L1: Basics of OP-AMP L2: Parameters of ideal and practical OP-AMP L3: Concept of positive and...

L1: Basics of OP-AMP

L2: Parameters of ideal and practical OP-AMP

L3: Concept of positive and negative feedback.

L4: Advantages of negative feedback.

L5: Applications- Inverting, non-inverting, difference,

summing, differentiator, integrator, comparators.

L6: OP-AMP waveform generator sine, square and triangular

UNIT – III : OP-AMPS AND APPLICATIONS

• Op-amp is basically a multistage amplifier which uses a number of amplifier stages interconnected to each other.

• The amplifier which could be configured to perform a variety of operations such as amplification, addition, subtraction, differentiation and integration.

• Hence the name is operational amplifier (OP-AMP)

• The integrated Op-amp offers all the advantages of monolithic integrated circuits such as small size, high reliability, reduced cost, less power consumption.

• μA741 is extremely popular and was used in a variety of applications.

Symbol and terminal

741

-Inverting input

Symbol and terminal

741

-Inverting input

+Non-Inverting input

Symbol and terminal

741

-Output

Inverting input

+Non-Inverting input

Symbol and terminal

741

-

+

+VCC positive supply voltage

-VEE negative supply voltage

Output

Inverting input

Non-Inverting input

Symbol and terminal

741

2

34

76

-

+

+VCC positive supply voltage

-VEE negative supply voltage

Output

Inverting input

Non-Inverting input

Input and output signals 1800 phase shift when the input signal is applied to the inverting (-) terminal

741

2

34

76

-

+

+VCC

-VEE

Vo

Inverting inputinput

Inverted Output signal

Input and output signals 00 phase shift when the input signal is applied to the Non-inverting (+) terminal

741

2

34

76

-

+

+VCC

-VEE

Vo

Non-Invertinginput

inputNon-Inverted Output signal

DC power supply for an OP-AMP

741

2

34

76

-

+

+ VCC

-VEE

Output

Inverting input

Non-Inverting input


741

2

34

76

-

+

+ VCC= +15V

-VEE = -15V

Output

Inverting input

Non-Inverting input


741

2

34

76

-

+

+ VCC

-VEE

Output

Inverting input

Non-Inverting input

+15V

-15V

Dual polarity supply


741

2

34

76

-

+

+ VCC

-VEE

Output

Inverting input

Non-Inverting input

+15V

Negative supply is connected to ground

Single polarity supply

• Ideal differential amplifier

• An ideal differential amplifier is expected to amplify the differential signal present between its two input signal.

• It is also the basic stage of an integrated Op-amp with differential input.

V2V1

Vd

Vo = V1 – V2++

--

Block diagram of an ideal differential amplifier

Ideal DifferentialAmplifier


V2V1

Vd

Vo = V1 – V2++

--

• Differential input signal :

• The difference between the input signals V1 and V2 is called as the differential signal Vd

• Differential signal Vd = V1 – V2

• From the equation it is clear that the amplifier output will be non-zero if and only if the differential signal is non-zero value


V2V1

Vd

Vo = V1 – V2++

--

Differential gain :• Vo = Ad ( V1 – V2 )• Where Ad is called as the differential gain.• The differential gain can be defined as the gain with which the

differential amplifier amplifies the differential signal.• Vo = Ad Vd as Vd = V1 – V2• Therefore the expression for the gain Ad = Vo / Vd• In decibels Ad (dB) =20 log10 [ Vo / Vd ]


V2V1

Vd

Vo = V1 – V2++

--

Common mode signal :

• A common signal to both the input terminals ( i.e. V1=V2=V) is called as common mode signal.

• The output voltage produced by an ideal differential amplifier is zero for the common mode signal.


V2V1

Vd

Vo = V1 – V2++

--

• Common mode gain :• The output voltage of an ideal differential amplifier will be

zero if V1 = V2 = V.• However equation does not describe a practical differential

amplifier.• In practice output voltage Vo of a differential amplifier

depends not only on the differential signal ‘Vd’ but it also depends on an average voltage level called “ common mode signal Vc”.

