What is an Operational Amplifiers? | Amplifier gain, characteristic & Types

These are linear devices having all the properties that are required for ideal DC amplification. It used for the signal condition, filtering or to perform mathematical operations such as add, subtract, differentiation and integration.

Operational amplifiers

An Operational Amplifier is a voltage amplifying device designed to be used with external feedback components such as capacitors and resistors between its input and output terminals.

These feedback components determine the operation of the amplifier and by the impact of the different feedback configurations, whether it is resistive, capacitive or both, the amplifier can perform a different verity of operations, giving rise to its name of Operational Amplifier.

An Operational Amplifier is basically a three terminal device which consists of two high impedance inputs and one input.

One of the inputs is Inverting Input, which marked with a negative or minus sign (–).

The other input is Non-inverting Input, which marked with a positive or plus sign (+).

A third terminal behaves the operational amplifiers output port which can both source and sink either a voltage or a current.


Operational Amplifier Gain

In a linear operational amplifier, the amplification factor is the output signal, known as the amplifier’s gain ( A ) that is multiplied by the value of the input signal and depending upon the nature of these input and output signals, there are four different classifications of operational amplifier gain.

  • Voltage– Voltage input and Voltage output
  • Current– Current input and Current output
  • Trans-conductance– Voltage input and Current output
  • Trans-resistance– Current input and Voltage output


The output voltage signal of the Operational Amplifier is the difference between the signals of two individual inputs.

In other words, an output signal of an operational amplifier is the difference between the two input signals as the input stage of an Operational Amplifier is, in fact, a differential amplifier.

Equivalent Circuit of an Ideal Operational Amplifier

Equivalent Circuit of an Ideal Operational Amplifier


Op-amp Parameter and Idealized Characteristic

  • Open Loop Gain, (Avo)

Infinite case

The main function of an op-amp is the amplification of the input signal and the more open-loop gain it has the better.

Open-loop gain is the gain of the operational amplifier without positive or negative feedback.

For such an amplifier, the gain will be infinite but the typical real values range from about 20,000 to 200,000.

  •   Input impedance, (ZIN)

Infinite case

The input impedance is the ratio of input voltage to the input current.

It is considered to be infinite to prevent any current flowing from the source into the amplifier’s input circuitry (IIN = 0).

The real op-amps have input leakage currents from the range of a few pico-amper to a few milli-amper.

  • Output impedance, (ZOUT)

Zero Impedance

The output impedance of the ideal op-amp is assumed to be zero and it acting as a perfect internal voltage source having no internal resistance so that it can supply enough current as necessary to the load.

This internal resistance is useful in series with the load and it reducing the output voltage available to the load.

Real op-amps have output impedances in the range of 100-20kΩ.

  • Bandwidth

Infinite Case

An ideal op-amp has an infinite frequency response and amplifies any frequency signal from DC to the high AC frequencies so it is assumed to have infinite bandwidth.

In real, the operational amplifier bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the frequency where the amplifier’s gain becomes unity.

  • Offset Voltage, (VIO)

Zero amplification

The output of the amplifiers will be zero when the voltage difference is zero between the inverting and the non-inverting inputs. The amplification is the same or when both inputs are grounded also included in this case. In real operational amplifiers have some amount of output offset voltage.

In idealized characteristics, we can see that the input resistance is infinite, so there are no current flows into either input terminal and that the differential input offset voltage is zero.

The real Op-amp do not have infinite gain or bandwidth but it has a typical Open Loop Gain which is defined as the amplifiers output amplification without any external feedback signals connected to it and for a typical op-amp is about 100dB at DC (zero Hz).

This output gain decreases linearly with respect to frequency down to Unity Gain, at about 1MHz

Operational Amplifiers Summary

The operational amplifier is a very high DC gain differential amplifier that uses one or more than one external feedback network to control the response and characteristics of an op-amp.

Inverting, Non-Inverting, Summing, Voltage Follower, Integrator,

We connect external resistors or capacitors to the op-amp in different ways to form basic building Block circuits such as, Inverting, Non-Inverting, Summing, Voltage Follower, Integrator, Differential and Differentiator type amplifiers.

A perfect ideal operational amplifier is a device with certain special characteristics such as infinite input resistance RIN, infinite open-loop gain AO, zero output resistance ROUT, infinite bandwidth zero to infinite and zero offset voltage.Op-amp is available in integrated circuit packages of single, dual or quad op-amps within one single device.

The most commonly available and used of all operational amplifiers in basic electronic kits are μA-741.

operational amplifiers in basic electronic kits are μA-741.


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