Transcript Chapter 6-JFET

Acknowledged to:
Shahrul Ashikin Azmi (PPKSE)
Objectives
 Explain the operation and characteristics of junction
field effect transistors (JFET).
 Understand JFET parameters
 Discuss and analyze how JFETs are biased
 Explain the operation and characteristics of
metal oxide semiconductor field effect transistors
(MOSFET)
 Discuss and analyze how MOSFET are biased
 Troubleshoot FET circuits.
Introduction
• FET – a three-terminal voltage-controlled
device used in amplification and switching
application.
• Field effect transistors controls current by
voltage applied to the gate. The FET’s major
advantage over the BJT is high input resistance.
• 2 basic type of FET: JFET and MOSFET
JFET
The junction field effect transistor, like a BJT, controls current
flow. The difference is the way this is accomplished. The
JFET uses voltage to control the current flow. As you
will recall the transistor uses current flow through the baseemitter junction to control current. JFETs can be used as an
amplifier just like the BJT.
VGG voltage levels control current flow in theVDD, RD circuit.
JFET
• The terminals of a JFET are the source, gate, and
drain.
• A JFET can be either p channel or n channel.
JFET
• VDD provide a drain-to-source voltage.
• VGG sets the reverse-bias voltage between gate and source.
JFET is always operated with gate-source pn junction reversebiased. Reverse-biasing of the gate-source junction with a –ve
gate voltage produces a depletion region along pn junction.
JFET Biasing
 Gate-to-source junction of JFET always reverse-
biased under normal condition.
 Gate-to-source junction never allowed to become
forward-biased because the gate material is not
designed to handle any significant amount of current
 may destroy the component.
 The fact gate is always reverse-biased leads to
important feature  JFET has high gate input
impedance; typically in high megaohm range.
 This feature result to JFET extensively being used in
integrated circuits. Low current draw helps IC remain
cool, thus allowing more components to be placed in
a smaller physical area.
JFET Characteristics and Parameters
Let’s first take a look at the effects with a VGS of 0V. This is
produced by shorting the gate to source junction.
JFET Drain Curve
 Refer to JFET drain curve from point A to B, ID increases
proportionally with increases of VDD (VDS increases as
VDD increases). This is called the ohmic region (point A
to B) because VDS and ID are related by Ohm’s Law.
 At point B, the curve levels off and ID becomes
constant. The point when ID ceases to increase
regardless of VDD increases is called the pinch-off
voltage, VP (point B). This current is called maximum
drain current (IDSS) and always specified for the
condition, VGS=0V. This area is called constant-current
area.
 Breakdown (point C) occur when ID begins to increase
rapidly with any increase in VDS. This of course
undesirable, so JFETs operation is always well below this
value.
JFET Characteristics and Parameters
JFET Characteristics and Parameters
From this set of curves you can see with increased voltage
applied to the gate, ID decrease and JFET reaches pinch-off at
values of VDS less than VP.
JFET Characteristics and Parameters
JFET Characteristics and Parameters
• We know that as VGS is increased ID will decrease. The point
that ID ceases increase is called cutoff. The amount of VGS
required to do this is called the cutoff voltage (VGS(off ) ).
• The more negative VGS, the smaller ID becomes. When VGS
has sufficiently large negative value, ID is reduced to zero.
It is interesting to note that
pinch-off voltage (Vp) and cutoff
voltage (VGS(off)) are both the
same value only opposite
polarity.
JFET Transfer Characteristic
 For n-channel JFET, VGS(off) is negative and for p




