Transistors Opamp

Transistors &

Opamp

Ms.S.M.Hosamani. Assistant Professor

Department of E&TC PICT, Pune.

17 September 2019 1

Contents

• Transistor (BJT) Structure

• Transistor characteristics and parameters

• DC operating point

• Transistor as an amplifier

• Transistor as a switch

• MOSFET

• Operational Amplifier

17 September 2019 2

Introduction

• The semiconductor device like a diode cannot amplify a

signal, therefore its application area is limited.

• The next development of semiconductor device after diode is a

BJT (bipolar junction transistor).

• It is a three terminal device. The terminals are – collector,

emitter, and base. Out of which the base is a control terminal.

• A signal of small amplitude applied to the base is available in

the “magnified” form at the collector of the transistor.

• Thus the large power signal is obtained from a small power

signal. 17 September 2019 3

http://www.bellsystemmemorial.com/belllabs_transistor.html

History of Transistors

1948 – The year of establishment of E&TC - COEP

17 September 2019 4

Why is it called transistor ?

• The term transistor was derived from the words TRANSFER & RESISTOR.

• Transfers input signal current from a low resistance path to a high resistance path.

17 September 2019 5

N-P-N transistor

N

N

P

C

E

B

Collector Base Junction JC

Emitter Base Junction JE

E Emitter

B Base

C Collector

17 September 2019 6

The BJT – Bipolar Junction Transistor

Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping.

npn pnp

n p n E

B

C p n p E

B

C

Cross Section Cross Section

B

C

E

Schematic Symbol

B

C

E

Schematic Symbol

17 September 2019 7

transistor currents

N

P

E

B

Collector Base Junction JC

Emitter Base Junction JE

E Emitter

B Base

17 September 2019 8

Number of P-N junctions and equivalent circuit

N

P

E

B P

N

B

C

E Emitter

C Collector

B Base

17 September 2019 9

An unbiased Transistor – Depletion region

• For an unbiased transistor no external power supplies are

connected to it

P

Junction JEB

Emitter collector

N

Base Junction JCB

N

Depletion region

Depletion region

-

-

-

-

-

+

+

+

+

+

-

-

-

-

-

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

17 September 2019 10

Transistor biasing in the active region

Sr.

No.

Region of

operation

Base emitter

junction

Collector base

junction

application

1 Cutoff region Reverse

biased

Reverse

biased

transistor is OFF

2 Saturation

region

Forward

biased

Forward

biased

transistor is ON

3 Active

region

Forward

biased

Reverse

biased

Amplifier


Transistor operation in the active region P-N-P

P

Junction JEB

Emitter collector

N

Base

Junction JCB

VEE

RE

+ -

RC

VCC

-

holes emitted holes collected

Conventional current

conventional current

+

P P N

IE = IC + IB 17 September 2019 12

Transistor configuration

• Depending on which terminal is made common to input and

output port there are three possible configurations of the

transistor. They are as follows:

• Common base configuration

• Common emitter configuration

• Common collector configuration


Transistor operation in the active region N-P-N common base configuration

P

Junction JEB

Emitter collector

N

Base

Junction JCB

N

VEE

RE

+ -

RC

VCC

+ -

Electron emitted Electron collected

Emitter electron current

Direction Conventional Current IC (INJ)

Direction Conventional Current IB

Direction Conventional Current IE

(injected collector current)


Transistor operation in the active region N-P-N common base configuration

P

JEB

Emitter collector

N

Base

JCB

N

Depletion region

Depletion region

-

-

-

-

-

+

+

+

+

+

-

-

-

-

-

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

RC

VCC

+ -

IC=ICBO

ICBO

Is a collector to base leakage current With open emitter

ICBO is a reverse saturation Current flowing due to the Minority carriers between Collector and base when the Emitter is open. ICBO flows due to the reverse Biased collector base junction. ICBO is neglected as compared to IC


Current relations in CB configuration

• Current amplification factor ( αdc)

• the current amplification factor is the ratio of collector current

due to the injection of total emitter current

IC = IC(INJ) + ICBO

αdc = IC(INJ) / IE

IC(INJ) = αdc IE

But ICBO is negligibly small

Therefore the current amplification factor

IC = αdcIE + ICBO

IC = αdcIE

αdc= IC / IE 17 September 2019 16

Characteristics of a transistor in CB configuration

Input characteristics

N P

JC JE

N

B

C E

+ -

B

C E

+ -

VEE

VEE

RE

RE

IE

IE

VBE

-

+

VBE

-

+ VCB =8V

+

-

+

-

VBE

IE

VCB =8V

VCB 4V

VCB 8V

ΔVBE

ΔIE

Input resistance Ri = ΔVBE / ΔIE

As the change in emitter current is very large for a Small change in input voltage, the input resistance Ri is small



“Early effect” or “base width modulation”.

