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UNIT I
Power Supplies
Biasing BJT and MOSFET
Outline
• Rectifier
• BJT Biasing
• FET Biasing
Block diagram of Power Supply Unit
RECTIFIER
• Transformer: To Step down AC voltage amplitude to the desired DC voltage (by selecting an appropriate turn ratio N1/N 2 for the transformer) Isolate equipment from power-line .
• Rectifier: Converts an ac input to a unipolar output Filter.
Filter
• Convert the pulsating input to a nearly constant dc output Regulator.
• Reduce the ripple of the dc voltage.
Half wave Rectifier
Half wave Rectifier- contd…
Half wave Rectifier -contd…
• The input is an alternating current. This input voltage is stepped down using a transformer. The reduced voltage is fed to the diode ‘D’ and load resistance RL.
• During the positive half cycles of the input wave, the diode ‘D’ will be forward biased.
Half wave Rectifier-contd…
• During the negative half cycles of input wave, the diode ‘D’ will be reverse biased. We take the output across load resistor RL.
Half wave Rectifier-contd…
• The diode passes current only during one half cycle of the input wave.
• The output is positive and significant during the positive half cycles of input wave.
Half wave Rectifier-contd…
• At the same time output is zero or insignificant during negative half cycles of input wave. This is called half wave rectification.
• Dc current given by
Half wave Rectifier-contd…
• The ratio of dc power output to the applied input a.c power is known as rectifier efficiency, denoted by η.
Half wave rectifier with filter
Half wave rectifier with filter-contd…
Half wave rectifier with filter-contd…
• Output of half wave rectifier is not a constant DC voltage. It is a pulsating dc voltage with ac ripples.
• In real life applications, we need a power supply with smooth wave forms. we desire a DC power supply with constant output voltage.
Half wave rectifier with filter-contd…
• We can make the output of half wave rectifier smooth by using a filter (a capacitor filter or an inductor filter) across the diode.
• We can also use an resistor-capacitor coupled filter (RC).
Full wave rectifier
Full wave rectifier-contd…
• In a Full Wave Rectifier circuit two diodes are used, one for each half of the cycle.
• A multiple winding transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, (C).
Full wave rectifier-contd…
• When point A of the transformer is positive with respect to point C, diode D1 conducts in the forward direction as indicated by the arrows.
• When point B is positive (in the negative half of the cycle) with respect to point C, diode D2 conducts in the forward direction.
• The current flowing through resistor R is in the same direction for both half-cycles.
Full wave rectifier-contd…
• This configuration results in each diode conducting in turn its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier so it is 100% efficient.
Full wave rectifier-contd…
Unidirectional current given by
Bridge Rectifier
Bridge Rectifier-contd…
• This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output.
• The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost.
Bridge Rectifier-contd…
• The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.
• The four diodes labelled D1 to D4 are arranged in “series pairs” with only two diodes conducting current during each half cycle.
Bridge Rectifier-contd…
During positive cycle of the input
Bridge Rectifier-contd…
• During the positive half cycle of the supply, diodes D1 and D2 conduct in series
• Diodes D3 and D4 are reverse biased and the current flows through the load as shown below.
Bridge Rectifier-contd…
During Negative cycle of the input
Bridge Rectifier-contd…
• During the negative half cycle of the supply, diodes D3 and D4 conduct in series.
• Diodes D1 and D2switch “OFF” as they are now reverse biased.
• The current flowing through the load is the same direction as before.
Bridge Rectifier-contd…
• The current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier.
• The average DC voltage across the load is 0.637Vmax.
Bridge Rectifier-contd…
The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage.
Bipolar Junction Transistor(BJT)
• Bell Labs (1947): Bardeen, Brattain, and Shockley
• Originally made of germanium
• Current transistors made of doped silicon
Bipolar Junction Transistor(BJT)
• The basic of electronic system nowadays is semiconductor device.
• The famous and commonly use of this device is BJTs (Bipolar Junction Transistors).
