Operational Amplifier

Operational Amplifier (Op-Amp)

The popular name of operational amplifier is op-amp. The operational amplifier is an active device used to design circuits that perform useful operations. The main operation of operational amplifier is

  • to generating sine waves or square waves
  • amplifying,
  • combining,
  • integrating,
  • differentiating and
  • removing noise; and
  • transforming alternating current into direct current and vice-versa.

Description of Op-Amp

  • It can also change the shape of a waveform, produce a change in the output when an input signal reaches a certain level.
  • It provides constant voltage or current and performs various other important circuit operations.
  • Op-amp circuits are very important as we develop a valuable perception of how electronic circuits work in general.
Opamp, Op-amp, Operational Amplifier
Basic Op-amp
  • An op-amp is a very high-gain differential amplifier with high input impedance and low output impedance.
  • The above figure shows a basic op-amp with two inputs and one output. The negative terminal is known as the inverting input terminal (Input 1), and the positive terminal is known as the non-inverting input terminal (Input 2).
  • Each input results in an output, which further depends upon the input that is being applied to positive (+) or negative (−) input terminals.
  • The op-amp is known as the differential amplifier because it amplifies the voltage difference of the inverting and non-inverting terminals.

Properties of The Ideal Operational Amplifier

An ideal op-amp should have the following properties:

  1. Gain must be infinite.
  2. Output voltage must be zero when input voltages are the same or when both are zero.
  3. The input resistance must be infinite.
  4. The output resistance must be zero.
  5. The common mode rejection ratio (CMMR) must be infinite.
  6. Infinite bandwidth, i.e., it must allow all frequencies to pass.
  7. Op-amp characteristics should not drift with temperature.

All these parameters for an ideal op-amp are different from those of a practical op-amp, as illustrated by the comparison given in table

Op-amp Parameters Ideal Op-amp Parameters Practical Op-amp Parameters
Gain Infinite 103 to 106 order
Output voltage Zero (0 volt) Few volts (in μV or nV), due to offset
Input resistance Infinite 103 Ω to 106 Ω order
Output resistance Zero (0 Ω) Few ohm (Ω) order
CMRR Infinite 100 dB order
Bandwidth Infinite Mega Hz order
Slew rate Infinite 0.5 V/μs order

Table Comparison between the parameters of an ideal op-amp and a practical op-amp

SPECIFICATIONS OF IC 741C

The op-amp popularly used in the laboratory is IC 741C. It is an eight (8) pin DIP (dual input package) IC, as shown in the figure.

Op-Amp IC 741C

Block diagram of 8 pin IC 741C

Description of Op-Amp 741 IC Pins

Pins 1 and 5: These two pins are used for the offset null process.
Pin 2: Inverting input terminal, i.e., when a sinusoidal signal is applied to the input pin 2, the inverted output is obtained at the output terminal 6.
Pin 3: Non-inverting input terminal, i.e., when a sinusoidal signal is applied to the input pin 3, the waveform of the same phase output is obtained.
Pin 4: Vcc, i.e., the negative terminal of the supply voltage is connected to this pin.
Pin 6: Output terminal.
Pin 7: + Vcc, i.e., positive terminal of the supply voltage is connected to this pin.
Pin 8: No electrical connection is there in this pin; this pin is just for balance and the symmetric dual-input package look.

APPLICATIONS OF THE OPERATIONAL AMPLIFIER

The op-amp is used either in inverting mode or non-inverting mode. In many practical applications, the op-amp is used as an adder, subtractor, intergrator, integrator, differentiator, voltage follower, phase changer, etc.

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