CH-1 OPERATIONAL AMPLIFIERS (Op-Amp) Marks: 20 (30)
Need of Op-Amp:
The gain of single stage amplifier is generally insufficient for practical use. Hence in order to obtain the desired high gain two or more amplifier stages are coupled together. Two important methods of coupling are RC coupling and direct coupling. The frequency response of these amplifiers is wide. The gain of RC coupled amplifiers falls at low and high frequencies. In direct coupled amplifiers output changes with age and change in power supply. These drawbacks are removed by using two identical stages of direct coupled amplifiers to form differential amplifiers.
Op-Amp is high gain DC amplifier. It consists of differential amplifier. Its performance can be changed as desired by changing external components.
An operational amplifier is a circuit that can perform mathematical operations such as addition, subtraction, integration and differentiation.
Block diagram of Op-Amp:
Working:
Differential amplifier is the basic building block of Op-Amp. The output of this stage is double ended i.e. it is taken between two collectors. This stage reduces noise and change in output voltage due to changes in temperature, power supply and ageing.
The emitter follower has very high input impedance and low output impendence. This stage acts as a buffer between two stages and avoids overloading.
The next stage is DC level shifter. DC level of output may change due to supply voltages. This stage brings o/p zero in absence of i/p.
The output stage is push-pull power amplifier. This stage provides sufficient current to load without distortion.
Ideal Characteristics of Op-Amp:
1. Input impedance is infinite.
2. Output impedance is zero.
3. Open loop gain is finite.
4. Input bias current is zero.
5. Input offset current is zero.
6. Input offset voltage is zero.
7. Common mode rejection ratio (CMRR) is infinite.
8. Infinite bandwidth.
Op-Amp parameters:
1. Input bias current: In an ideal Op-Amp any current is not drawn from applied input signal. Practically it absorbs a small value of direct current to bias the transistors. The input bias current is defined as the average of biasing currents of inverting and non-inverting inputs. Typically it is 80nA.
2. Input offset voltage: When the inputs of Op-Amp are grounded, the output must be zero.
But there is a small voltage at the output. The amount of input voltage to make output zero is called input offset voltage.
3. Input offset current: The difference between the biasing currents of inverting and non-inverting inputs is called input offset current.
4. Open loop gain: The gain of Op-Amp without feedback is called open loop gain. For ideal Op-Amp it is infinite. Gain is ratio of output to input.
5. Closed loop gain: A feedback resistor is connected between inverting input terminal and output terminal. The gain of Op-amp with feedback is called closed loop gain.
6. Slew rate: There is a limit to the rate at which output voltage of op-amp changes with change in input voltage. It is termed as slew rate. Slew rate is defined as the maximum rate of change of o/p voltage to a change sudden change in input voltage. It is expressed in volts/microseconds. A
7. Common mode rejection ratio (CMMR): It is the ratio of differential voltage gain to common mode gain. Ideally CMMR is infinite. It indicates the ability to reject the common mode signal.
8. Frequency response: It shows the change in voltage gain with frequency.
Op-amp symbol and IC:
Op-amp has two input terminals, inverting and non-inverting input terminals. Inverting input has minus sign. It indicates out of phase (1800 phase shift) between input and output. The non-inverting input has a plus sign which indicates input and output in phase. Op-amp needs dual power supply. IC 741 is widely used op-amp.
Op-Amp Symbol
Pin connections of IC741
Concept of virtual ground:
In above circuit, input signal is applied to inverting input terminal and non-inverting input terminal is grounded.
Negative feedback is obtained through R2. The feedback current If from output is almost same value as input current Ii. But feedback current is out of phase with input current.
At junction point S, these currents meet. They cancel each other. A state of almost zero potential is formed. Hence point, ‘S’is called as "virtual ground’.
Thus, virtual ground is that point in a circuit that has zero voltage and draws no current. An ordinary ground has zero voltage and can sink infinite current. But a virtual ground has zero voltage and zero current.