• Vc = ( V1 + V2 ) / 2• The gain with which a practical differential amplifier

amplifies the common mode signal ( Vc) is called as the “ common mode gain Ac”

• Vo = Ac Vc


V2V1

Vd

Vo = V1 – V2++

--

• Common mode gain :

• The total voltage of a differential amplifier is given by :• Vo = Ad Vd + Ac Vc


V2V1

Vd

Vo = V1 – V2++

--

• Common mode gain :

• The total voltage of a differential amplifier is given by :

• Vo = Ad Vd + Ac Vc

• For an ideal differential amplifier the differential gain Ad should be infinite and the common mode gain Ac should be zero, so the output voltage is proportional only to the differential input signal.


V2V1

Vd

Vo = V1 – V2++

--

• Common mode rejection ration (CMRR) :

• Common mode rejection ration (CMRR) is the ability of a differential amplifier to reject the common mode signal successfully.

• CMRR is defined as the ratio of differential gain Ad and common mode gain Ac. It is denoted by letter “ρ”

• CMRR = ρ = Ad / Ac

• Ideally CMRR should be infinite and practically it should be as high as possible.

Pin configuration of OP-AMP IC 741

Equivalent circuit of an OP-AMP

-

+

+ VCC

-VEE

Output

Inverting input

Non-Inverting input

RL

Ro

AVVd

+

- Vo

Ri

-

+Vd

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-

RiVd= 0Zero differentialInput voltage

Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

• Important characteristics of Op-Amp

1. Infinite voltage gain ( )

the open loop gain of an ideal OP-AMP is denoted by Av. It is the

differential voltage gain and its value for an ideal OP-AMP is infinite.

AV 8

Vo = AVVD

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

2. Infinite input resistance (Ri ∞)

the input resistance Ri of an ideal OP-amp is infinite. Due to this,

the current flowing in each input terminal will be zero.

due to infinite input resistance, almost any source can drive it and there is no loading of the source.

IB1= 0 IB2= 0

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

3. Zero output resistance ( = )

The output resistance Ro of an ideal OP-amp is zero. Due to this,

the ideal Op-amp can handle infinite number of other devices.

Ro 0

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVd

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

4. Zero offset voltage

In practical Op-amps a small output voltage is present even though

both the inputs V1 ad V2 are having a zero value.

This voltage is called as the offset voltage.

for ideal Op-amp the offset voltage is zero.

That means output voltage is zero when input voltage is zero.

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

5. Infinite Bandwidth

Bandwidth of an amplifier is the range of frequencies over which all

the signal frequencies are amplified almost equally.

The bandwidth of an ideal Op-amp is infinite. So it can amplify any

frequency from zero to infinite hertz.

Thus the gain of an ideal amplifier is constant from zero to infinite hertz.

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

6. Infinite CMRR

for an Op-amp, the common mode rejection ratio (CMRR) id defined

as the ratio of differential gain to common mode gain.

CMRR is infinite for the ideal Op-amp.

Thus the output voltage corresponding to the common mode noise is zero.

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

7. Infinite slew rate.

the slew rate of an ideal Op-amp is infinite so that the output voltage

changes occur simultaneously with the input voltage changes.

The ideal OP-AMP

-

+

Output

V2

V1

Ro

AVVD

+

-


Ri 8

Ro 0

AV 8

IB2= 0

IB1= 0

Vo = AVVD

8. Zero power supply rejection ratio (PSRR).

PSSR is a parameter which specifies the degree of the dependence

of the Op-amp output on the changes in power supply output. For an

ideal Op-amp, PSRR = 0. that means the output voltage does not

Change due to fluctuation in supply voltage

The practical characteristics of OP-AMP

• The Op-amp characteristics are important in practice because we can use them to compare the performance of various Op-amp ICs and select the best suitable from them for the required application.

• Op-amp characteristics are classified into two categories namely DC characteristics and AC characteristics.

