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
a) Ri=∞
b) Ro=0
c)
A0L=∞
d) BW=∞
e)
Zero offset voltages
f) CMRR=∞
Ri=∞:
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.
R0=0:
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.
A0L=∞:
It
implies there is a finite amount of output voltage for the zero differential
input voltages.
BW=∞:
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.
Iios=|Ib1-Ib2|
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.
Ib=(Ib1+Ib2)/2
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
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Article By
--------------
Mr.K.Pithamber
Assistant Professor
CSE Department
LAQSHYA Institute of Technology & Sciences
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