Operational Amplifier(OPAMP) - Directorate of Higher Education

Operational Amplifier(OPAMP) - Directorate of Higher Education
Operational Amplifier(OPAMP)
Operational amplifier is a direct-coupled
amplifier used to perform wide variety of linear
and some nonlinear operations which is usable
in the frequency range from 0 to few MHz.
(TOAL No of Slides is 14)
Some features of OPAMP
 OPAMP
was designed to perform mathematical operations
such as summation, subtraction, multiplication, differentiation,
integration etc. in analog computer.
 OPAMP can also be used for the purpose of solution of
simultaneous linear algebraic equation as well as differential
equation.
 Now a days OPAMPs are available almost all in IC forms having
comparatively low price.
 Many useful circuits can be designed using OPAMP and so this
has become very popular in electronic industry.
Pin Diagram & Circuit symbol of
OPAMP
Characteristics of ideal OPAMP
 Open loop voltage gain is infinite
 Input impedance is infinite
 Output impedance is zero
 Bandwidth is infinite
 Perfect balance i.e, ouput is zero when two input
voltage are equal.
 Characteristics do not drift with temperature
 Common mode rejection ratio is infinite
 Slew rate is infinite
Deviations of practical amplifier from
ideal one
In practical OPAMP
 Open Loop Voltage gain is not infinite
 Input impedance is not infinite
 CMRR is not infinite
 Output is not zero even if two input voltages are identical. The
voltage which should be applied between the input terminals
to balance the amplifier is called input offset voltage. where
as the input offset current is the difference between the two
bias currents entering into the input terminals of balanced
amplifier and input bias current is the average of two
separate currents entering the input terminals of a balanced
amplifier.
Applications of OPAMP
OPAMP can be used both in inverting mode and non inverting
mode. However OPAMP can be used for designing
i) Scale changer
ii) Phase Shifter
iii) Unity gain follower
iv) Adder
v) Subtractor
vi) Differential amplifier
vii) Integrator
viii) Differentiator
Inverting Amplifier
•



Op-amp are almost always used with a negative feedback:
Part of the output signal is returned to the input with negative sign
Feedback reduces the gain of op-amp
Since op-amp has large gain even small input produces large output, thus for
the limited output voltage (lest than VCC) the input voltage vx must be very
small.
 Practically we set vx to zero when analyzing the op-amp circuits.
with vx =0 i1 = vin /R1
i2 = i1
and
vo = -i2 R2 = -vin R2 /R1
so
Av=vo /vin =-R2 /R1 ……………(1)
Scale changer and Phase Shifter
•
From (1)
,
R2
v0   vs  kvs
R1
i.e the circuit multiplies the input by –k and so such circuit can be treated as scale
changer.
If , R1 and R2 is replaced by impedances z1 and z2 and they
are chosen in such a way that they have equal magnitude but
different phase then
v0
Iz2 Ie j2
j ( 2 1 )


e
vs
Iz2 Ie j1
Where 1and  2 are respectively the phase angles of z1 and z2.Thus the OPAMP
can shift the phase of input voltage by the angle  2 1
Non Inverting Amplifier
Here signal is applied in non-inverting terminal. Since gain
A
There is a virtual short at the input terminals and so,
v0  v2
v2  0

R2
R1
or ,
v0
v
R
 0  1 2
vs
v2
R1
.........(1)
Unity gain follower
From eq (2)it is found that the closed loop
gain becomes unity if we choose R1   and/or
. R2  0 . The amplifier then acts a voltage
follower i.e, a non-inverting amplifier with
unity gain.
OPAMP as Adder
Thus the output is proportional to the algebraic sum of the inputs.
OPAMP used for difference of two
signals
OPAMP as Integrator
OPAMP as differentiator
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