• The DC characteristics include input bias current, input offset current, input offset voltage and thermal drift.

• Where as the AC characteristics include the frequency response, stability of Op-amp, frequency compensation, slew rate etc.


• Open loop gain (Av) :

• Open loop gain of a practical Op-amp is not infinite. It is in the range of a few thousands.

• The open loop gain of IC 741 is 2 X 105.


• Input resistance (Ri) :

• Input resistance of a practical Op-amp is few MΩ.

• For IC 741 the input resistance is 2 MΩ.

• For Op-amps having FET differential input stage the input resistance can be in the GΩ range.

• (1 GΩ = 1 X 109Ω)


• Output resistance (Ro) :

• Output resistance of a practical Op-amp is few ohms.

• For IC 741 the output resistance is 75Ω.


• Bandwidth (BW) :

• Practical Op-amp do not have infinite bandwidth, they have the bandwidth of a few hundred KHz.

• For IC 741 the bandwidth is 1 MHz.


• Input offset voltage (VIOS) :• Ideally, for a zero input voltage, the Op-amp output voltage

should be zero.• But practically it is not so. This is due to the unavoidable

unbalances inside the Op-amp, specially the unbalances in its differential input stage.

• So we have to apply a small differential voltage at the input of the Op-amp to make the output voltage zero, which is called Input offset voltage.

• The input offset voltage is denoted by Vios.• This input offset voltage is normally in a few mV range.• The value of input offset voltage is temperature dependent.


• Input bias current (IB) :

• Input bias current IB is the average of the currents flowing into the two input terminals of the Op-amp.

• Ideally the currents IB1 and IB2 must be zero.

• But for practical Op-amp they do exist due to the finite value of input resistance Ri. Due to slight difference in the characteristics of the transistors used in the input stage of an Op-amp, the two currents IB1 and IB2 are not equal.

• The maximum value of IB is 50nA for IC 741.

• It can reduced to pA level using FET Op-amps.

• The value of input bias current is temperature dependent.


• Input offset current (IIOS) :• The algebraic difference between the currents flowing into the

inverting and non-inverting terminals of Op-amp is called as “input offset current”.

• IIOS = IB1 – IB2

• Ideally, the input offset current must be zero and practically it should be as small as possible.

• The input offset current exists due to the unequal currents IB1 and IB2 flowing into the input terminals of the Op-amp.

• The input offset currents for the Op-amp is few tens or hundreds of nA.

• For IC 741 the maximum input offset current is 6 nA.


• Common mode rejection ratio (CMRR) :

• CMRR of a practical Op-amp is not infinity.

• However it is very high.

• For IC 741 the CMRR is 90 dB or 31622.

• Such high CMRR is helps to reject the common mode signals such as noise successfully.


• Power supply rejection ratio (PSRR) :

• The change in the Op-amps input offset voltage caused by variation in the supply voltage is called as power supply rejection ratio (PSRR).

• It is also called as supply voltage rejection ratio (SVRR) .

• PSRR is expressed either in microvolt per volt or in decibels.

• For IC 741, PSRR = 150 μV/V


• Slew rate :

• Slew rate is defined as the maximum rate of change of output voltage per unit time and it is expressed in volts/microseconds.


• Importance of Slew rate :

• Slew rate decides the capability of Op-amp to change its output rapidly, hence it decides the highest frequency of operation of a given Op-amp.

• Slew rate changes with change in voltage gain. Therefore it is generally specified at unit gain.

• Slew rate should be ideally infinite and practically as high as possible.

• Slew rate of IC 741 op-amp is only 0.5 V/μV

Block Diagram of Op-Amp

Input Stage

• Dual i/p, balanced o/p differential amplifier• High voltage gain• High i/p impedance• Two i/p terminals• Small i/p offset voltage (Vios)• Small i/p offset current (Iios)• High CMRR• Low input bias current

Intermediate Stage

• Another differential amplifier with dual i/p, unbalanced o/p i.e. Single ended o/p.

