ECA1212Introduction to Electrical &

Electronics EngineeringChapter 7: Operational Amplifier

by Muhazam Mustapha, November 2011

Learning Outcome

• Be able to explain and solve some problems involving operational amplifier

By the end of this chapter students are expected to:

Chapter Content

• Reason for Op Amp

• Regular Op Amp Circuits

Why Operational Amplifier

CO1 Text Book page 410

Need for Semi-Finished Amplifier

CO1 Text Book page 412

• The kind of amplifier that we can build from BJT as we learnt in Chapter 5 was constructed using raw components

• We need something more robust, flexible and easier to install

Need for Semi-Finished Amplifier

CO1 Text Book page 413

• Operational Amplifier (Op-Amp) is a semi-finished amplifier fabricated on IC

• Its gain and operation is easily adjustable using some combination of resistors

Fundamentals

CO1 Text Book page 413

• Input current iin is assumed to be zero because the input resistance is very high

• Virtual ground: inverting (v−) and noninverting (v+) inputs are assumed to be at the same voltage but no current going to ground (close loop mode)

Inverting input

Noninverting input

OutputVin

v+

v−

Vout

iin

• Symbol:• Gain (A, open loop gain):

vout = Avin

– Value is high ranging 105 ~ 107

Op-Amp Circuits

CO1 Text Book page 414

Close Loop Mode

• Most of the time op-amp is used in close loop mode

• This is done by putting a resistor from the output to the input (mostly the inverted one)

• The resistor is the called feedback resistor

• This feedback resistor pulls down the voltage gain to a more manageable value– But we won’t discuss the detail

CO1

Close Loop Mode

• In close loop mode the previous assumptions are always applied:

CO1

iin = 0

v− = v+

Text Book Eq. 8.19, 8.37, etc

• For all circuits that we are going to cover, we won’t go through into the derivation details, but references will be made to the respective pages in the text book

Inverting Amplifier

• Circuit:

CO1 Text Book page 413-417, Example 8.1 partly

• Voltage Gain:

S

F

S

out

R

R

v

v

Summing Amplifier

• Circuit:

CO1 Text Book page 418

• Voltage Gain:

N

nS

S

Fout n

n

vR

Rv

1

Noninverting Amplifier

• Circuit:

CO1 Text Book page 419

• Voltage Gain:

S

F

S

out

R

R

v

v1

Voltage Follower

• Circuit:

CO1 Text Book Example 8.2

• Voltage Gain:

outS vv

Differential Amplifier

• Circuit:

CO1 Text Book page 421-422

• Voltage Gain:

)( 121

2 vvR

Rvout

Integrator

• Circuit:

CO1 Text Book page 433, Example 8.7

• Output Voltage:

dtvCR

v SFS

out

1

vout and vS are functions of time

Differentiator

• Circuit:

CO1 Text Book page 449

• Output Voltage:

dt

dvCRv s

SFout

vout and vS are functions of time