Base P

JE

Emitter collector

Emitter N

JC

Collector N

Wider Depletion Region for larger values of VCB

-

-

-

-

-

+

+

+

+

+

Total base width

zero effective base width At larger values of VCB

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

VCB increases extremely

For extremely large VCB the effective base width may be reduced to zero, causing voltage breakdown of a transistor.

This phenomenon is known as punch through

-

-

-

-

-

-

-

-

-

-



Output characteristics

N P

JC JE

N

B

C E

+ -

B

C E

+ -

VEE

VEE

RE

RE

Constant IE=3mA

Constant IE=3mA

VEB

-

+

VCB +

RC

+ -

VCC

IC

+ -

VCC

RC

IC

VCB +

-

VCB

IC (mA)

0 5 10 -1

IE=0 IC=ICBO

Cutoff region Both the junctions are reverse biased

IE=1 mA 1

2 IE=2 mA

3 IE=3 mA

Active region (high output dynamic resistance)

saturation region Both the junctions are forward biased



Transfer characteristics

0 1 2 3 4

4 3 2 1

IE (mA)

IC (mA) VCB constant

Slope = ΔIC / ΔIE = αdc

α dc = ΔIC / ΔIE


Characteristics of a transistor in CE configuration

• It is a graph of input current (IB) versus input voltage (VBE) at a constant output voltage (VCE).

N

N

P

C

E

B JC

JE

+

-

-

+

RB

IC

IB

IE

VCE constant

VBE

VBB

VCC

N-P-N Transistor

VBE

IB (μA) VCE = 4V 10V

0 0.7 1 2

The value of dynamic input resistance “Ri” is low for CE

ΔIB

ΔVBE Ri=ΔVBE/ΔIB

VCE Constant

Input characteristics



• It is a graph of output current (Ic)

versus output voltage (VCE) at a

constant input current (IB)

E

C

N-P-N Transistor

-

+

RE

B

+

-

RB

IC

IE

VCC VCE VBE

VBB

VCE

IB = 0

4 3 2 1

IC (mA)

1 2 3 4

Cutoff region

IB = 2μA

IB = 4μA

IB = 3μA

IB = 4μA

Saturation region

Active region

βdc = IC /IB

Output characteristics



Transfer characteristics

0 1 2 3 4

4 3 2 1

IB (μA)

IC (mA) VCE constant

Slope = ΔIC / ΔIB = βac

β ac = ΔIC / ΔIB

β dc = IC / IB VCE constant 17 September 2019 23

Comparison of configurations

Sr. No. Parameter CB CE CC

1 Common terminal

between input and output

Base

Emitter

Collector

Conduction Angle 0 o 180 o 0 o

2 Input current IE IB IB

3 Output current IC IC IE

4 Current gain αDC = IC/IE

Less than one

βDC = IC/IB

High

γ = IE/IB

HIGH

5 Input Voltage Veb Vbe Vbc

6 Output voltage Vcb Vce Vec

7 Current gain Less than unity High High

8 Input resistance Very low (20Ω) Low (1KΩ) High(500kΩ)

9 Output resistance Very high (1M) High(40kΩ) Low (50Ω)

10 Application As

preamplifier

Audio

amplifier

Impedance

matching

11. Voltage gain

Medium Large Less than 1 17 September 2019 24

Transistor Biasing

• What is meant by dc biasing of a transistor ?

• Depending on the application, a transistor is to be operated in

any of the three regions of operation namely cutoff, active and

saturation region.

• To operate the transistor in these regions the two junctions of a

transistor should be forward or reverse bias


DC Load Line

IC = [-1/RC] VCE + VCC/RC

• and substituting IC = 0 in above equation

• VCE = VCC or point “B”

VCE

IB = 0

A 3 2 1

IC (mA)

1 2 3 4

IB = 2μA

IB = 3μA

IB = 4μA

E

C

N-P-N Transistor

-

+

RC IC

VCC VCE

IC (MAX)

VCE=VCC

B

DC load line



Typical Junction Voltages Voltage Silicon Transistor Germanium Transistor

VBE (Cut-off) 0 -0.1V

VBE (Cut-in)

0.5v 0.1V

VBE (Active)

0.7V 0.2V

VBE (Saturation)

0.8V 0.3V

VCE (Saturation)

0.2V 0.1V


Biasing circuits

To avoid a shift of Q-point, bias-stabilization is

necessary. Various biasing ckts can be used for this

purpose.