Linear applications of Op-Amp:
1. Inverting operational amplifier (Op-Amp as inverting amplifier):
Working:
In inverting op-amp, input signal is applied to the inverting terminal through resistorR1. Non-inverting input terminal is grounded. Inverting voltage feedback is obtained through Rf. The output signal is 1800 out of phase with the input.
2. Non-inverting operational amplifier (Op-Amp as non-inverting amplifier):
Working:
In non-inverting op-amp, the input signal is applied to non-inverting input of an amplifier Inverting input terminal is grounded through resistor R1 . A fraction of output voltage is feedback to inverting input through resistor Rf.
The combination of Rf and Rl acts as voltage divider.
Voltage at point S is,
3. Op-amp as a buffer/voltage follower (unity gain amplifier):
A buffer amplifier is a stage that isolates the preceding stage from the following stage. As gain is l, it is called ‘unity gain voltage follower. It is non-inverting amplifier. Input signal is applied to the non-inverting input terminal. Output is feedback directly to the inverting input terminal
Therefore, Rf=0.
Here, Rf = 0
Gain = l.
4. Op-Amp adder (Inverting summing amplifier):
Op-amp is an inverting amplifier. Let V1, V2, V3, be applied at the input terminals through Rl, R2, and R3 respectively. The non-inverting input terminal is grounded. The inverting input terminal is at virtual ground potential.
From circuit,
These currents meet at inverting input terminal. Due to very high input impedance, any current is not drawn by op-amp. Total current If flows through feedback resistor Rf only.
The output is sum of input voltages but with a negative sign. The negative sign indicates that output is 1800 out of phase with input.
5. Op-amp subtractor (Difference amplifier):
The input voltages are V1 and V2 are applied to inverting and non-inverting inputs of Op-Amp.
Output voltage is proportional to difference between two voltages. Input voltages may be ac or dc signal.
Output, Vo = V1 – V2
6. Op-Amp as inverter or sign changer:
In inverter, input is applied to inverting input terminal. In inverting op-amp gain is - Rf/R1. In inverter Rf=R1=R, therefore gain= -1. Output voltage, Vout = -Vin. Thus, the gain of this amplifier is -1 and it can be used to change the sign of input signal.
Non linear applications of Op-Amp:
1. Op-amp as an Integrator:
If the input to the integrator is a constant DC voltage, the output increases linearly with time. If input is a rectangular pulse, the output is a ramp signal. Integrator is called ‘low pass filter’.
Input and output waveforms:
Input Output
2. Op-amp as differentiator:
If input to the differentiator is ramp signal, we get an inverted pulse at the output. Differentiator is called ‘high pass filter’.
Input and output waveforms:
Input Output
3.
Voltage comparator: (Principle):
A comparator is a circuit with two input voltages (inverting and non-inverting) and one output voltage. When the non-inverting input is less than the inverting input, the comparator produces low output voltage. When the non-inverting voltage is larger than the inverting voltage, the output is high. It is an op-amp circuit without feedback. lt takes the advantage of very high open loop gain of op-amp.
For example, consider a op-amp with open lop gain=l0000. A voltage difference of 25mV between input terminals will produce an output voltage of 25mVxl0000=25V. However, most of op-amps have output voltages less than ±15V because of their DC supply voltages.
Therefore, a very small differential input voltage will drive the op-amp to saturation.
Saturation voltage: Maximum output voltage swing is normally 2volt less than applied voltage. This is called ‘saturation voltage’ (VSAT)
For dual power supply of ±15V, VSAT = ± l3V
Circuit and working of comparator:
Above fig. illustrates the action of comparator. The input voltages are V1 and V2. V1 is called signal input and V2 is called reference input. If differential input (Vi=Vl─V2) is positive, the circuit is driven to saturation and output goes maximum positive value i.e. +VSAT. When differential input Vi goes to negative, output goes to maximum negative value i.e. ─VSAT. When Vi is 0, output is 0. This circuit is called comparator because it compares V1 to V2 to produce saturated positive or negative output voltage. Output voltage rapidly changes from +VSAT to ─VSAT and vice-versa.