• It can be used as amplifier and logic switches
Point-Contact Transistor – first transistor ever made
Bipolar Junction Transistor(BJT)
• BJT consists of three terminal: collector : C base : Bemitter : E
• Two types of BJT : pnp and npn
• 3 layer semiconductor device consisting:– 2 n- and 1 p-type layers of material npn transistor– 2 p- and 1 n-type layers of material pnp transistor
• The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material
Basic models of BJT
Diode
Diode
Diode
Diode
npn transistor
pnp transistor
Transistor currents
-The arrow is always drawn on the emitter
-The arrow always point toward the n-type
-The arrow indicates the direction of the emitter current:
pnp:E Bnpn: B E
IC=the collector currentIB= the base currentIE= the emitter current
Understanding BJT working
force – voltage/currentwater flow – current - amplification
Transistor working PNP
Transistor working PNP
• Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated.
• Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.
• A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes.
• The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal.
Transistor working PNP
• Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material.
• Applying KCL to the transistor :
IE = IC + IB
• The comprises of two components – the majority and minority carriers
IC = ICmajority + ICOminority
• ICO – IC current with emitter terminal open and is called leakage
current.
Operation Operation RegionRegion
IB or VCE
Char. BC and BE BC and BE JunctionsJunctions
ModeMode
Cutoff IB = Very small
Reverse & Reverse
Open Switch
Saturation VCE = Small Forward & Forward
Closed Switch
Active Linear
VCE = Moderate
Reverse &
Forward
Linear Amplifier
Break-down
VCE = Large Beyond Limits
Overload
Operation region summary
Transistor Bias Circuits
Objectives
Discuss the concept of dc biasing of a transistor for linear operation
Analyze voltage-divider bias, base bias, and collector-feedback bias circuits.
Basic troubleshooting for transistor bias circuits
Introduction
For the transistor to properly operate it must be biased.
Several methods to establish the DC operating point.
We will discuss some of the methods used for biasing transistors as well as troubleshooting methods used for transistor bias circuits.
Common Emitter Configuration
•The circuit has been re-configured with input at Base & output at Collector
• The Emitter is common to input & output
•This is called Common Emitter configuration
Input
Output
_ + _ +
Ie Ic
Ib
Vee Vcc
Re
Rc
Rb1
Rb2
Vcc
Input
Output
E
B
C
Input Characteristics
Output Characteristics
Beta () or amplification factor
• The ratio of dc collector current (IC) to the dc base current (IB) is dc beta (dc ) which is dc current gain.
• where IC and IB are determined at a particular operating point, Q-point (quiescent point).
Common Base Configuration
------ --- -- -- --
Vbe
_ +
E
B
Ie Ic
Ib
--
--
--- -
--
--- -
----
-----
---
--
Vcb
C
_ +
•Here the input is applied at the Emitter & the output taken from the Collector
•In this arrangement Base is common to the input & output
•This is called Common Base configuration
Input Output
Common Base
Common Base Configuration
• In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha
=
IC = IE + ICBO
• It can then be summarize to IC = IE (ignore ICBO due to small value)
E
C
II
E
C
II
Common Base Configuration
• Alpha a common base current gain factorcommon base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector.
• The value of is typical from 0.9 ~ 0.998.
Common – Collector Configuration
• Also called emitter-follower (EF).
• It is called common-emitter configuration since
both the signal source and the load share the
collector terminal as a common connection point.
Common – Collector Configuration
• The output voltage is obtained at emitter terminal.
• The input characteristic of common-collector
configuration is similar with common-emitter.
configuration.
Common Collector characteristics
Output characteristics of CC configuration
Region of operation
E – B
junction
C – B
junction
Cut off Reverse Biased
Reverse Biased
Active Forward Biased
Reverse Biased
Saturation Forward Biased
Forward Biased
Operating Regions
Ic
Vce
24 V0 V
Ib = 20μAIc = 2mA
Ib = 30μAIc = 4mA
Ib = 40μAIc = 6mA
Ib = 50μAIc = 8mA
Ib = 60μAIc = 10mA
Active RegionSaturation Region
Cut-off Region
Transistor as an amplifier
Simulation of transistor as an amplifier
The DC Operating Point
The goal of amplification in most cases is to increase the amplitude of an ac signal without altering it.
