Saturday, June 21, 2014

Basics of Operational Amplifier (EEE Department @ LAQSHYA College)

Basics of Operational Amplifier
Operational amplifier is so named as it is used to perform mathematical operations such as addition, subtraction, multiplication, differentiation, integration and many more. As op-amp has wide range of applications, some of its various applications are in industrial, communication, computer, control, and medical applications in additional with them in military applications too.
            An integrated circuit manufacturing industries incorporates integrated transistors, diodes, resistors and capacitors within op-amp ICs. So it is an extremely versatile device which has countless applications in many more areas.

            Since the op-amp is an integrated device, we don’t find any discrete components like active components such as transistors and diodes and passive components like R, L & C. consequently, it offers small size, low cost, high reliability, more temperature stability and low power consumption.
Block Diagram of A typical Op-Amplifier:

1à Non inverting input terminal
2à Inverting input terminal
3àDual input balanced output differential amplifier
4à Dual input unbalanced output differential amplifier
5à Emitter follower with constant current source. It is used to shift the DC level to ground in order to keep Q-point stable and also to limits the output voltage swing.
6à Complementary symmetry push-pull amplifier.
Schematic Symbol of an Op-Amplifier
Five basic terminals of op-amp :

                                Pin2àNon inverting terminal
                                Pin3à Inverting terminal
                               Pin7&pin4à Power supplies
                                Pin6à Output terminal
Equivalent Circuit of an Op-Amplifier:
Vdà Differential input voltage
Rinà Input resistance of an op-amp
Routà Output resistance of an op-amp
Avd & Routà  Thevenin’s voltage source and Thevenin’s resistance respectively looking back into the output terminals of an op-amp
Note: An electrical equivalent circuit is used to analyze basic operating principles of op-amp and in observing the effects of feedback.

Ideal Op-Amp Characteristics :
  a) Ri=∞     
   b) Ro=0 
              c) A0L=∞         
  d) BW=∞       
  e) Zero offset voltages           
  f) CMRR=∞

     Since input resistance is infinite, Ib1&Ib2 bias currents are ideally zero and practically very small. Due to Ri is very large loading effect is avoided.
     Since output resistance is zero the voltage across output terminals is independent of current flowing through the load. If Ro=0 , it is used to drive infinite number of sources.
     It implies there is a finite amount of output voltage for the zero differential input voltages.
     It shows, op-amp is used for both DC&AC where the frequency ranges from 0 HZ to high frequency.
Zero offset :
     It means for V1=V2=0 the Vo  must be zero.

CMRR(Common Mode Rejection Ratio):
     For an ideal op-amp,                                  CMRR=ρ=Ad / Ac=∞  
      Adàdifferential mode gain                        Acàcommon mode gain

DC Characteristics of Op-Amp:
a)      Vios: The spurious i/p voltage causes to get small mv of output even in the presence of both the inputs are grounded. For an ideal op=amp it should be zero.
b)     Iios:  The algebraic difference between the two bias currents is called input offset currents.

c)      I/P Bias Current: The average sum of two bias currents flowing into an op-amp for the two bases of the transistors is called as input bias current.
d)     Thermal Drift: The effect of variation in temperature causes changes in Vios, Iios & Ib is referred as thermal drift.

AC Characteristics :
a)      Gain Bandwidth Product: The range of operating frequencies of an op-amp at its unity gain is called gain bandwidth product. It also describes frequency response where variation in magnitude and phase of the gain due to change in frequency.

b)     Slew Rate: The maximum rate of change of output voltage is known as slew Rate.
SR=dVo/dt  | max
                 SR= dVo/dt | max=Imax/C
Article By
Assistant Professor
CSE Department
LAQSHYA Institute of Technology & Sciences              


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