Types of Comparators:
(a) Zero-reference or zero crossing comparators:
In this circuit, the inverting input terminal is grounded and input voltage is above ground level (0V), the op-amp is saturated. A positive input voltage produces positive saturation and negative input voltage produces negative saturation. In short,
When Vjn>0, Vo = +VSAT
When Vin<0, Vo = ─VSAT and when Vin = 0, Vo = 0.
(b) Non-zero reference comparator (limit detector):
In this type of circuit, the trip point is equal to Vref. When Vin is slightly more than Vref the output goes into positive saturation. When Vin is less than Vref the output goes into negative saturation. In short,
When Vin>Vref, Vo = +VSAT,
When Vin<Vref, Vo= ─VSAT and when Vin=Vref , Vo=0.
4. Schmitt trigger using op-amp:
A Comparator with positive feedback is called Schmitt trigger. Schmitt trigger is used to convert irregular signal wave into square wave/pulse.
Positive feedback is obtained using voltage divider of R1 and R2. The feedback factor is β = When o/p is positively saturated, the reference voltage applied to the non-inverting input is Vref = + βVSAT. When the o/p is negatively saturated, the reference voltage is Vref = ─ βVSAT. These are trip points of circuit.
The o/p will retain in a state until the input exceeds the reference voltage for that state. For example, if o/p is positively saturated, the input voltage Vin must be increased to slightly more than + βVSAT. Then error voltage reverses and o/p voltage changes to the low state i.e. ─βVSAT. If Vin becomes more negative than ─βVSAT , output reverses to + βVSAT.
Applications of op-amp:
Ans (A) Linear applications-
Inverting op-amp
Non inverting op-amp
Inverter
Buffer
Adder
Subtractor
(B) Non-linear applications-
Integrator
Differentiator
Comparator
Schmitt trigger.
Advantages and disadvantages of Op-Amp:
Advantages:
OP-amp is an universal amplifier.
High input impedance.
Low output impedance.
High bandwidth.
It can be used as voltage comparator.
It can be used as analog to digital converter.
Disadvantages:
Op-amps are designed for low power operation.
Specially designed op-amps are required for high output.
Op-amp shuts off when output resistance is below specific value.
Questions
Marks wise Questions: 1M(7), 3M(3), 4M(2), 6M(1)
1. Fill in the blanks-
An Op-Amp Schmitt trigger uses —————.
(i) Positive feedback (ii) Negative feedback
(iii) Compensating capacitor (iv) Pull up resistor
The Op-Amp can amplify ———.
(i) DC signals only (ii) AC signals only
(iii) Both AC and DC signal (iv) None
lf input voltages to inverting adder are V1, V2, V3 then, the output is given by --------.
(i) Vo= ─ (2Vl+V2 +3V3) (ii) Vo = ─Vl─V2 ─V3
(iii) Vo= V1 + V2 +V3 (iv) None
ln Op-Amp inverting circuit, the inverting voltage is equal to —————— .
(i) lnput voltage (ii) Output voltage (iii) Supply voltage (iv) Zero voltage
Slew rate of Op—Amp is measured in ——————.
(i) microseconds/V (ii) V/microseconds (iii) microvolts/sec (iv) None
When Op-Amp is used to detect the voltage level then circuit is called —————
(i) Integrator (ii) Differentiator (iii) Comparator (iv) Buffer
—————— stage of Op—Amp rejects the noise signal.
(i) Level shifter (ii) Differential amplifier (iii) Emitter follower (iv) None
Differential amplifier has ---------- number of inputs. [O-09, M-14]
(i) 1 (ii) 2 (iii) 3
Unity gain voltage follower is also called -------------. [M-09]
(i) Rectifier (ii) Buffer (iii) Oscillator (iv) Doubler
The voltage gain of buffer amplifier using Op-Amp is __________.
