IB affected IC; in FETs it is VGS that controls ID.
Voltage-Divider Bias Calculations
VG 
Using Kirchoff’s Law:
Rearranging and using ID =IS:
R2VDD
R1  R2
VG VGS VRS  0
VGS  VG  I D RS
Again the Q point needs to be established by plotting a line that intersects the transfer
curve.
Voltage-Divider Q-point
1.
2.
3.
Plot the line: By plotting two points: VGS = VG, ID =0 and VGS = 0, ID =
VG/RS
Plot the transfer curve by plotting IDSS, VP and calculated values of ID.
Where the line intersects the transfer curve is the Q point for the circuit.
Voltage-Divider Bias Calculations
Using the value of ID at the Q-point, solve for the other variables in the Voltage-Divider
Bias circuit:
VDS  VDD  I D ( RD  RS )
VD  VDD  I D RD
VS  I D RS
VDD
IR1  IR2 
R1  R2
Example 5
Determining the Q-point for the network
Example 6
Determining the network equation for the configuration
Determining the Q-point for the network
Summary Table
P-Channel FETs
For p-channel FETs the same calculations and graphs are used, except that the voltage
polarities and current directions are the opposite. The graphs will be mirrors of the nchannel graphs.
Practical Applications
• Voltage-Controlled Resistor
• JFET Voltmeter
• Timer Network
• Fiber Optic Circuitry