The DC Operating Point
• For a transistor circuit to amplify it must be properly biased with dc voltages.
• The dc operating point between saturation and cutoff is called the Q-point.
• The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.
Q-Point (Static Operation Point)
• When a transistor does not have an ac input, it will have specific dc values of IC and VCE.
• These values correspond to a specific point on the dc load line. This point is called the Q-point.
• The letter Q corresponds to the word (Latent) quiescent, meaning at rest.
• A quiescent amplifier is one that has no ac signal applied and therefore has constant dc values of IC
and VCE.
DC Biasing Circuits
RC
RB
+VCC
ic
vceib
v in
v out
• The ac operation of an amplifier depends on the initial dc values of IB, IC, and VCE.
• By varying IB around an initial
dc value, IC and VCE are made
to vary around their initial dc values.
• DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device.
The DC Operating Point
The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.
Collector characteristic curves
The collector characteristic curves graphically show the relationship of collector current and VCE for different base currents.
The DC Operating Point
With the dc load line superimposed across the collector curves for this particular transistor we see that 30 mA of collector current is best for maximum amplification, giving equal amount above and below the Q-point.
C
CCCE
cc R
VV
R
)
1(I
The DC Operating Point-contd…Effect of a superimposed ac voltage has on the circuit.
The collector current swings do not exceed the limits of operation(saturation and cutoff).
Applying too much ac voltage to the base would result in driving the collector current into saturation or cutoff resulting in a distorted or clipped waveform.
Voltage swing
• Both AC and DC load lines are shown as drawn on the collector characteristics of an NPN transistor.
• Note that both of the lines have to pass through the operating point, Q.
Voltage swing-contd…
• AC load line defines the range the collector current and voltage swings that can take place around the operating point.
• The range limited on by the saturation region of the transistor characteristics and on the right by its cut-off point.
Voltage swing-contd…
• If the swings ten exceed these limits, the waveform is clipped, creating severe distortion in the amplified signal. The undisto (unclipped) voltage swing is restricted to ∆vMAX+ and ∆vMAX+ around the operating point
Voltage swing-contd…
Voltage-Divider Bias
Voltage-divider bias is the most widely used type of bias circuit.
voltage-divider bias is more stable( independent) than other bias types.
Voltage-Divider Bias-contd…
R1 and R2 are used to provide the needed voltage to point A(base).
The voltage at point A of the circuit in two ways, with or without the input resistance(point A to ground) considered.
Voltage-Divider Bias-contd…
Voltage-Divider Bias-contd…
•The voltage across R2(VB) by the proportional method.
VB = (R2/R1 + R2)VCC
CCEDC
EDC
RRR
RV
)||(
||RV
21
2B
Voltage-Divider Bias-contd…
Base voltage and subtract VBE to find out what is dropped across RE ,determine the current in the collector-emitter side of the circuit.
The current in the base-emitter circuit is much smaller, IE≈ IC
Base Bias
This type of circuit is very unstable since its changes with temperature and collector current. Base biasing circuits are mainly limited to switching applications.
DCC )(I B
BECC
R
VV
Collector-Feedback Bias
Collector-feedback bias is kept stable with negative feedback, although it is not as stable as voltage-divider or emitter.
With increases of IC, less voltage is applied to the base.
With less IB ,IC comes down as well..
•IB = (VC - VBE)/RB
•IC = (VCC - VBE)/(RC + RB/DC)
Vc = Vcc – (Ic + Ib) x Rc
Also, Vc = (Ib x Rb) + Vbe
Equating the two equations
Vcc – (Ic + Ib)Rc = (Ib Rb) + Vbe
Or, Ib(Rc + Rb) = Vcc – IcRc - Vbe
Ib = Vcc – IcRc - Vbe
Rc + Rb . ..