(i) 1 (ii) 0 (iii) (Infinity )
For an ideal OP-AMP, input impedance is _________. [O-11,M-12]
2. Match the following-
A B
(l) Op-Amp/IC741[O-09,11,M-16](l) High pass filter
(2) Virtual ground [O-09] (2) Low pass filter
(3) Inverting Op-Amp (3) lnverting summer
(4) Non inverting Op-Amp (4) Unity gain amplifier
(5) Buffer/voltage follower (5) Positive feedback/square wave output
(6) Op-Amp adder (6) Dual power supply/o/p pin no.6/
Many analog applications/ [M-10]
Differential output/Schmitt trigger/Infinite gain
(7) Op-Amp integrator (7) Output is in phase with input
(8) Op-Amp differentiator (8) Output is out of phase with input
(9) Voltage comparator (9) Zero voltage
(10) Schmitt trigger [M-10] (10) Infinite
(11) CMRR [O-11, M-12] (11) Level detector
3. True or false-
Basic building block of an Op-Amp is differential amplifier. [O-11]
Op-Amp uses positive or negative power supply.
In ideal op-amp, input impedance is zero and output impedance is infinite.
[M-09,M-10]
4. Output impedance of an ideal Op-Amp is zero. [M-11]
ln inverting op-amp, output is in phase with input.
ln op-amp, virtual ground is point having zero voltage and no current
Gain of buffer (voltage follower) using op-amp is infinite.
When the non-inverting input is less than the-inverting input, the comparator produces low output voltage.
In Schmitt trigger using op-amp, negative feedback is used.
Op-amp as integrator is an example of linear amplifier. [O-09]
Op-amp is linear IC. [O-09, M-10]
Comparator circuit is an example of open-loop amplifier. [M-11]
Input impedance of an Op-Amp is zero. [M-13]
Op-Amp has two inputs and one output. [M-16]
4. Answer the following-
Define op-amp. State the characteristics of ideal op-amp.
[O-09, M-10, M-11, O-11, M-12]
Draw block diagram of op-amp and explain it. [M-O-09, M-10, M-O-11, M-12, 14.16]
Explain the following parameter/characteristics of an op-amp.
(l) Input bias current (2) Input offset current (3) Input offset voltage (4) CMMR
(5) Slew rate (6) Frequency response (7) Open loop gain (8) Closed loop gain.
[M-09, O-09, M-10, M-11, O-11, M-12, M-13, M-14, 16]
Explain the working of inverting op-amp. Derive/write an expression for its gain. [M-09, M-10, M-12, M-13, 17]
Explain working of non-inverting op-amp with circuit diagram. [M-11, O-11]
Explain concept of virtual ground in op-amp. [M-14, M-16]
Explain an inverting adder with the help of op-amp. Derive/write the equation for output voltage. [O-09, M-12, M-13, M-16]
Explain an inverting subtractor with the help of op-amp. Derive/write the equation for output voltage. [M-09, O-11, M-14]
Explain application of op-amp as buffer/unity gain voltage follower. [M-14]
Explain use of op-amp as integrator and differentiator with circuit diagrams./ Draw ckt of differentiator using Op-Amp. Write its output equation/waveforms.
[O-09, M-10, O-11, M-12, M-13,16]
What is comparator? Explain different types of voltage comparators using op—amp.
[O-09]
Explain working of Schmitt trigger using op-amp. [M-09, M-10, M-11]
Explain with the help of circuit diagram, the working of a comparator using IC74l (op-amp). [M-09]
State four advantages of op-amp over normal amplifier.
Explain in brief any two applications of op-amp.
Draw schematic symbol of op-amp and pin diagram of IC74l.
Draw circuit diagram of integrator using Op-Amp. State expression of its output. [M-11]
State applications of differential amplifier. [M-13]
