Operating Amplifier

8/9/2019 Operating Amplifier

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Operational Amplifier

Operational Amplifier • A combination of transistors, resistors, and (sometimes)

capacitors that Amplifies the difference between two inputvoltages and produces a single output – Original application — analog computers

– Original construction — discrete components• Vacuum tubes• Transistors• Now — an integrated circuit

– Applications now — extremely broad

– Called operational amplifier due to the use of this amplifier to performspecific electronic circuit functions or operations, such as summation,integration, differentiation, etc.


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• Notch – Counterclockwise numbering

• Input – #2 V -; #3 V +

• Output – #6 V out

• Power supply – #4 –V DD; #7 +V CC

• #1, #5, offset adjustment

741 Operational Amplifier


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Operational Amplifier • Op-Amp Parameters

– Aod = differential or open-loopgain

– Output:• 180° out of phase with v1

(inverting)• In phase with v2 (noninverting)

• Op-amp responds only todifferences between v2 and v1 – Common-mode signal when v2 =

v1 ≠ 0 – Characteristic called “common-

mode rejection”


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Op-amp Parameters

Inverting Amplifier


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Subtractor • Used to subtract analog

signals• Output signal is

proportional to differencebetween two inputs

If all resistors are equal:

Integrator • Replace feedback resistor of

inverting op-amp withcapacitor

• A constant input signalgenerates a certain rate ofchange in output voltage

• Smoothes signals over time


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Differentiator • Input resistor of inverting

op-amp is replaced with acapacitor

• Signal processing methodwhich heighten noise overtime

• Output signal is scaledderivative of input signal

Non-Ideal Characteristics• Offset

– DC input is zero but output is not zero – Adjust using pins 1 and 5

-+

DM

1 k Ω

10 k Ω

10 k pot

-15 V


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OpAmp non-idealities II• Output voltage swing: real OpAmp has a maximum and minimum

limit on the output voltagesOpAmp transfer characteristic is nonlinear, which causesclipping at output voltage if input signal goes out of linear rangeThe range of output voltages before clipping occurs depends onthe type of OpAmp, the load resistance and power supplyvoltage.

• Output current limit: real OpAmp has a maximum limit on the outputcurrent to the load

The output would become clipped if a small-valued loadresistance drew a current outside the limit

• Slew Rate (SR) limit: real OpAmp has a maximum rate of change ofthe output voltage magnitude

limitSR can cause the output of real OpAmp very different from anideal one if input signal frequency is too highFull Power bandwidth: the range of frequencies for which theOpAmp can produce an undistorted sinusoidal output with peakamplitude equal to the maximum allowed voltage output

SRdt

dvo ≤

max2 oFP v

SR f

π =

Slew Rate

Linear RC Step Response: the slope of the step response is proportional to the final value of the output, that is, if weapply a larger input step, the output rises more rapidly.

If Vin doubles, the output signal doubles at every point,therefore a twofold increase in the slope.

But the problem in real OpAmp is that this slope can notexceed a certain limit.

Copyright © Mcgraw Hill Company


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OpAmp non-idealities III• DC imperfections: bias current, offset current and offset voltage

bias current : the average of the dc currents flow into the noninvertingterminal and inverting terminal ,offset current: the difference of the two currents,offset voltage: the DC voltage needed to model the fact that the output isnot zero with input zero,

• The three DC imperfections can be modeled using DC current and voltagesources

• The effects of DC imperfections on both inverting and noninverting amplifieris to add a DC voltage to the output. It can be analyzed by considering theextra DC sources assuming an otherwise ideal OpAmp

• It is possible to cancel the bias current effects. For the inverting amplifier, wecan add a resistor to the non-inverting terminal

B I

−+ −=

B Boff I I I )(2/1 −+ += B B B I I I + B I − B I

off V

B I

+ B I

− B I

off V

2/off I

B I Ideal

21 // R R R =

Copyright © Mcgraw Hill Company

When Vin=0, Vout is NOT 0 due to mismatch of transistors in real circuitdesign.

It is more meaningful to specify input-referred offset voltage, defined asVos,in=Vos,out / A.

Offset voltage may causes a DC shift of later stages, also causes limitedprecision in signal comparison.

DC offset of an differential pair


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Important amplifier circuits I• Inverting amplifer

• Summing amplifier

• Noninverting amplifier

0

/

1

12

=

=

−=

out

in

v

Z

R Z

R R A

0

/

2

1

/

=

=

=

−=

out

B Bin

A Ain

B A f v

Z

v for R Z

v for R Z

R R A

0

/1 12

=

∞=

+=

out

in

v

Z

Z

R R A

Graphs from Prentice Hall

Important amplifier circuits II• Differential amplifier

• Instrumentation qualify Diff Amp

• Voltage-to-current converter

• Howland voltage-to-currentconverter for grounded load

• Current-to-voltage amplifier

• Current amplifier

0

143=

+=

out

in

Z

v for R R Z

0=

∞=

out

in

Z

Z

∞=

∞=

−==

out

in

f inom

Z

Z

RviG /1/

∞=

+=

−=

out

Lin

m

Z

R R R R Z

RG

)/(

/1

221

2

0

0=

=

−=

out

in

f m

Z

Z

R R

∞=

=

+−=

out

in

vi

Z

Z

R R A

0

)/1( 12



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Important amplifier circuits III• Integrator circuit: produces an

output voltage proportional tothe running time integral of theinput signal

• Differentiator circuit: producesan output proportional to thetime derivative of the inputvoltage


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Operating Amplifier