Operational AmplifiersLecture (11)

1

• What is an IC?

• What does an AMPLIFIER do?

Integrated Circuits

Inside an IC

OP-Amp is an amplifier IC

2

What can you do with Op amps? • You can make music louder when they are used

in stereo equipment.

• You can amplify the heartbeat by using them in medical cardiographs.

• You can use them as comparators in heating

systems. • You can use them for Math operations like

summing, integration etc.3

OP-AMP (operational amplifier)

• An OP-AMP is an integrated circuit (IC) used for amplification of signals.

• It is the most widely used analog IC.• It is used in control systems, instrumentation, signal

processing etc.

4

• The Operational Amplifier or "op-amp" is an amplifier with two inputs and one output. One input is the inverting input and the other is a non inverting input.

OP-AMP BLOCK DIAGRAM

Figure 1 Op Amp Block Diagram

Inverting Input (- VIN)

+V

-V

Output

Noninverting Input (+ VIN)

Differential Amplifier

Differential Amplifier

Voltage AmplifierVoltage

Amplifier

Output AmplifierOutput

Amplifier

5

OP-AMP HAS 3 –STAGE AMPLIFIER CIRCUITS

• First Stage : Differential Amplifier -it gives the OP-AMP high input impedance (resistance)

• Second Stage: Voltage Amplifier – it gives high gain

• Third Stage : Output Amplifier (Emitter Follower) – gives low output impedance (resistance)

6

OP-AMP CHARACTERISTICS

1. Very high input impedance 2. Very high gain 3. Very low output impedance

OP-AMP is a differential, voltage amplifier with high gain.

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OP-AMP is a differential, voltage amplifier with high gain. Why????

• Differential Amplifier: Because it amplifies the difference between 2 voltages

• Voltage Amplifier: Because input and output are voltages

• High Gain Amplifier: Because the voltage gain is very high (> 100,000)

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The 741 Op-amp

The most common op-amp is the 741 IC.

9

Packaging Types

Figure : Op Amp packages

)b (OPA547FKTWT

DIP SMT package)a (Op Amp 741

8-pins DIP package(c) TO-5 metal can

8-Leads package

OP-AMP pins identification

Figure 3 Op Amp pins Identification

1

2

3

4 5

6

7

8+VCC

-VEE

-

+

741

b( Notched Package

1

2

3

4 5

6

7

8+VCC

-VEE

-

+

a( Dot marked Package

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What are these pins?

Figure 4 Op Amp pins Description

1

2

3

4 5

6

7

81 .Offset Null

-VEE

8 .N / C

6.Output

741

7+ .VCC

3 .Noninverting Input +VIN

2 .Inverting Input –VIN

5.Offset Null

http://www.quia.com/pp/200743.html 11

What are these pins?• Pin 1 and Pin 5 : Offset null input, are used to remove the

Offset voltage. • Pin 2: Inverting input (-VIN), signals at this pin will be inverted

at output Pin 6.• Pin 3: Non-inverting input (+VIN), signals at pin 3 will be

processed without inversion.• Pin 4: Negative power supply terminal (-VEE). • Pin 6: Output (VOUT) of the Op-Amp• Pin 7: Positive power supply terminal (+VCC)• Pin 8: No connection (N\C), it is just there to make it a

standard 8-pin

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Symbol of OP-AMP

Figure 5 Op Amp Schematic Symbols

-VIN

+

-

+VIN

VOUT

-VIN

+

-

+VIN

VOUT

+VS

-VS

(a )Without power connection (b )With power connection

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Most Op Amps require dual power supply with common ground

Positive Supply (+15V) to pin7Negative Supply (-15V) to pin4

Figure 6 Dual Supply Voltages connection

-VIN

+

-

+VIN

VOUT

+VS

-VS

7

4

Common Ground

14

Some Op Amps work on single supply also

Figure 7 Single Supply Voltages connection

-VIN

+

-

+VIN

VOUT

-VS

7

4

-VIN

+

-

+VIN

VOUT

+VS

7

4

(a )Single Positive Voltage (b )Single Negative Voltage

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Advantage of dual power supply

Using dual power supply will let the op amp to output true AC voltage.