Ic = ( Vcc – IcRc – Vbe)
Rc + Rb As Ic = Ib
Base Bias
Disadvantages of fixed bias circuit
• Ic increases with temperature & there is no control over it
• Hence there is poor thermal stability Ic = Ib
• Hence Ic depends on may change from transistor to transistor
• This will shift the operating point
• Hence stabilization is very poor in fixed bias circuit
Advantages of fixed bias circuit
• Simple circuit with minimum components
• Operating point can be fixed conveniently in the active region, by selecting appropriate value for Rb
• Hence fixed bias circuit provides flexibility in the design
Emitter Bias
•This type of circuit is independent of making it as stable as the voltage-divider type. The drawback is that it requires two power supplies.
•IB ≈ IE/
•IC ≈ IE ≈( -VEE-VBE)/(RE + RB/DC)
Summary
The purpose of biasing is to establish a stable operating point (Q-point). The Q-point is the best point for operation of a transistor for a given collector current. The dc load line helps to establish the Q-point for a given collector current.
The linear region of a transistor is the region of operation within saturation and cutoff.
Stability Factor
Stability• Temperature & Current gain variation may change
the Q point
• Stability refers to the design that prevents any
change in the Q point
• Temperature effect
• When the temperature increases it results in the
production of more charge carriers
• This increases the forward bias of the transistor
and Ib increases
Temperature effect
• When the temperature increases it results in the production
of more charge carriers
• This increases the minority charge carrier and hence the
leakage current as
Iceo = (+1) Icbo
• Icbo doubles for every 100 C
As Ic = Ib + Icbo
• The increase in the temperature increases Ic
• This in turn increases the power dissipation and again
more heat is produced
Stability Factor
• It indicates the degree of change in the operating point due to variation in temperature
• There are 3 stability factors corresponding to the 3 variables – Ico, Vbe &
S Ic
Ico =
Vbe, constant
S’ Ic
Vbe =
Ico, constant
S’’ Ic
=
Ico, Vbe constant
The stability factor should be as minimum as possible
Techniques
• Stabilization technique• Resistive biasing circuits change Ib suitably and
keep Ic constant • Compensation technique• Temperature sensitive devices such as diodes,
thermistors & transistors are used to provide suitable compensation and retain the operating point without shifting
Stability Factor S
For the fixed Bias Circuit Ib = Vcc / Rb
S = IcIco Vbe, constant
=( I + )
1 - Ib
Ic
Ib
Ic. ..
= 0
S = 1 + . ..
S =( I + )
1 - (0) . ..
For Fixed Bias Circuit
Stability Factor S’
S’ IcVbe =
Ico, constant
S = - / Rb. ..
= Vcc - Vbe
Rb+ ( + 1) Icbo
= - + ( + 1) IcboRb
Vcc
Rb
Vbe
. ..
+Ib
Vbe= 0
Rb
_ 0
= Ib + ( + 1) Icbo
Ic = Ib + Iceo
For Fixed Bias Circuit
Stability Factor S’’
S’’ = Ic / . ..
