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Operational Amplifier (Op-Amp)

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.

Some Important terms related to Operational Amplifier

Slew Rate

Slew rate (SR) is defined as the maximum rate of change in output voltage per unit of time, and is expressed in volts per μ-seconds. The slew rate can be expressed as:

SR=(\frac{dV_{o}}{dt})_{maximum} V/\mu s

Slew rate indicates how rapidly the output of an op-amp changes in response to changes in the input frequency. The slew rate changes with change in voltage gain and is normally specified at unity (+1) gain. The slew rate of an op-amp is fixed. Therefore, if the slope requirements of the output signal are greater than the slew rate, then distortion occurs. Thus, slew rate is one of the important factors in selecting the op-amp for ac applications; particularly at relatively high frequencies.

Common-Mode Rejection Ratio (CMRR)

So, the CMRR is defined as the ratio of the differential voltage gain Ad to the common-mode voltage gain Acm. Therefore:

CMRR=\frac{Differential voltage gain}{Common-mode gain}=\frac{A_{d}}{A_{cm}}

The value of CMRR can be expressed in logarithmic form as:

CMRR=20 log_{10}\left | \frac{A_{d}}{A_{cm}} \right |

The differential voltage gain Ad is the same as the large signal voltage gain A, however, the common-mode voltage gain is given by:

A_{cm}=\frac{V_{ocm}}{V_{cm}}

where, Vocm is the output common-mode voltage, and Vcm is the input common-mode voltage.

Supply Voltage Rejection Ratio (SVRR)

The change in an op-amp’s input offset voltage Vio caused by a variation in one supply voltage when the other supply voltages remain constant in the circuit is called the supply voltage rejection ratio (SVRR). It is also referred to as power supply rejection ratio (PSRR) or power supply sensitivity (PSS). These are expressed in terms of microvolts per volt or in decibels. If we denote the change in supply voltages by Δ V, and the corresponding change in the input offset voltage by ΔVio, then SVRR can be defined as:

SVRR=\frac{\Delta V_{io}}{\Delta V}

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.

The op-amp is the basic component of analog computers. The typical uses of op-amp include providing amplitude changes in oscillators, active filter circuits and amplifier circuits of electronic instruments. In analog computers it is used to solve mathematical equations, simulate physical systems and to control physical process. The programming of analog computer is to arrange op-amps in different modes to solve mathematical, logical problems of certain equations. Mathematical functions for the solution of equation include integration, differentiation, summation, subtraction, etc. Op-amp is also used in many electronic devices as an amplifier, voltage multiplier, etc. Calculators, phase changer circuits also use the op-amp. It should be kept in mind that the op-amp is only used for low-power devices. It cannot be used in high-power applications.

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