• Fixed bias

• Collector-to-base bias

• Self Biased or Voltage divider bias

• Fixed bias with emitter resistor

• Emitter bias


RC

RE CE R2

R1

+VCC

C1

C2 VO

Vi Signal to be Amplified RL

Amplified signal output Signal

R1 & R2 are Biasing Resistor

C1 & C2 are Coupling

Capacitors

Bypass Capacitor

Single Stage RC Coupled CE Amplifier


BJT Switch

• When operated in

saturation, the BJT

acts as a closed

switch.

• When operated in

cutoff, the BJT acts as

an open switch.


MOSFET


FIELD-EFFECT TRANSISTORS ( FET)

• FETs are the uni polar devices because, unlike BJTs

that use both electron and hole current, they operate

only with one type of charge carrier.

• The two main types of FET’s are -

Junction Field Fffect Transistor (JFET) and

Metal Oxide Semiconductor Field Effect Transistor

(MOSFET)


Current Controlled & Voltage Controlled Devices


Field Effect Transistors - Classification


MOSFET (IGFET)

• The MOSFET (metal oxide semiconductor field effect transistor) is the category of FET.

• The MOSFET differs from the JFET in that it has no PN junction structure; instead, the gate of the MOSFET is insulated from the channel by a silicon dioxide (Sio2) layer.

• Two basic types of MOSFETS are :

Depletion ( D ) MOSFET and

Enhancement ( E ) MOSFET

• Because of the insulated gate, these devices are also called as IGFET.


ENHANCEMENT MOSFET ( E-MOSFET)

MOSFET was invented by Atalla & Dawon at Bell Labs in 1959 17 September 2019 37

Linear & Saturation Regions



Transfer & Drain Characteristics


BJT MOSFET It is a current controlled device. It is a voltage controlled device.

It is a bipolar device (Current flows due

to both majority & minority carriers).

It is a unipolar device (Current flows

due to only majority carriers).

Thermal Runaway can damage the BJT Thermal Runaway does not take place

Input resistance (Ri) is very low. Output resistance (Ro) is very high.

Transfer characteristics are linear in

nature.

Transfer characteristics are non-linear in

nature.

BJT is More sensitive than MOSFET MOSFET is less Sensitive

AC Voltage Gain is HIGH AC Voltage Gain is Less

Bigger in size. Smaller in size.

Regions of operation: Saturation – ON

Switch , Cut off – OFF Switch

Active – Amplifier

Regions of operation: Ohmic – ON

Switch ,Saturation – Amplifier ,

Cut off – OFF Switch

It is more noisy. It is less noisy.

Switching speed is less. Switching speed is high.

Symbol Symbol



Operational Amplifier

An operational amplifier (often op-amp or opamp) is a DC coupled high-gain electronic voltage amplifier with a differential input and usually, a single-ended output


• Op-amp is basically a multistage amplifier which is uses a number of amplifier stages interconnected to each other in a complicated manner.

• The amplifier which could be configured to perform a variety of operations such as amplification, addition, subtraction, differentiation and integration.

• Hence the name is operational amplifier (OP-AMP)

• The integrated Op-amp offers all the advantages of monolithic integrated circuits such as small size, high reliability, reduced cost, less power consumption.

• IC 741 is extremely popular and was used in a variety of applications.


Pin configuration of OP-AMP IC 741


Symbol and terminal

741

2

3

4

7

6 -

+

+VCC positive supply voltage

-VEE negative supply voltage

Output

Inverting input

Non-Inverting input

The Operational Amplifier (op amp) was invented in the 40's. Bell Labs filed a patent in 1941 and many consider the first practical op amp to be the vacuum tube K2-W invented in 1952 by George Philbrick. 17 September 2019 46

Op-amp symbols and packages.

Thomas L. Floyd

Electronic Devices, 6e and Electronic

Devices: Electron Flow Version, 4e


Manufactures of OP-AMP IC 741

• The manufactures of Op-amp ICs are companies like Fairchild, National semiconductor, Motorola, Texas Instruments and signetics.

• The identifying initials for some other companies are as follows:

1. National semiconductors : LM 741

2. Motorola : MC 741

3. RCA : CA 741

4. Texas instruments : SN 52741

5. Signetics : N 5741 17 September 2019 48

Ideal differential amplifier

• An ideal differential amplifier is expected to amplify the differential signal present between its two input signal.

• It is also the basic stage of an integrated Op-amp with differential input.