Ic = Ib + Iceo S’’ Ic
=Ico, Vbe constant
For Fixed Bias Circuit
= Ib + (+1)Icbo
= Vcc - Vbe
Rb+ ( + 1) Icbo
= - + ( + 1) IcboRb
Vcc
Rb
Vbe
. .. Ic
= - + Icbo
Rb
Vcc
Rb
Vbe
= Ib + Icbo
= Ib (approx)
= Ic /
Stability Factor S’
S’ IcVbe =
Ico, constantIb =
Vcc – IcRc - Vbe
Rc + Rb
= Vcc – IcRc - Vbe
Rc + Rb
Ic
Ic+
Rc + Rb
IcRc
Rc + Rb
Vcc - Vbe=
Rc + Rb + RcIc
(Rc + Rb) Rc + Rb
Vcc - Vbe=
S’ Ic
Vbe =
Ic =Rb + ( + 1) Rc
(Vcc – Vbe)
=Rb + ( + 1) Rc
-
For Collector-Base Bias
Stability Factor S’’
S’’ Ic
=Ico, Vbe constant
Vcc = (Ib + Ic)Rc + IbRb + Vbe
Vcc –Vbe = (Ib + Ic)Rc + IbRb
= Ib [(1 + )Rc +Rb]
Ib =. .. Vcc – Vbe
(1 + ) Rc + Rb
Ic =. .. ( Vcc – Vbe)
(1 + ) Rc + Rb
For Collector-Base Bias
. .. Ic
=[(1 + )Rc +Rb](Vcc –Vbe) - (Vcc –Vbe) Rc
[(1 + ) Rc + Rb]2
(Vcc –Vbe)[(1 + )Rc +Rb] - Rc
[(1 + ) Rc + Rb]2=
(Vcc –Vbe)(Rc +Rb)
[(1 + ) Rc + Rb]2=
= Vcc – Vbe
(1 + ) Rc + Rb
Rc + Rb
(1 + ) Rc + Rb x
= Ib(Rc + Rb)
(1 + ) Rc + Rb
= Ic(Rc + Rb)
[(1 + ) Rc + Rb] . ..
S’’
= Ic(Rc + Rb)
[(1 + ) Rc + Rb] S’’
= (Rc + Rb)
(1 + ) Rc + Rb
Ic
1+
1+
=(1 + ) Rc + Rb
Ic
1
1+
(Rc + Rb)(1+ )
= Ic
S
1+
If S is small, S’’ will also be small
Hence if we provide stability against Ico variations, it will take care of variation as well
Vb = IbRb +Vbe + IeRe S
IcIco =
Vbe, constant
0 = IbRb + 0 + IbRe + IcRe
i.e. Ib(Rb + Re) = - IcRe
Ib
Ic. .. -Re
Rb + Re= S =
(I + )
1 - Ib
Ic
=(I + )
1 + Re
Re + Rb
where Rb = Rb1 ll Rb2
Stability Factor SFor Voltage Divider Bias
= IbRb +Vbe + (Ib + Ic)Re
Differentiating,
• In the above equation, if Rb << Re, then S becomes 1
Rb = Rb1 ll Rb2
• Hence either Rb1 or Rb2 must be << Re
• Since Vb << Vcc, Rb2 is kept small wrt Rb1
S =(I + )
1 + Re
Re + Rb
• Re cannot be increased beyond a limit, as it will affect Ic and hence the Q point
• If Rb-Re ratio is fixed, and if Rb >> Re, S increases with
• Thus stability decreases with increasing
S =(I + )
1 + Re
Re + Rb
S =(I + )
1 + 1
1 + Rb/Re
S = (I + )
• If Rb << Re, then S becomes independent of
• Stability factor S for Voltage Divider circuit is less compared to other circuits
• Hence it is preferred over other circuits
S =(I + )
1 + Re
Re + Rb
S =(I + )
1 + 1
1 + Rb/Re
S = I
Stability Factor S’
S’ IcVbe =
Ico, constant
S’ IcVbe
= =Rb + ( + 1) Re
-
= Ib(Rb + Re) + Vbe + IcRe
= Ic / (Rb +Re) + Vbe + IcRe
Or, Vb = Ic(Rb +Re) + Vbe + IcRe
= Ic[Rb +( + 1)Re] + Vbe
Differentiating, 0 = Ic[Rb +( + 1)Re] + Vbe
For Voltage Divider Bias
Vb = IbRb +Vbe + IeRe
= IbRb + Vbe + (Ib + Ic)Re
Or, Vbe = - Ic [Rb +( + 1)Re]
Stability Factor S’’
= Ib(Rb + Re) + Vbe + IcRe
= Ic / (Rb +Re) + Vbe + IcRe
Or, Vb = Ic(Rb +Re) + Vbe + IcRe
Differentiating,
S’’ Ic
=Ico, Vbe constant
Or, (Vb – Vbe) = Ic(Rb +Re) + IcRe
(Vb – Vbe) = Ic(Rb +Re) + IcRe + Ic Re
(Vb – Vbe – IcRe) = Ic[Rb + Re+ Re]
. .. Ic
=S’’ =
Vb – Vbe - IcRe
Rb + Re(1+ )
Vb = IbRb +Vbe + IeRe
For Voltage Divider Bias
Hence Rb / Re must be small to make S’’ smaller
Ic
=S’’ =
Vb – Vbe - IcRe
Rb + Re(1+ )
=Vb – Vbe - IeRe
Rb + Re(1+ )As Ie = Ic
=Ib Rb
Rb + Re(1+ )
=Ib
1 +(Re/Rb)(1+ )
Bias Compensation• The biasing circuits seen so far provide stability of