channel, VGS(off) is positive.
Bottom end of the curve is at a point on VGS axis equal
to VGS(off) and the top end of the curve is at a point on
ID axis equal to IDSS (shorted-gate drain current rating
of the device).
The operating limits of JFET are:
ID=0
when VGS=VGS(off)
ID=IDSS
when VGS = 0
Transfer characteristic curve can be developed from
drain characteristic curves by plotting values of ID for
the values of VGS taken from the family of drain curves
at pinch-off.
JFET Characteristics and Parameters
The transfer characteristic curve illustrates the control VGS has on
ID from cutoff (VGS(off) ) to pinchoff (VP). Note the parabolic shape.
The formula below can be used to determine drain current. All
these values are usually available from data sheet.
ID = IDSS(1 - VGS/VGS(off))2
JFET Characteristics and Parameters
Forward transfer conductance, gm of JFETs is sometimes
considered. It is the changes in ID based on changes in VGS with
VDS is constant.
Input resistance for a JFET is high since the gate -source
junction is reverse biased, however the capacitive effects can
offset this advantage particularly at high frequencies.
The value is larger at the top of the curve (near VGS=0)
but become smaller as you increase VGS (near VGS(off)).
Transconductance
Transconductance
Forward transfer conductance referred to as
gm = ∆ID /∆VGS.
At VGS =0, the parameter is known as minimum
transfer conductance, gmo and can be calculated using
this equation:
gmo = 2IDSS/|VGS(off)| and gm = gmo(1 - VGS/VGS(off))
gmo can be read from the datasheet
as gfs or yfs and sometimes written
as Forward Transfer Admittance.
JFET Input Resistance
Since JFET is reverse-biased for operation, its input
resistance becomes so large. This is an advantage of using
JFET. Looking at the datasheet, you may calculate the
resistance value by using the Gate Reverse Current IGSS.
This internal input resistance can be calculated at different
VGS :
RIN=|VGS/IGSS|
As IGSS increases with temperature, RIN will decrease.
JFET Input Resistance
Example 1:
Calculate RIN if IGSS=-2nA and VGS=-20V
Solution:
RIN=|VGS/IGSS|=|-20/-2n|=10G
JFET Biasing Circuit
• Just as we learned that the bi-polar junction transistor
must be biased for proper operation, the JFET also must be
biased for operation. Let’s look at some of the methods for
biasing JFETs. In most cases the ideal Q-point will be the
middle of the transfer characteristic curve which is about
half of the IDSS.
• 4 types of bias method are self-bias, gate-bias, voltagedivider bias and current-source bias.
JFET Biasing- Self bias
• Self-bias is the most common type of biasing method for
JFETs. Notice there is no voltage applied to the gate, VG=0V.
However, the voltage from gate to source (VGS) will be negative
for n channel and positive for p channel to keep the junction
reverse biased.
• Uses a source resistor to help reverse biase JFET gate. The gate
is returned to ground via RG, and RS has been added to source
circuit.
• This voltage can be determined using the formulas below. ID =
IS for all JFET circuits. VG=0 and VS=IDRS.
VGS = VG - VS
(n channel) VGS = 0-IDRS =-IDRS
(p channel) VGS = 0-(-IDRS )=IDRS
JFET Biasing- Self bias
JFET Biasing – self bias
 Keep in mind that analysis of p-channel is the same as n-
channel except for opposite polarity voltages.
 The drain voltage with respect to ground is:
VD = VDD – IDRD
 Since VS = IDRS, VDS is:
VDS = VD – VS = VDD – ID(RD+RS)
JFET Biasing-self bias
• Setting the Q-point requires us to determine
a value of RS that will give us the desired ID
and VGS. The formula below shows the
relationship.
RS = | VGS/ID |
• For a desired value of VGS, ID can be
determined from the either the transfer
characteristic curve or more practically from
the formula below. The data sheet provides
the IDSS and VGS(off).
ID = IDSS(1 - VGS/VGS(off))2
JFET Biasing-self bias
Midpoint biasing- desirable to bias a JFET near the
midpoint of its transfer characteristic curve where ID =IDSS /
2. ID is approximately one-half of IDSS when:
VGS  VGS(off)/3.4
2
2




V
/
3
.
4
V
GS
(
off
)
  0.5I DSS
I D  I DSS 1  GS   I DSS 1 
 V



V
GS ( off ) 
GS ( off )



JFET Biasing-self-bias
The value of RS needed to establish VGS can be determined
by the relationship below.
RS = | VGS/ID |
To set the drain voltage at midpoint (VD=VDD/2), select a
value of RD to produce the desired voltage drop.
The value of RD needed can be determined by taking half of
VDD and dividing it by ID.
RD = (VDD/2)/ID
JFET Biasing- self-bias
Remember the purpose of biasing is
to set a dc operating point (Q-point).
In a self-biasing type JFET circuit, the
Q-point is determined by the given
parameters of the JFET itself and
values of RS and RD. Setting it at
midpoint on the drain curve is most
common.
One thing not mentioned in the
discussion is RG. It’s value is arbitrary
large to prevent loading on the driving
stage in a cascaded amplifier
arrangement.
JFET Biasing-graphical
The transfer characteristic curve along with other parameters
can be used to determine the mid-point bias Q-point of a selfbiased JFET circuit.
First, establish dc load line by calculating VGS.
VGS = -IDRS for ID=0 and ID=IDSS
With 2 points (ID=0 and ID=IDSS), draw dc load line on the
transfer characteristic curve.
The point where the two lines intersect gives us the ID and VGS
(Q-point) needed for mid-point bias. Note that load line
extends from VGS(off)(ID= 0A) to VP(ID = IDSS)
JFET Biasing
JFET Biasing- gate-bias
JFET Biasing – gate-bias
 Gate supply voltage (-VGG) is used to ensure gate-to-