• Gain requirement from Op-Amp is very high.• Alone single i/p stage can’t provide this req.• So this stage provides additional high gain.• It provides chain of cascaded amplifiers.

Level shifting stage• All stages are coupled to each other directly

without using coupling capacitors.• Hence DC quiescent voltage level of previous

stage gets applied as the i/p to the next stage.• Hence stage by stage DC level increases well

above ground potential.• Hence transistors may drives into saturation &

produces distortions in the o/p.• This level shifting stage brings DC level down to

ground potential.

Output Stage

• Low o/p impedance.• Short circuit protection.• Low quiescent power dissipation.• Large o/p voltage swing capability.• Low o/p current swing capability.• Push-pull amplifier in Class AB or Class B satisfies

all above requirements.

Manufactures of OP-AMP IC 741

• The manufactures of Op-amp ICs are companies like Fairchild, National semiconductor, Motorola, Texas Instruments and signetics.

• The identifying initials for some other companies are as follows:

1. National semiconductors : LM 741

2. Motorola : MC 741

3. RCA : CA 741

4. Texas instruments : SN 52741

5. Signetics : N 5741

Important characteristics of OP-AMP IC 741

Sr. No. Characteristics Value for IC 741 Ideal value

1 Input resistance Ri 2 MΩ

2 Output resistance Ro 75 Ω 0

3 Voltage gain Av 2 X 105

4 Bandwidth BW 1 MHz

5 CMRR 90 dB

6 Slew rate S 0.5 V/μS

7 Input offset voltage 2 mV 0

8 PSRR 150 μV/V 0

9 Input bias current 50 nA 0

10 Input offset current 6 nA 0

88

88

8

Open loop configuration of OP-AMP

• The meaning of open loop operation is that there is absolutely no feedback present from the output to input.

Vd

Vo = Av Vd++

--

Op amp

-

+V1

V2

0ba

+V(SAT)

-V(SAT)

Vo = Av Vd

Vd

Why Op-amp not used as an amplifier in the open loop configuration ?

• Due to very large open loop gain, distortion is introduced in the amplified output signal.

• The open loop gain does not remain constant, it varies with change in temperature and power supply as well as due to mass production technique.

• The bandwidth of an Op amp in open loop mode is very very small – almost zero,

• For this reason the Op-amp is not used in practice as an amplifier.

• However the Op-amp in open loop configuration is used in application such as comparator.

Close loop configuration of OP-AMP

• In the closed loop configuration some kind of “feedback” is introduced in the circuit.

• A part of output is returned back or fed back to the input.

• Types of feedback

• Positive feedback or regenerative feedback

• Negative feedback or degenerative feedback.

Concept of Feedback

Positive feedback or regenerative feedback

• If the feedback signal and the original input signal are in phase with each other then it is called as the positive feedback.

• Positive feedback is used in the application such as “Oscillators” and Schmitt triggers or regenerative comparators.

Negative feedback or Degenerative feedback

• If the signal is fed back to the input and the original input signal are 1800 out of phase, then it is called as the negative feedback.

• In the application of Op-amp as an amplifier, the negative feedback is used.

Negative feedback or Degenerative feedback

• In the amplifier circuits using Op-amp, a feedback resistor RF is connected between the output and inverting terminal as shown in figure to introduced a negative feedback.

• As the voltages V2 and VO are 1800 out of phase, a fraction of output voltage fed back to the input via RF will be 1800 out of phase with the input.

OP-AMP

2

3

6-

+

V2

V1

RF

input

Output

Feedback resistor

Vo

Advantages of Negative feedback

• Negative feedback is used in the amplifier circuits as they provide the following improvements in the operation of an amplifier:

• It reduces and stabilizes the gain.

• Reduces the distortion.

• Increases the bandwidth.

• Changes the values of input and output resistances.

• Reduces the effects of variation in temperature and supply voltage on the output of the Op-amp.

Inverting Amplifier

Non-Inverting Amplifier

L1: Basics of OP-AMP L2: Parameters of ideal and practical OP-AMP L3: Concept of positive and...

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