operating point for any change in Ico, Vbe or
• The collector- base bias & emitter bias circuits provide negative feedback & make the circuit stable, but the gain falls down
• In such cases it is necessary to use compensation techniques
Here diode D has been connected as shown
It is given forward bias through Vdd
The diode D is identical to the BE junction of the transistor
The charge carriers will increase in the BE jn. due to temperature or other variations
• Diode Compensation Technique
Rb 270 K 5.6 K
Vcc
Rc
Rd
+
-
Re
Vdd D
Since diode D has similar properties, its charge carrier also increases, for any change in the parameters
Thus the increase in current in the BE junction is compensated by the current flow through the diode in the reverse direction.
Rb 270 K 5.6 K
Vcc
Rc
Rd
+
-
Re
Vdd D
Another technique
Here the diode D has been connected in the bleeder path
When there is increase in current in the BE junction due to parameter changes, current through D also increases by the same amount
Rb1 270 K 5.6 K
Vcc
Rc
ReRb2
D
Ib1
Ib2
This increases Ib1, produces more
drop across Rb1& reduces Vb
As Vb decreases, Ib falls down
Thus the transistor currents are
arrested and not allowed to increase
Thus diode D provides suitable
compensation
Rb1 270 K 5.6 K
Vcc
Rc
ReRb2
D
Here a Negative Temperature Coefficient Resistor has been used
As temperature increases, its resistance decreases
This increases Ib1 & voltage drop across Rb1
This decreases Vb and hence Ib & Ic, thus keeping the circuit stable.
Thermistor Compensation
270 K 5.6 K
Vcc
Re
RcRb1
NTC
Ib
Ib1
Ib2
Re provides self bias
Vb is fixed depending on
the ratio of Rb1 & Rb2 &
the value of Vcc
Ve = Vb - Vbe
Vbe is fixed for a transistor
Hence Ve is fixed &
Ie = Ve / Re is also fixed
Hence it acts as a constant
current circuit
5.6 K
Vcc
Re
RcRb1
Rb2
Constant Current circuit
270 K 5.6 K
-20 V
Re 2K2
Rb1
Problem For the given Si transistor
find the constant current I
Answer I = 4.22 mA
Rb2 4K7
I
FET Biasing
Introduction• For the JFET, the relationship between input and output
quantities is nonlinear due to the squared term in Shockley’s equation.
• Nonlinear functions results in curves as obtained for transfer characteristic of a JFET.
• Graphical approach will be used to examine the dc analysis for FET because it is most popularly used rather than mathematical approach
• The input of BJT and FET controlling variables are the current and the voltage levels respectively
JFETs differ from BJTs:• Nonlinear relationship between input (VGS) and
output (ID)
• JFETs are voltage controlled devices, whereas BJTs are current controlled
Introduction -contd…
Common FET Biasing Circuits• JFET
– Fixed – Bias – Self-Bias – Voltage-Divider Bias
• Depletion-Type MOSFET– Self-Bias– Voltage-Divider Bias
• Enhancement-Type MOSFET– Feedback Configuration– Voltage-Divider Bias
FET Biasing
General Relationships
• For all FETs:
• For JFETs and Depletion-Type MOSFETs:
• For Enhancement-Type MOSFETs:
AIG 0
SD II
2
P
GSDSSD )
V
V(1II
2)( TGSD VVkI
Fixed-Bias Configuration
• The configuration includes the ac levels Vi and Vo and the coupling capacitors.