V2 V1

Vd

Vo = V1 – V2 + +

- -

Ideal

Differential

Amplifier


Ideal

Differential

Amplifier

V2 V1

Vd

Vo = V1 – V2 + +

- -

Differential gain - • Vo = Ad ( V1 – V2 ) Where Ad is called as the differential gain. • The differential gain can be defined as the gain with which the

differential amplifier amplifies the differential signal. Vo = Ad Vd as Vd = V1 – V2 Gain Ad = Vo / Vd Ad (dB) =10 log10 [ Vo / Vd ]


Ideal

Differential

Amplifier

V2 V1

Vd

Vo = V1 – V2 + +

- -

Common mode signal

• A common signal to both the input terminals ( i.e. V1=V2=V) is called as common mode signal.

• The output voltage produced by an ideal differential amplifier is zero for the common mode signal.


Block diagram of a typical OP-AMP

Input

Stage

Intermediate

stage

Level

shifting

stage

Output

Stage

Non-inverting

input

Inverting

input

+

-

Output

Dual input

Balanced

Output

Differential

amplifier

Dual input

unbalanced

Output

Differential

amplifier

Such as

Emitter follower

Using constant

Current source

Complementary

Symmetry

Push-pull

amplifier



Input and output signals 1800 phase shift when the input signal is

applied to the inverting (-) terminal

741

2

3

4

7

6 -

+

+VCC

-VEE

Vo

Inverting input input

Inverted Output signal


Input and output signals 00 phase shift when the input signal is

applied to the Non-inverting (+) terminal

741

2

3

4

7

6 -

+

+VCC

-VEE

Vo

Non-Inverting

input input

Non-Inverted Output signal


Ideal

Differential

Amplifier

V2 V1

Vd

Vo = V1 – V2 + +

- -

Common mode rejection ratio (CMRR)

• Common mode rejection ration (CMRR) is the ability of a differential amplifier to reject the common mode signal successfully.

• CMRR is defined as the ratio of differential gain Ad and common mode gain Ac. It is denoted by letter “ρ”

• CMRR = ρ = Ad / Ac

• Ideally CMRR should be infinite and practically it should be as high as possible.


Equivalent circuit of an OP-AMP

-

+

+ VCC

-VEE

Output

Inverting input

Non-Inverting input

RL

Ro

AVVd

+

- Vo

Ri

-

+ Vd


The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-

Ri Vd= 0 Zero differential Input voltage

Ri

8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

Important characteristics of Op-Amp

1. Infinite voltage gain ( )

the open loop gain of an ideal OP-AMP is denoted by Av. It is the

differential voltage gain and its value for an ideal OP-AMP is infinite.

AV

8

Vo = AVVD 17 September 2019 58

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

2. Infinite input resistance (Ri ∞)

the input resistance Ri of an ideal OP-amp is infinite. Due to this,

the current flowing in each input terminal will be zero.

due to infinite input resistance, almost any source can drive it and

there is no loading of the source.

IB1= 0 IB2= 0


The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

3. Zero output resistance ( = )

the output resistance Ro of an ideal OP-amp is zero. Due to this,

the ideal Op-amp can handle infinite number of other devices.

RO 0


The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

4. Zero offset voltage

in practical Op-amps a small output voltage is present even though

both the inputs V1 ad V2 are having a zero value.

This voltage is called as the offset voltage.

for ideal Op-amp the offset voltage is zero.

That means output voltage is zero when input voltage is zero. 17 September 2019 61

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

5. Infinite Bandwidth

Bandwidth of an amplifier is the range of frequencies over which all

the signal frequencies are amplified almost equally.

The bandwidth of an ideal Op-amp is infinite. So it can amplify any

frequency from zero to infinite hertz.

Thus the gain of an ideal amplifier is constant from zero to infinite hertz. 17 September 2019 62

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

6. Infinite CMRR

for an Op-amp, the common mode rejection ratio (CMRR) id defined

as the ratio of differential gain to common mode gain.

CMRR is infinite for the ideal Op-amp.

Thus the output voltage corresponding to the common mode noise is zero.


The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

7. Infinite slew rate

the slew rate of an ideal Op-amp is infinite so that the output voltage

changes occur simultaneously with the input voltage changes.


