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Operational Amplifier – IQ – 1

1. Define an operational amplifier.

An  operational  amplifier  is  a  direct-coupled,  high  gain  amplifier  consisting  of  one  or more differential amplifier. By properly selecting the external components, it can be used to perform a variety of mathematical operations.

2. Mention the characteristics of an ideal op-amp. 

Characteristics of an ideal op-amp :
1.  Open loop voltage gain is infinity. 
2. Input impedance is infinity. 
3. Output impedance is zero. 
4.  Bandwidth is infinity. 
5.  Zero offset.

3. What happens when the common terminal of V+ and V- sources is not grounded ? 

If the common point of the two supplies is not grounded, twice the supply voltage will get applied and it may damage the op-amp.

4. Define input offset voltage. 

A small voltage applied to the input terminals to make the output voltage as zero when the two input terminals are grounded is called input offset voltage. 

5. Define input offset current. State the reasons for the offset currents at the input of the op-amp. 

The difference between the bias currents at the input terminals ofthe op-amp is called as input offset current. The input terminals conduct a small value of dc current to bias the input  transistors  .Since  the  input  transistors  cannot  be  made  identical,  there  exists  a difference in bias currents. 

6. Define CMRR of an op-amp.

The relative sensitivity of an op-amp to a difference signal as compared to a common – mode signal is called the common –mode rejection ratio. It is expressed in decibels. 

CMRR= Ad/Ac 

7. Define slew rate. 

The slew rate is defined as the maximum rate of change of  output voltage caused by a step  input  voltage.  An  ideal  slew  rate  is  infinite  which  means  that  op-amp’s  output voltage should change instantaneously in response to input step voltage

8. What causes slew rate ? 

There  is  a  capacitor  with-in  or  outside  of  an  op-amp  to  prevent  oscillation.  It  is  this capacitor  which  prevents  the  output  voltage  from  responding  immediately  to  a  fast changing input. 

9. In practical op-amps, what is the effect of high frequency on its performance ? 

The  open-loop  gain  of  op-amp  decreases  at  higher  frequencies  due  to  the  presence  of parasitic capacitance. The closed-loop gain increases at higher  frequencies and leads to instability 

10. Why IC 741 is not used for high frequency applications ? 

IC 741  has  a  low  slew  rate  because  of  the  predominance  of  capacitance  present  in  the circuit  at  higher  frequencies.  As  frequency  increases  the  output  gets  distorted  due  to limited slew rate.

11. Mention some of the linear applications of op – amp.

Adder,  subtractor,  voltage  –to-  current  converter,  current  –to-  voltage  converters, instrumentation  amplifier,  analog  computation,  power  amplifier,  etc  are  some  of  the linear op-amp circuits. 

12. Mention some of the non – linear applications of op-amp.

Rectifier, peak detector, clipper, clamper, sample and hold circuit, log amplifier, anti –log amplifier, multiplier are some of the non – linear op-amp circuits. 

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