• The resistor is present to ensure that Vi appears at the input to the FET amplifier for the AC analysis.
Fixed-Bias Configuration-contd…
Investigating the input loop
• IG=0A, therefore
VRG=IGRG=0V
• Applying KVL for the input loop,
-VGG-VGS=0
VGG= -VGS
• It is called fixed-bias configuration due to VGG is a fixed power
supply so VGS is fixed
• The resulting current,
2)1(P
GSDSSD V
VII
• Investigating the graphical approach.• Using below tables, we can draw the graph
Fixed-Bias Configuration(Graphical approach)
Self Bias Configuration
• The self-bias configuration eliminates the need for two dc supplies.
• The controlling VGS is now determined by the voltage across the resistor RS
• For the indicated input loop:
• Mathematical approach:
SDGS RIV
2
2
1
1
P
SDDSSD
P
GSDSSD
V
RIII
V
VII
Self Bias Configuration-contd…
– Draw the device transfer characteristic– Draw the network load line
• First point,
• Second point, any point from ID = 0 to ID = IDSS. Choose
– the quiescent point obtained at the intersection of the straight line plot and the device characteristic curve.
– The quiescent value for ID and VGS can then be determined and used to find the other quantities of interest.
SDGS RIV
0,0 GSD VI
2
2
SDSSGS
DSSD
RIV
thenI
I
Graphical approach
Graphical approach
• For output loop
– Apply KVL of output loop
– Use ID = IS
RDDDSDSD
SDS
DSDDDDS
VVVVV
RIV
RRIVV
)(
Self Bias
Voltage-Divider Bias
• The arrangement is the same as BJT but the DC analysis is different
• In BJT, IB provide link to input and output circuit, in FET VGS does the same
Voltage-Divider Bias• The source VDD was separated into two equivalent sources to
permit a further separation of the input and output regions of the network.
• IG = 0A ,Kirchoff’s current law requires that IR1= IR2 and the series
equivalent circuit appearing to the left of the figure can be used to find the level of VG.
21
DD2G
RR
VRV
SDGGS
RSGSG
RIVV
VVV
0
Voltage-Divider Bias
• VG can be found using the voltage divider rule :
• Using Kirchoff’s Law on the input loop:
• Rearranging and using ID =IS:
• Again the Q point needs to be established by plotting a
line that intersects the transfer curve.
Procedures for plotting
1. Plot the line: By plotting two points: VGS = VG, ID =0 and VGS = 0,
ID = VG/RS
2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID.
3. Where the line intersects the transfer curve is the Q point for the
circuit.
• Once the quiescent values of IDQ and VGSQ are
determined, the remaining network analysis can be found.
• Output loop:
2121 RR
VII DD
RR
)( SDDDDDDS RIRIVV
DDDDD RIVV
SDS RIV
Voltage Divider Bias
Effect of increasing values of RS
•Depletion-type MOSFET bias circuits are similar to JFETs. The only difference is that the depletion-Type MOSFETs can operate with positive values of VGS and with ID values that exceed IDSS.
Depletion-Type MOSFETs
•Analyzing the MOSFET circuit for DC Analyzing the MOSFET circuit for DC analysisanalysis
How to analyze dc analysis for How to analyze dc analysis for the shown network?the shown network? It is a Type networkIt is a Type network Find VFind VG G or V or VGSGS
Draw the linear Draw the linear characteristicscharacteristics
Draw the transfer Draw the transfer characteristicscharacteristics
Obtain VObtain VGSQGSQ and I and IDQDQ from the from the
graph intersectiongraph intersection
1. Plot line for VGS = VG, ID = 0 and ID = VG/RS, VGS = 0
2. Plot the transfer curve by plotting IDSS, VP and calculated values
of ID.
