Chapter 10: Operational Amplifiers - الصفحات الشخصية | الجامعة...

Chapter 10:

Operational Amplifiers

Copyright ©2009 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/e

Robert L. Boylestad and Louis Nashelsky

Differential Amplifier

2

Differential amplifier has two identical transistors with two inputs

and two outputs.






3

Differential amplifier has two identical transistors with two inputs

and two outputs.





DC Analysis -Differential Amplifier

4

.





AC Analysis -Differential Amplifier

5

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Use of Current Source

6






Operational amplifier or op-amp, is a very high gain differential

amplifier with a high input impedance (typically a few meg-Ohms)

and low output impedance (less than 100 Ω).

Note the op-amp has two inputs and one output.

7





Single Ended Input

8





Double-Ended (Differential) Input

9





Single and Double-Ended Output

10





Differential Input and Differenial Output

11

Common Mode Operatin





Common Mode Rejection Ratio

Differential inputs

= −

Common inputs

=

+

Output Voltage

o = +

Common Mode Rejection Rate

=

= !"#

($%)

12





Common Mode Rejection Ratio

Example: Calculate the for the circuit measurements shown

13

.





Op-Amp Gain

Op-Amps have a very high gain. They can be connected open-loop or

closed-loop.

• Open-loop refers to a configuration where there is no feedback

from output back to the input. In the open-loop configuration

the gain can exceed 10,000.

• Closed-loop configuration reduces the gain. In order to control

the gain of an op-amp it must have feedback. This feedback is a

negative feedback. A negative feedback reduces the gain and

improves many characteristics of the op-amp.

14





Inverting Op-Amp

• The signal input is applied to the inverting (–) input

• The non-inverting input (+) is grounded

• The resistor Rfis the feedback resistor. It is connected from the output to

the negative (inverting) input. This is negative feedback.

15





Inverting Op-Amp GainGain can be determined from

external resistors: Rfand R

1

Unity gain—voltage gain is 1

The negative sign denotes a 180°

phase shift between input and

output.

1

f

i

ov

R

R

V

VA ==

1R

RA

RR

1

fv

1f

−=

−

=

=

Constant Gain—Rfis a multiple of R

1

16





Virtual GroundAn understanding of the

concept of virtual ground

provides a better

understanding of how an op-

amp operates.

The non-inverting input pin is

at ground. The inverting input

pin is also at 0 V for an AC

signal.

The op-amp has such high input impedance

that even with a high gain there is no

current from inverting input pin, therefore

there is no voltage from inverting pin to

ground—all of the current is through Rf.

17





Practical Op-Amp Circuits

Inverting amplifier

Noninverting amplifier

Unity follower

Summing amplifier

Integrator

Differentiator

18





Inverting/Noninverting Op-Amps

1

1

fo V

R

RV

−

=

Inverting Amplifier Noninverting Amplifier

1

1

fo V)

R

R1(V +=

19





Unity Follower

1o VV =

20





Summing Amplifier

Because the op-amp has a

high input impedance, the

multiple inputs are

treated as separate inputs.

++−= 3

3

f2

2

f1

1

fo V

R

RV

R

RV

R

RV

21





Integrator

The output is the integral

of the input. Integration

is the operation of

summing the area under

a waveform or curve over

a period of time. This

circuit is useful in low-

pass filter circuits and

sensor conditioning

circuits.

∫−= (t)dtvRC

1(t)v 1o

22





Differentiator

The differentiator

takes the derivative of

the input. This circuit

is useful in high-pass

filter circuits.

dt

(t)dvRC(t)v 1

o −=

23





Op-Amp Specifications—DC Offset

Parameters

• Input offset voltage

• Input offset current

• Input offset voltage and input offset current

• Input bias current

Even when the input voltage is zero, there can be an

output offset. The following can cause this offset:

24





Input Offset Voltage (VIO

)

The specification sheet for an op-amp indicate an

input offset voltage (VIO

).

The effect of this input offset voltage on the output

can be calculated with

1

f1IOo(offset)

R

RRVV

+

=

25





Output Offset Voltage Due to Input Offset

Current (IIO

)

• The input offset Current (IIO

) is specified in the specifications

for the op-amp.

• The effect on the output can be calculated using:

fIO)I to dueo(offset RIVIO

=

If there is a difference between the dc bias currents for the same

applied input, then this also causes an output offset voltage:

26





Output Offset Voltage Due to Input Offset

Current (IIO

)

fIO)I to dueo(offset RIVIO

=

27





Total Offset Due to VIO

and IIO

Op-amps may have an output offset voltage due to both

factors VIO

and IIO

. The total output offset voltage will be

the sum of the effects of both:

)I to due(offset V)V to due(offset V(offset)VIOoIOoo

+=

28





Example

Calculate the total offset voltage for the given circuit for the op-amp with

specified values of input offset voltage = 4 mV and input offset current

=150 nA.

)I to due(offset V)V to due(offset V(offset)VIOoIOoo

+=

29





Input Bias Current (IIB

)

A parameter that is related to input offset current (IIO

) is called

input bias current (IIB

)

The separate input bias currents are:

The total input bias current is the average:

2

III

IO

IBIB+=

+

2

II I

IO

IBIB−=

−

2

III

IBIBIB

+−+

=

30





An op-amp is a wide-bandwidth amplifier. The following

affect the bandwidth of the op-amp:

• Gain

• Slew rate

Frequency Parameters

31





Gain and Bandwidth

The op-amp’s high frequency

response is limited by

internal circuitry. The plot

shown is for an open loop

gain (AOL

or AVD

). This means

that the op-amp is operating

at the highest possible gain

with no feedback resistor.

In the open loop, the op-amp

has a narrow bandwidth. The

bandwidth widens in closed-

loop operation, but then the

gain is lower.

32





Slew Rate (SR)

Slew rate (SR) is the

maximum rate at which an

op-amp can change output

without distortion.

The SR rating is given in

the specification sheets as

V/µs rating.

s)V/ (in Δt

ΔVSR o

µ=

33





Slew Rate (SR) Example

Example: For an op-amp having a slew rate of SR = 2V/μs, what

is the maximum closed loop voltage gain that can be used when

the input signal varies by 0.5 V in 10 μs .

Solution

Since o = ,

∆o

∆=

∆

We get =

∆/∆=

34





Maximum Signal Frequency

The slew rate determines the highest frequency of

the op-amp without distortion.

where VP

is the peak voltage

pVπ2

SRf ≤

35





Maximum Signal Frequency

Example: For the circuit in the figure, determine the maximum

frequency that may be used. Op-amp slew rate is SR = 0.5 V/μs

Solution

For a gain of magnitude

,

.

. rad/s

36





General Op-Amp Specifications

Other ratings for op-amp found on specification sheets

are:

• Absolute Ratings

• Electrical Characteristics

• Performance

37





Absolute Ratings

These are common

maximum ratings

for the op-amp.

38





Electrical Characteristics

Note: These ratings are for specific circuit conditions, and they often

include minimum, maximum and typical values.

39





CMRR

One rating that is unique to op-amps is CMRR or common-mode

rejection ratio.

Because the op-amp has two inputs that are opposite in phase

(inverting input and the non-inverting input) any signal that is common

to both inputs will be cancelled.

Op-amp CMRR is a measure of the ability to cancel out common-mode

signals.

40





Op-Amp Performance

The specification sheets will also

include graphs that indicate the

performance of the op-amp over

a wide range of conditions.

41

Chapter 10: Operational Amplifiers - الصفحات الشخصية | الجامعة...

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