IEEE (mod 3 basics elx)

Question 1

what is a OP-AMP

Answer 1

An Operational Amplifier is defined as: direct coupled, multistage, very high gain differential amplifier with high input impedance and low output impedance
[impedance is the opposition to alternating current presented by the combined effect of resistance in a circuit]

Question 2

What are a few facts about OP-AMP

Answer 2

–The output of an OP-AMP is proportional to the difference between the two input signals hence it is named as differential amplifier.
–The OP-AMP is sometimes called as linear Integrated Circuit as the output signal is proportional to the input signal (till the output reaches saturation level).
–The OP-AMP is a voltage controlled voltage source with very high gain.

Question 2

Give us a intro on OP-AMP

Answer 2

-The Operational Amplifier (OP-AMP) is a key building block in analog integrated circuit design. An OP-AMP is composed by several transistors, passive elements (resistors and capacitors) and arranged such that its low frequency voltage gain is very high;
-The behavior of Op-Amp can be controlled by adding suitable feedback. They were mainly used to perform mathematical operations in analog computers before the invention of digital computer applications.
–The various practical applications of Op-Amp are Comparator, Precision rectifier, Instrumentation amplifier,Digital to analog converter etc.

Question 2

what is the dc gain of a OP-AMP

Answer 2

the dc gain of the OP-AMP-741 is around 10^5 V/V

Question 3

What are the distinct places in VTC graph

Answer 3

1. one around Vi= 0, the linear region (straight line graph)where the output changes linearly with respect to input
2. the other at which changes in Vi has little effect on Vo, the saturation region (non-linear behavior).
   Circuits with operational amplifiers can be designed to operate in both of these regions

Question 3

What is voltage transfer curve (VTC)

Answer 3

the output voltage to the input voltage is called the voltage transfer curve…The VTC of OP-AMP is fundamental in designing and understanding amplifier circuits

Question 3

Draw us the symbol of OP-AMP and label

Answer 3

Va =Input voltage applied at non-inverting terminal
Vb= Input voltage applied at inverting terminal
Vi = Va-Vb (differential input)
Vo= Gain times the differential input voltage
A = Open loop voltage gain of the OP-AMP (gain without feedback)

Question 3

Draw us the Pin Configuration of Op-Amp

Answer 3

1–> Offset Null
2–> Inverting Input
3–> Non-Inverting Input
4–> -Vee
5–> Offset null
6–>Output
7–>+Vcc
8–> No connection
[Remember that after 4 the 5-8 comes bottom to top and not top to bottom]

Question 4

What is A or open loop gain

Answer 4

In the linear region the slope of the line relating Vo to Vi is very large, indeed it is equal to the open loop gain A.

Question 5

What is Single ended operation

Answer 5

Single-ended input operation results when the input signal is connected to one input with the other input connected to ground
In this operation there can be two connections
1. Vi at inverting and the non- inverting connected to the ground…giving us the result as same phase amplified output
2.Vi at non-inverting and the inverting connected to the ground….giving us the result as opposite phase amplified output.

Question 6

What is Double ended operation

Answer 6

using only one input, it is possible to apply signals at each input—this being a double-ended operation
In this operation there can be two connections
1. same signal is connected to both inverting and non-inverting terminal…giving us result the resulting amplified output in phase
2.two diff signal is connected at inverting and non-inverting terminal…giving us result
the difference signal being V1 - V2

Question 7

Ideal Characteristics of Op-Amp

Answer 7

1. Infinite open loop voltage gain ( i.e., A =∞)
2. Infinite input impedance ( = ∞): An ideal Op-Amp does not draw any current from voltage sources connected to its input terminals.
3. Zero output impedance (Ro =0): The output impedance is independent of the current drawn from it.
4. Infinite Bandwidth (B.W. = ∞): Amplifies signals of any frequency ranges with constant gain.
5. Infinite Common mode rejection ratio (i.e., CMRR =∞) the ratio of differential voltage gain to common-mode voltage gain.
   CMRR = Ad/Ac
6. Infinite slew rate (i.e., S=∞); the rate of change of output voltage per unit time.
   S=dVo/dt
7. Zero power supply rejection ratio (PSRR = 0); output voltage is zero when power supply VCC =0.
8. Zero offset voltage: when the input voltages are zero, the output voltage will also be zero.
9. Perfect balance: the output voltage is zero when the input voltages at the two input terminals are equal.
10. The characteristics are temperature independent.

