UNIT I Power Supplies Biasing BJT and MOSFET Outline Rectifier BJT Biasing FET Biasing

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.


• 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.


• 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


• 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…


• 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.


• 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).


• 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.


• 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.


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.


• 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.


During positive cycle of the input


• 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.


During Negative cycle of the input


• 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.


• 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.


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


• 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


• 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


• 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.


• 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


• 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


• 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.


• 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 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.


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.


• 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-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.



•The voltage across R2(VB) by the proportional method.

VB = (R2/R1 + R2)VCC

CCEDC

EDC

RRR

RV

)||(

||RV

21

2B


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


Stability Factor S’’

S’’ = Ic / . ..

Ic = Ib + Iceo S’’ Ic

=Ico, Vbe constant


= Ib + (+1)Icbo

= Vcc - Vbe

Rb+ ( + 1) Icbo

= - + ( + 1) IcboRb

Vcc

Rb

Vbe

. .. Ic

= - + Icbo

Rb

Vcc

Rb

Vbe

= Ib + Icbo

= Ib (approx)

= Ic /


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


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


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]


= 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


• 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


• 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

Documents

UNIT I Power Supplies Biasing BJT and MOSFET Outline Rectifier BJT Biasing FET Biasing