The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

8. Zero power supply rejection ratio (PSRR).

PSSR is a parameter which specifies the degree of the dependence

of the Op-amp output on the changes in power supply output. For an

ideal Op-amp, PSRR = 0. that means the output voltage does not

Change due to fluctuation in supply voltage 17 September 2019 65

Important characteristics of OP-AMP IC 741

Sr. No. Characteristics Value for IC 741 Ideal value

1 Input resistance Ri 2 MΩ

2 Output resistance Ro 75 Ω 0

3 Voltage gain Av 2 X 105

4 Bandwidth BW 1 MHz

5 CMRR 90 dB

6 Slew rate S 0.5 V/μS

7 Input offset voltage 2 mV 0

8 PSRR 150 μV/V 0

9 Input bias current 50 nA 0

10 Input offset current 6 nA 0

8

8

8

8

8


Open loop configuration of OP-AMP

• The meaning of open loop operation is that there is absolutely no feedback present from the output to input.

Vd

Vo = Av Vd + +

- -

Op amp

-

+ V1

V2

0 b a

+V(SAT)

-V(SAT)

Vo = Av Vd

Vd


Open loop configuration of OP-AMP


Why Op-amp not used as an amplifier in the open loop configuration ?

• Due to very large open loop gain, distortion is introduced in the amplified output signal.

• The open loop gain does not remain constant, it varies with change in temperature and power supply.

• The bandwidth of an Op amp in open loop mode is very very small – almost zero

• For this reason the Op-amp is not used in practice as an amplifier.

• However the Op-amp in open loop configuration is used in application such as comparator.


Close loop configuration of OP-AMP

• In the closed loop configuration some kind of “feedback” is introduced in the circuit.

• A part of output is returned back or fed back to the input.

• Types of feedback

Positive feedback or Regenerative feedback

Negative feedback or Degenerative feedback.


Positive feedback or regenerative feedback

• If the feedback signal and the original input signal are in phase with each other then it is called as the positive feedback.

• Positive feedback is used in the application such as “Oscillators” and Schmitt triggers or regenerative comparators.


Negative feedback or Degenerative feedback

• If the signal is fed back to the input and the original input signal are 1800 out of phase, then it is called as the negative feedback.

• In the application of Op-amp as an amplifier, the negative feedback is used.


Negative feedback or Degenerative feedback

• In the amplifier circuits using Op-amp, a feedback resistor RF is connected between the output and inverting terminal as shown in figure to introduced a negative feedback.

• As the voltages V2 and VO are 1800 out of phase, a fraction of output voltage fed back to the input via RF will be 1800 out of phase with the input.

OP-AMP

2

3

6 -

+

V2

V1

RF

input

Output

Feedback resistor

Vo


Advantages of Negative feedback

• Negative feedback is used in the amplifier circuits as they provide the following improvements in the operation of an amplifier:

• It stabilizes the gain.

• Reduces the distortion.

• Increases the bandwidth.

• Changes the values of input and output resistances.

• Reduces the effects of variation in temperature and supply voltage on the output of the Op-amp.


Virtual short

-

+

Output

V2

V1

Ri Vd

Ri 8

I = 0

Vo = AVVD

-VEE

+VCC

The input impedance Ri of an Op-amp is ideally infinite. Hence current “I” flowing from one input terminal to the other will

be zero. Thus the voltage drop across Ri will be zero and both the input terminals will be at same potential, in other words they are virtually shorted to each other. 17 September 2019 75

OP-AMP

-

+

VS

V1

RF

Vo

R1 +

-

V2

Vd

IB2 = 0

+ -

- +

AV =

8

input

t

t 0

0

VS

VO

Expression for the closed loop voltage gain (AVF)

AVF = - RF / R1

The negative sign indicates that there is a phase shift of 1800 Between the input and output voltages.

I

The Inverting Amplifier


OP-AMP

-

+

VS

V1

RF

Vo

R1

+

-

V2

I2 = 0

+ - - +

AV =

8

input

t

t 0

0

VS

VO

I1 = 0

As input impedance of ideal Op-amp is infinite, the current entering into both the input terminals of

Op-amp will have zero values. (I1 = I2 = 0 )

The Non-Inverting Amplifier


The Voltage follower (unity gain buffer)

OP-AMP

-

+

VS

V1

RF = 0

Vo

+

-

V2

I2 = 0

+ - - +

AV =

8

I1 = 0

When R1 is infinite and RF = 0 the non-inverting amplifier gets converted into a voltage follower or unity gain.

R1 =

8


Conclusion

• Read the Instruction Manuals of equipment i.e. Car, Washing m/c, Microwave oven, Cell phone, Laptop etc.

• BJT is used rarely.

• MOSFET is matured technology & used everywhere.

• MOSFET ckts have low dissipations, high swing & integration.

• Device / Ckt / Chip / Application designers are well respected. Less effect of recession.

• Classrooms may diminish; Hands on has only meaning.

• Knowledge of E&TC is must for every branch.

• Opamp is hot topic forever !


Thank You

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