Question 8

Characteristics of Practical Op-Amp

Answer 8

1. Open loop Voltage gain- 2x 105
2. Input Resistance 2MΩ
3. Output Resistance 75Ω
4. Input offset current 20nA
5. Input bias current 80 nA
6. CMRR 90 DB
7. Bandwidth 1 MHz at unity gain
8. Slew rate 0.5 V/µs
9. PSRR 30 µV/V

Question 8

Differential gain of Op-amp

Answer 8

When two inputs and are applied to an op-amp such that Va is not equal to Vb then output voltage is given by A=Vo/Vd

Question 9

Explain Op-Amp parameters Slew
rate

Answer 9

Slew rate of an Op-amp is defined as the maximum time rate of change of its output voltage, expressed in volt per microsecond.
dVo/dt I max
Slew rate is a measure of how fast the op-amp output can change in response to changes in the input signal. This limits the maximum operating frequency of op-amp. Slew rate depends on factors such as
load capacitance. Typical rate for µA741 is 0.5V/µs. fmax= slew rate/2piVm

Question 10

Common mode gain of op-amp

Answer 10

When both inputs of op-amp are at equal voltages i.e., , . Va=Vb , Vo=Ad(Va-Vb)=0
But practically a small non zero voltage will appear.

Question 11

Input offset voltage (Vos)

Answer 11

It is the differential input required to make the output voltage zero (typically 1mV)

Question 11

Open loop voltage gain (A)

Answer 11

When op-amp is used without feedback the differential voltage gain is known as open loop voltage gain A= Vo/Va-Vb
Ideally, A is infinite but practically ranges from 10^3 to 10 ^5

Question 11

Input resistance (Ri)

Answer 11

Open loop incremental resistance looking into the input terminals. Typically 2MΩ

Question 12

Common Mode Rejection Ratio (CMRR)

Answer 12

The ratio of differential mode gain (Ad) to the common mode gain (Ac) is termed as CMRR.

Question 12

Input bias current (IB) :

Answer 12

It is the average of separate currents entering the two input
terminals of balanced amplifier and given as IB = (IB1 + IB2 ) / 2 with Vo = 0V.

Question 13

Output offset voltage

Answer 13

It is the output voltage obtained when both the inputs are grounded.

Question 13

Negative feedback concept

Answer 13

Any input signal slightly greater than zero drive the output to saturation level because of very high gain.
So, when operated in open-loop, the output of the OPAMP is either negative or positive saturation or switches between positive and negative saturation levels (comparator)
Therefore open loop op-amp is not
used in linear applications.
With negative feedback, the voltage gain ( ) can be reduced and controlled so that op-amp can function as a linear amplifier.
for an amplifier the product of gain and bandwidth called the ‘Gain-Bandwidth
product’ is constant. An Op-amp with very large gain and small bandwidth is of less use. So, a negative feedback path automatically reduces the gain while increasing the bandwidth.

Question 13

Advantages of negative feedback

Answer 13

Amplifier becomes more linear
input and output impedances are modified to their ideal values
gain of the amplifier becomes more stable
Bandwidth of amplifier increases

Question 13

Input offset current (Iio)

Answer 13

: It is the difference between the two input currents entering the two terminals of a balanced amplifier.
Iio =(IB1 - IB2 ) with Vo = 0V

Question 13

Virtual ground concept

Question 14

Applications of OP-AMP

Answer 14

An Op-Amp can be used as
1. Inverting Amplifier
2. Non-Inverting Amplifier
3. Voltage follower
4. Adder ( Summer)
5. Difference amplifier(Subtractor)
6. Integrator
7. Differentiator

Question 14

Power Supply Rejection Ratio (PSRR)

Answer 14

: It is the measure of ability of op-amp to disregard the changes in power supply voltage

Question 14

Inverting Amplifier

Answer 14

–An inverting amplifier is one whose output is amplified and is out of phase by1800 with
respect to the input
The input is applied to the inverting terminal through resistance R1 and the non-inverting terminal is grounded. The feedback is established from output to inverting terminal through Rf.
Since Vin is applied to the inverting terminal, Vin and Vo are out of phase.
Therefore the feedback is negative. Due to the virtual short both the input terminals of op - amp are at
the same potential. i.e., Va=Vb.Therefore current flowing through the inverting terminal is zero and
hence the current flowing through is equal to the current flowing through Rf page 18 refer

Question 14

Non Inverting Amplifier

Answer 14

A non-inverting amplifier is one whose output is amplified and is in-phase with the input. The input signal is applied to the non-inverting terminal with respect to ground
The feedback is established from output to inverting terminal through Rf. Due to the virtual short both the input
terminals of op - amp are at the same potential. i.e., Va=Vb. Therefore current flowing through the
inverting terminal is zero and hence the current flowing through R1 is equal to the current flowing through Rf refer 20