Lecture Opamp

8/8/2019 Lecture Opamp

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Institute of Space TechnologyInstitute of Space Technology Communication Systems Engineering DepartmentCommunication Systems Engineering Departmenthttp://www.ist.edu.pk

Electronic DevicesBatch 07, Fall 2009

Operational Amplifiers


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Operational Amplifiers

Early, OpAmps were used primarilyto performsmathematical operations such as addition,

subtraction, integration and differentiation Hence the name OPERATIONAL

Earlymade from vacuum tubes and consumeshigh power

Nowa days, these are linear Integrated Circuitsand consumes less power


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Symbols and Terminals

It has two inputs

(-) inverting input

(+) non-inverting input Typically, operates with two power supplies

One positive and other negative


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Ideal Op-Amp

Infinite Voltage gain

Infinite bandwidth

Infinite input impedance (open)

Zero output impedance

Input is applied between the two terminals

Av is the voltage gain


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Practical Op-Amp

Op Amp has both voltage and current limitationsas the case with all the devices

Peak-to-peak output voltage is usuallylimited toslightlyless than the two power supplies

Practical Op-Amp

Veryhigh voltage gain

Veryhigh Input impedance Verylow output impedance

Wide bandwidth


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Internal block diagram


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Differential Amplifier

Amplifies the difference between the two signalsapplied to the inputs

The first stage ofan Op-Amp is a differentialamplifier


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Modes of Operation

There are three modes of operation for adifferentialamplifier

Single E

nded mode Differential or Double ended

Common Mode

Since the input stage ofan Op amp is adifferentialamplifier

Op-Amp has the same input modes


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Single-Ended Mode

One input is grounded

Signal is applied to the other input

When the signal is applied to the inverting input,the output is inverted and amplified

When the signal is applied to the non-invertinginput, the output is amplified but not inverted


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Differential Mode

Two opposite polaritysignals are applied to theinputs

The amplified difference between the inputsappears at the output

Also called double ended


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Common Mode

Two signal voltages of the same frequency, phaseand amplitude are applied to the two inputs

Theycancel each other and the out put is zero The action is called

COMMOM MODEREJECTION

An unwanted signalappears at both the inputs

But it will not appear at the output Unw anted signal-noise


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Common Mode Rejection Ratio

The abilityofan amplifier to reject the commonunwanted signals is called the CMRR

Zero gain for the unwanted noisysignals


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Op Amp 741


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Inverting Amplifier

Open loop gain maybe veryhigh i.e. 1,000,000

Of no use as it is unstable

Onlya verysmall input produces veryhigh input Noise

1mv input produces 1mv x 1,000,000 = 1,000 V

Means the Op Amp can easilybe saturated by

noise We can compromise some gain to achieve the

stability


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Inverting Amplifier

Some portion of the output is fed back via feedbackresistor Rf to the inverting input introducing anegative feed back

Input signal is now the sum of the actual inputsignaland the some portion of the inverted output

The gain of the amplifier is now called closed loopgain


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Inverting Amplifiers

Two important facts to remember

No input current flows into the terminal

And V1 = V2 Theyare at the same potential


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Closed loop gain

x Closed loop gain is given as


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Example

Find the closed loop gain


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Non Inverting Amplifier


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Voltage gain


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Voltage Follower

Unitygain buffer


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Summing Amplifier


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Example


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Differential Amplifier


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Integrator


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Differentiator

Lecture Opamp

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