source junction is reverse-biased. Since there is no
gate current, there is no voltage dropped across RG.
So, VGS = -VGG.
RG  to prevent input signal from being shorted to
gate supply through low reactance of input coupling
capacitor.
To find ID, use ID = IDSS(1 - VGS/VGS(off))2
VDS= VDD – IDRD
Diadvantage: Gate bias does not provide a stable Qpoint value of ID from one JFET to another.
JFET Biasing- voltage divider bias
• Voltage-divider bias can also be used
to bias a JFET. R1 and R2 are used to keep
the gate-source junction in reverse bias.
Operation is no different from self-bias.
Determining VGS for a JFET voltage-divider
circuit with givenVD can be calculated with
the formulas below.
Source voltage,VS = IDRS
Gate voltage, VG =(R2/R1+R2)VDD
Gate-to-source voltage.VGS=VG –VS
Source voltage, VS = VG - VGS
JFET Biasing- voltage-divider bias
 VS must be more +ve than VG in order to keep VGS
reverse-biased (-ve value).
Drain current, ID = (VDD – VD)/RD or
Since ID=IS, then ID=VS/RS
JFET Biasing-graphical
• In using the transfer characteristic curve to determine the
approximate Q-point we must establish the two points for
the load line.
• The first point is ID = 0 and VGS =VG.
VS=IDRS=(0)RS=0V
VGS=VG-VS=VG-0=VG
For VGS=0,
ID=(VG-VGS) / RS = VG / RS
• The second point is ID=VG / RS and VGS=0.
• The point at which the load line intersect with transfer characteristic
curve is Q-point.
Dc load line for JFET with voltage-divider bias
JFET Biasing – Current Source Bias
 Current source bias  provides high Q-point stability by




making value of ID independently of JFET.
From figure, JFET drain current equals BJT collector
current. IDQ = IC
The value of IC is less than the lowest value of IDSS for
JFET. Assume JFET in the figure has a value of IDSS=512mA, so as long as IC < 5mA, value of IDQ is
independently of JFET itself.
Advantage: provide the most stable Q-point value of ID.
Disadvantage: circuit complexity makes it undesirable
for most applications.
JFET Biasing- Current source bias
JFET Biasing
Transfer characteristics can vary for JFETs of the
same type. This would adversely affect the Qpoint for self-bias analysis. Q-point is much more
stable using voltage-divider bias and current
source bias.
MOSFET
The metal oxide semiconductor field effect transistor (MOSFET) is the
second category of FETs. The difference is that there no pn junction
structure instead gate of MOSFET is insulated from the channel by silicon
dioxide layer. MOSFETs are static sensitive devices and must be handled
by appropriate means.
There are depletion MOSFETs (D-MOSFET) and enhancement MOSFETs
(E-MOSFET). Note the difference in construction. The E-MOSFET has no
structural channel.
D-MOSFET
The D-MOSFET can be
operated in depletion or
enhancement modes. To be
operated in depletion mode,
a negative gate-to-source
voltage is applied. With
negative gate voltage,
negative charges on the
gate repel electrons from
channel, leaving +ve ions in
their place. N-channel is
depleted of some electron,
thus decreasing channel
conductivity.
D-MOSFET
To be operated in the
enhancement mode the
gate-to-source is made
more positive, attracting
more electrons into the
channel for better current
flow and thus enhancing
the channel conductivity.
Remember we are using n
channel MOSFETs for
discussion purposes. For p
channel MOSFETs,
polarities would change.
E-MOSFET
The E-MOSFET or
enhancement MOSFET can
operate in only the
enhancement mode. With a
positive voltage above a
threshold value on the
gate, an induced channel of
thin layer of –ve charges is
created.
The conductivity of channel
is enhanced by increase
VGS and thus pulling more
electrons into channel area.
POWER MOSFET
The lateral double diffused MOSFET (LDMOSFET) and the Vgroove MOSFET (VMOSFET) are specifically designed for high
power applications.
LDMOSFET
VMOSFET
POWER MOSFET
 Dual gate MOSFETs have two gates which
helps control unwanted capacitive effects at
high frequencies.
MOSFET Characteristics and Parameters
Since most of the characteristics and parameters
of MOSFETs are the same as JFETs we will cover
only the key differences.
D-MOSFET Characteristics and Parameters
The D-MOSFET operate in either +ve or –ve gate voltages. The point
on the curves where VGS=0 corresponds to IDSS. The point where
ID=0 corresponds to VGS(off). As with JFET, VGS(off)=-VP.
The equation to find drain current also the same as JFET:
ID = IDSS(1 - VGS/VGS(off) )2
Remember n and p channel polarity differences.
Example 7-13

