Q. 1 Classification of IC ?

Q. 2 What is an operational amplifier ?

Ans. : The operational amplifier is basically an excellent high gain differential amplifier. It
amplifies the difference between its two inputs. Due to its use in performing mathematical
operations it has been given a name operational amplifier.

Q. 3 Draw the op-amp symbol and state its important terminals.

Ans. : For the op-amp symbol, refer the Fig. 2.2.1. The important terminals are,

1. Positive supply 2. Negative supply 3. Output 4. Inverting input 5. Noninverting input.

Q. 4 State the various blocks of IC op-amp.

Ans. : 1. Internal stage 2. Intermediate stage

3. Level shifting stage 4. Output stage.

Q. 5 Which circuits are commonly used for the various stages of an 1C op-amp ?

Ans. :

1. Internal stage : Dual input balanced output differential amplifier.

2. Intermediate stage : Multistage amplifiers without coupling capacitors and single ended

3. Level shifting stage : Various transistor circuits to reduce the d.c. output level.

4. Output stage: Push-pull complementary symmetry amplifier in Class AB operation.

Q. 6 List the requirements of the internal stage of IC op-amp.

Ans. : 1. High voltage gain 2. High input impedance 3. Two input terminals

4. Small offset voltages 5. Small offset currents 6. High CMRR.

Q. 7 Why level shifting is required in op-amp ?

Ans. : As coupling capacitors are not used in the intermediate stage of the op-amp, the d.c.
biasing voltage level propagates through the chain of the amplifiers. Finally it appears as a
significant d.c. component present at the output along with the desired a.c. output. Such a d.c.
level distorts the output and limits the maximum output voltage swing. Hence it is necessary to
reduce this d.c. level before the output stage for which level shifting circuit is used.

Q.8 List the requirements of the output stage of IC op-amp.

Ans. : 1. High output voltage swing 2. High output current swing 3. Low output impedance 4.
Low quiescent power dissipation 5. Various protection circuits.

Q.9 List the important characteristics of ideal op-amp.

Ans. : 1. Infinite voltage gain 2. Infinite input impedance 3. Zero output impedance

4. Infinite bandwidth 5. Infinite CMRR 6. Infinite slew rate 7. Zero PSRR

Q.10 Draw the equivalent circuit of practical op-amp.

Q. 11 What is input offset voltage ?

Ans. : When both the input terminals of the op-amp are grounded, ideally the output should be
zero. But practically op-amp produces the small output voltage. To nullify this voltage, some
voltage is required to be applied to either of the two input terminals in the particular direction.
This voltage is called input offset voltage.

How much voltage, to which input and with what polarity is specified by the manufacturer in the
data sheet of the op-amp.

Q.12 What is input bias current and input offset current ?

Ans. : The base currents of the transistors used in the input stage of the differential amplifier are
practically finite and the two currents differ in magnitude as perfect matched transistors are not
possible in practice. These currents are responsible for the input bias current and input offset
current of op-amp.

The input bias current is the average of the two base currents Ib,and Ib2 while the input offset
current is the difference between the two base currents Ib and Ib2.

Q.13 What is output offset voltage ? Which parameters are responsible to produce it ?

Ans. : The output voltage present when both the input terminals of the op-amp are grounded is
called output offset voltage. The input offset voltage, input bias current and input offset current
are responsible to produce output offset voltage.

Q. 14 What is thermal drift ?

Ans. : The dependence of the op-amp parameters on the temperature is indicated by a factor
called temperature drift. The change in the op-amp parameter corresponding to the change in the
temperature is defined as the thermal drift of that parameter. Thus Δ Vios / ΔT is called input
offset voltage temperature drift.

Q. 15 What is slew rate ? State slew rate equation.

Q.16Compare the ideal and practical op-amp characteristics.

Q.17 What is frequency response of the op-amp ?

Ans. : The plot showing the variations in magnitude and phase angle of the gain of op-amp due
to the changes in the input frequency is called the frequency response of the op-amp.

Q.18 What is frequency compensation ?

Ans. : The method of modifying loop gain frequency response of the op-amp so that it behaves
like single break frequency response which provides sufficient positive phase margin is called
frequency compensation technique.

Q.19 List the methods used to provide the external frequency compensation.

Ans. : The methods used to provide the external frequency compensation are,

1. Dominant pole compensation

2. Pole-Zero compensation

3. Feed-Forward compensation.

Q.20 Why operational amplifier open loop configurations are not used in linear
applications ? May-07

Ans. : Op-amp has a saturable property in open loop mode. Its output saturates at Vsat which is
0.9 times its supply voltages. Its open loop gain is very very high, of the order of 2 × 10 5. Hence
for very small range of input, of the order of few µV the output varies linearily with the input.
This range is not sufficient for the practice linear applications. Hence the open loop op-amp is
not used in the linear applications.

Q. 21 What is the need for compensating network in op-amps ?

Ans. : Due to slight mismatch in the first stage of op-amp, there exists input offset voltage and
input bias current. Both are responsible to produce output offset voltage. This voltage must be
ideally zero. For making this voltage as small as possible, the compensating network is necessary
in op-amps.

Q. 22 Discuss the cause of output offset voltage in op-amp.

Ans. : The output offset voltage (Voos) is the difference between d.c. voltage present at the output
terminals when both the input terminals are grounded. The two main causes for output offset
voltage in op-amp are,

1. Input offset voltage (Vios) 2. Input bias current (Ib)

The output voltage caused by input offset voltage can be either positive or negative with respect
to ground. Similarly, the output voltage caused by input bias current can be either positive or
negative with respect to ground. If these voltages are of the same polarity then the output offset
voltage is large.

The output offset voltage due to these two parameters are given by,

Voos = (l + Rf / R1) Vios + Ib Rf

where Rf is feedback resistance and Rx resistance in series with the source.

To make this Voos as small as possible, compensating network is used in the op-amps.