3. Where the line intersects the transfer curve is the Q-point.
Use the ID at the Q-point to solve for the other variables in the
voltage-divider bias circuit. These are the same calculations as used by a JFET circuit.
Depletion type MOSFET
1. Plot line for VGS = VG, ID = 0 and ID = VG/RS, VGS = 0
2. Plot the transfer curve by plotting IDSS, VP and calculated values
of ID.
3. Where the line intersects the transfer curve is the Q-point.
Use the ID at the Q-point to solve for the other variables in the
voltage-divider bias circuit. These are the same calculations as used
by a JFET circuit.
Q-Point- Enhancement MOSFET
•The transfer characteristic for the enhancement-type MOSFET is very different from that of a simple JFET or the depletion-type MOSFET.
Enhancement-Type MOSFET
• Transfer characteristic for E-MOSFET
2)( )( ThGSGSD VVkI
2)()(
)(
)( ThGSonGS
onD
VV
Ik
Enhancement MOSFET
Feedback Biasing Arrangement
• IG =0A, therefore VRG = 0V
•Therefore: VDS = VGS
•Which makes DDDDGS RIVV
1. Plot the line using VGS = VDD, ID = 0 and ID = VDD / RD and VGS =
0
2. Plot the transfer curve using VGSTh , ID = 0 and VGS(on), ID(on); all
given in the specification sheet.
3. Where the line and the transfer curve intersect is the Q-Point.
4. Using the value of ID at the Q-point, solve for the other variables
in the bias circuit.
Feedback Biasing Q-Point
DC analysis step for Feedback Biasing DC analysis step for Feedback Biasing Enhancement type MOSFETEnhancement type MOSFET
Find k using the datasheet or specification given;Find k using the datasheet or specification given;
ex: Vex: VGS(ON)GS(ON),V,VGS(TH)GS(TH)
Plot transfer characteristics using the formula Plot transfer characteristics using the formula
IIDD=k(V=k(VGSGS – V – VTT))22. Three point already defined that is . Three point already defined that is
IID(ON)D(ON), V, VGS(ON)GS(ON) and V and VGS(TH)GS(TH)
Plot a point that is slightly greater than VPlot a point that is slightly greater than VGSGS
Plot the linear characteristics (network bias line)Plot the linear characteristics (network bias line) The intersection defines the Q-pointThe intersection defines the Q-point
Again plot the line and the transfer curve to find the Q-point.
Using the following equations: 21
DD2G
RR
VRV
)( DSDDDDS
SDGGS
RRIVV
RIVV
Input loop :
Output loop:
Voltage-Divider Biasing
1. Plot the line using VGS = VG = (R2VDD)/(R1 + R2), ID = 0 and ID = VG/RS and VGS = 0
2. Find k
3. Plot the transfer curve using VGSTh, ID = 0 and VGS(on), ID(on); all given in the specification sheet.
4. Where the line and the transfer curve intersect is the Q-Point.
5. Using the value of ID at the Q-point, solve for the other variables in the bias circuit.
Voltage-Divider Bias Q-Point
TroubleshootingTroubleshooting
N-channel VN-channel VGSQGSQ will be 0V or negative if will be 0V or negative if
properly checkedproperly checked Level of VLevel of VDSDS is ranging from 25%~75% of V is ranging from 25%~75% of VDDDD. .
If 0V indicated, there’s problemIf 0V indicated, there’s problem Check with the calculation between each Check with the calculation between each
terminal and ground. There must be a reading, Rterminal and ground. There must be a reading, RGG
will be excludedwill be excluded
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 n-channel graphs.
P-Channel FETs
• Voltage-Controlled Resistor
• JFET Voltmeter
• Timer Network
• Fiber Optic Circuitry
• MOSFET Relay Driver
Practical Applications