For a certain D-MOSFET, IDSS=10mA and
VGS(off)=-8V.
a)
b)
c)
Is this n-channel or a p-channel?
Calculate ID at VGS=-3V.
Calculate ID at VGS=+3V.
Solution:
a)
The device has a –ve VGS(off), this is an n-channel
MOSFET.
b)
ID=IDSS(1-VGS/VGS(off))2=(10mA)(1- (-3/-8))2 =3.91mA
c)
ID=(10mA)(1- (+3/-8))2=18.9mA
E-MOSFET Characteristics and Parameters
The E-MOSFET for all practical
purposes does not conduct
until VGS reaches the threshold
voltage (VGS(th)). ID when
conducting can be determined
by the formulas below. The
constant K must first be
determined from data sheet
by taking ID(on) at any given
value of VGS on a particular
MOSFET.
K = ID(on) /(VGS - VGS(th))2
ID = K(VGS - VGS(th))2
MOSFET Biasing- zero bias
The three ways to bias a MOSFET are zero-bias, voltage-divider
bias, and drain-feedback bias.
For D-MOSFET zero biasing as the name implies has no applied
bias voltage to the gate. The input voltage swings it into depletion
and enhancement mode.
Zero bias
 Since VGS=0 and ID=IDSS, the drain-to-source voltage
is:
VDS = VDD – IDSSRD
 The purpose of RG is to accommodate ac signal input
by isolating it from ground as shown in figure (b)
above. Since there is no dc gate current, RG does not
affect the zero gate-to-source bias.
MOSFET Biasing- voltage divider bias
For E-MOSFETs zero biasing cannot be
used. Voltage-divider bias must be
used to set the VGS greater than the
threshold voltage (VGS(th)). ID can be
determined as follows. To determine
VGS, normal voltage divider methods
can be used. The following formula
can be applied.
VGS = (R2 / (R1+R2))VDD
VDS = VDD - IDRD
K = ID(on)/(VGS - VGS(th))2
ID = K(VGS -VGS(th))2
VDS can be determined by application of
Ohm’s law and Kirchhoff’s voltage law to
the drain circuit.
Example 7-16
 Determine VGS and VDS for E-MOSFET circuit below.
Assume MOSFET has minimum values of
ID(on)=200mA at VGS=4V and VGS(th)=2V.
Solution example 7-16
 R2 
 15k 


VGS  
V DD  
24  3.13V

 115k 
 R1  R2 
I D( on)
200mA
2
K


50
mA
/
V
(VGS  VGS ( th) ) 2 (4  2) 2
I D  K (VGS  VGS ( th) ) 2  50mA / V 2 (3.13  2) 2  63.8mA
VDS  VDD  I D RD  24  (63.8m )( 200)  11.2V
MOSFET Biasing- drain feedback bias
With drain-feedback bias there
is no voltage drop across RG
making VGS = VDS. With VGS
given determining ID can be
accomplished by the formula
below.
ID = (VDD – VDS)/RD
Troubleshooting
As always, having a thorough knowledge of the devices
makes for easier troubleshooting circuits utilizing them.
We will discuss some the common faults associated with
FET circuits. Experience in troubleshooting is the best
teacher having basic theoretical knowledge is extremely
helpful.
Troubleshooting
If VD = VDD in a self-biased
JFET circuit it could be one of
several opens. It is a clear
indication of no drain current.
Use of senses to check for
obvious failures the first and
easiest step. Replace the FET
only if associated components
are known to be good.
If VD is less than normal in a
self-biased JFET circuit an
open in the gate circuit is
more than likely the problem.
The low drain voltage would
be indicative of more drain
current flowing than normal.
Troubleshooting
In a zero-biased D-MOSFET or drain-feedback biased E-MOSFET an
open in the gate circuit is more difficult to detect. It may seem to be
biased properly with dc voltages but will fail to work properly when
an ac signal is applied.
Troubleshooting
With a voltage-divider biased E-MOSFET circuit faults are more
easily detected. With an open R1 there is no drain current, so the
VD = VDD. With an open R2 full VDD is applied to the gate turning
it on fully. VD = 0
Summary
 JFETs are unipolar devices.
 JFETs have three terminals: Source, Gate, and Drain.
 JFETs have a high input resistance since the gatesource junction is reverse biased.
 Unwanted capacitance associated with FETs can be
dealt with by using dual gate type FETs.
 IDSS for all FETs is the maximum amount of current
flow in the drain circuit when VGS is 0V.
 All FETs must be biased for proper operation.
Midpoint is most common for use in amplifiers.
Summary
 MOSFETs differ in construction in that the gate is
insulated from the channel.
 D-MOSFETs can operate in both depletion and
enhancement modes. E-MOSFETs can only operate in
the enhancement mode.
 E-MOSFETs have no physical channel. A channel is
induced with VGS greater than VGS(th).
 E-MOSFETs have no IDSS parameter.
 There are special MOSFET designs for high
power applications.