0V

+15V

-15V

Output

Figure 8a Op Amp powered from Dual supply

+30V

0V

Output

Figure 8b Op Amp powered from Single supply

30 V30 V

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What is dual power supply?

Single Power Supply Single Power Supply

Figure 18 Dual Power Supply

Common +15V–15V

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How can you make a dual power supply using two 9V batteries?

What is the voltage between + of first battery and – of second battery?

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OP-AMP CONFIGURATIONS

Figure Types of Feedback

(a) No Feedback (open loop comparator circuit)

(b) Negative Feedback (c) Positive Feedback

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Feedback

• No feedback : Open loop (used in comparators)

• Negative feedback : Feedback to the inverting input (Used in amplifiers)

• Positive feedback : Feedback to the non inverting input (Used in oscillators(

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OP AMP as a Comparator )compares 2 voltages and produces a signal to indicate which is greater(

+VS

VO

+VIN = –VIN

–VS

+VIN –VIN

+VIN –VIN

0VO

+VIN

+VS

–VS

–VIN

(a )Comparator Circuit (b )Comparator Output

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Applications of Comparators

• Analog to digital converters (ADC)• Counters (e.g. count pulses that exceed a

certain voltage level).• Cross Over Detectors

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OP-AMPS WITH NEGATIVE FEEDBACK

The two basic amplifier circuits with negative feedback are:

• The non-inverting Amplifier.• The inverting Amplifier

(Note: Negative feedback is used to limit the gain)

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NON-INVERTING AMPLIFIER• The input signal is applied to the non-inverting input (+VIN).

The output is fed back to the inverting input through resistor RF.

VOVIN

RFR1

Figure 11 Closed-Loop Noninverting Amplifier Circuit

Where;

VO = Output voltage

Vin= Input voltage= Vf

Ain = Non inverting Gain

IN

F

F

Oin

FIN

FINO

R

R 1

V

V A

V R

R R V

24

Problem:• Calculate the gain of a non inverting amplifier

if Rin=3K and Rf=30K. If Vin=4mV, calculate the output voltage.

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mV 444 3

303V

V R

R R V

113

30 1A

R

R 1

V

V A

O

FIN

FINO

NI

IN

F

F

ONI

INVERTING AMPLIFIER

• The input signal is applied through a series input resistor RI to the inverting input. Also, the output is fed back through RF to the same input. The noninverting input is grounded.

R

R-

V

V A

V R

R V

IN

F

F

OI

ININ

FO

Where;VO = Output voltageVIN = Input voltageAI = Inverting Gain

VO

VIN

RFRIN

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Examples of Negative Feedback Applications:

A) Inverting Amplifiers1.Summing Amplifier2.Differentiator3.Integrator B) Non Inverting Amplifiers1. Voltage Follower

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Summing Amplifier (Adder) : the inputs are added and the sum is inverted . If all resistors are of equal value, then

Vo = -(V1 + V2+ V3)

Figure 14 Summing Amplifier

V1

VO

V2

V3

0V

R1

R2

R3

RF

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Differentiator (the input is differentiated with respect to time)

Figure 16 Inverting Op-Amp as Differentiator

C

R

VINVO

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Integrator (the input is integrated with respect to time)

Figure 15 Inverting Op-Amp as Integrator

R

C

VIN

VO

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Voltage Follower (Non Inverting)

• It is a non inverting amplifier with gain=1• So the output is the same as input.

VIN

VO

Figure 17 Voltage Follower

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Positive Feedback is used in oscillators

• Astable Multivibrator (Relaxation Oscillator)

VO

R2

R3

R1C1

Figure 13 A stable Multi-vibrator32

Offset Null Adjustment (practical)

•

Figure 22 Offset Null adjustment

-VS

N/C

VO

+VS

VIN = 0

A741

1 2 3 4

5678

V

33

• offset voltage can be defined as the slight amount of voltage that appears at the output when the voltage differential (ΔVIN) between the input pins is 0 V.

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http://holbert.faculty.asu.edu/ece201/opamp.html

For more reading on op. Amplifiers