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Basic Block Diagram of Op-Amp. V 1. Out put Stage (Buffer). Level Shifter. Gain Stage (C E Amp.). Input Stage (Diff. Amp.). I / P. V O. An Op-Amp can be conveniently divided in to four main blocks An Input Stage or Input Diff. Amp. The Gain Stage The Level Translator An Out put Stage

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Basic block diagram of op amp

Basic Block Diagram of Op-Amp

V1

Out put Stage (Buffer)

Level Shifter

Gain Stage (C E Amp.)

Input Stage

(Diff. Amp.)

I/P

VO

An Op-Amp can be conveniently divided in to four main blocks

An Input Stage or Input Diff. Amp.

The Gain Stage

The Level Translator

An Out put Stage

Note: It can be used to perform various mathematical operations such as Addition, Subtraction, Integration, Differentiation, log etc.

V2

Op-Amp IC


An ideal op amp
An IDEAL OP AMP

An ideal op amp has the following characteristics:

  • Infinite open-loop voltage gain, AV≈ ∞.

  • Infinite input resistance, Ri≈∞.

  • Zero output resistance, Ro≈ 0.

  • Infinite CMRR, ρ =∞

  • The output voltage Vo=0; when Vd = V2-V1= 0

  • Change of output with respect to input, slew rate = ∞

  • Change in out put voltage with Temp., ∂Vo/∂Vi=0



The Operational Amplifier

+VS

_

i(-)

Inverting

RO

A

vid

Output

Ri

vO = AdVid

Noninverting

i(+)

+

-VS

  • i(+), i(-) : Currents into the amplifier on the inverting and non-inverting lines respectively

  • vid : The input voltage from inverting to non-inverting inputs

  • +VS , -VS : DC source voltages, usually +15V and –15V

  • Ri : The input resistance, ideally infinity

  • A : The gain of the amplifier. Ideally very high, in the 1x1010 range.

  • RO: The output resistance, ideally zero

  • vO: The output voltage; vO = AOLvid where AOL is the open-loop voltage gain


Operational amplifier model

+

-

Operational Amplifier Model

  • An operational amplifier circuit is designed so that

    • Vout = Av(V1-V2) (Avis a very large gain)

    • Input resistance (Rin) is very large

    • Output resistance (Rout) is very low

V1

Rout

Vout

Rin

Av(V1- V2)

V2


Practical op amp circuits
Practical Op-Amp Circuits

These Op-amp circuits are commonly used:

  • Inverting Amplifier

  • Noninverting Amplifier

  • Unity Follower

  • Summing Amplifier

  • Integrator

  • Differentiator



Noninverting Amplifier

Notice the output formula is similar to Inverting Amplifier, but they are not the same.


Summing Amplifier

Because the op-amp has a high input impedance the multiple inputs are treated as separate inputs.



Inverting Amplifier: Input and Output Resistances

Rout is found by applying a test current (or voltage) source to the amplifier output and determining the voltage (or current) after turning off all independent sources. Hence, vs= 0

But i1=i2

Since v- = 0, i1=0. Therefore vo= 0 irrespective of the value of io.


Differential Amplifier Using Op Amp

I/P Current to op amp is zero



The Unity-Gain Amplifier or “Buffer”

  • This is a special case of the non-inverting amplifier, which is also called a voltage follower, with infinite R1 and zero R2.

  • Hence Av = 1.

  • It provides an excellent electrical isolation while maintaining the signal voltage level.

  • The “ideal” buffer requires no input current and can drive any desired load resistance without loss of signal voltage.

  • Such a buffer is used in many sensor and data acquisition system applications.


Unity-Gain Buffer

Closed-loop voltage gain

Used as a "line driver" that transforms a high input impedance (resistance) to a low output impedance. Can provide substantial current gain.



Op-Amp Integrator Cont…

Since the inverting input is at virtual ground

Applying KCL at the inverting input

i1+i2 = 0


Op amp differentiator circuit
Op-Amp Differentiator Circuit


Op-Amp Differentiator Cont…

Since the inverting input is at virtual ground

Applying KCL at the inverting input

i1+i2 = 0

Differentiators are avoided in practice as they amplify noise


Instrumentation Amplifier

NOTE

Combines 2 non-inverting amplifiers with the difference amplifier to provide higher gain and higher input resistance.

Gain can be varied by varying single resistor R1

Ideal input resistance is infinite because input current to both op amps is zero. The CMRR is determined only by Op Amp 3.


Finite Open-loop Gain and Gain Error

This is the “ideal” voltage gain of the amplifier. If Ab is not >>1, there will be “Gain Error”.

is called the feedback factor.


  • Gain Error is given by

    GE = (ideal gain) - (actual gain)

    For the non-inverting amplifier,

  • Gain error is also expressed as a fractional or percentage error.


Output Voltage and Current Limits

Practical op amps have limited output voltage and current ranges.

Voltage: Usually limited to a few volts less than power supply span.

Current: Limited by additional circuits (to limit power dissipation or protect against accidental short circuits).

The current limit is frequently specified in terms of the minimum load resistance that the amplifier can drive with a given output voltage swing. Eg:

For the inverting amplifier,



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