Op amp or Operational Amplifier | Working Principle of Operational Amplifier or Op amp

An op amp is a high-gain differential amplifier, normally with a single-ended output. Depending on the design, its integrated circuit may use bipolar transistors, FETs and resistors in several internal stages.
External feedback and input networks determine the closed-loop function of the Integrated Circuit (IC). These networks can implement amplification, subtraction, addition, integration and differentiation within the device’s bandwidth and signal limits. The IC 741 is an older general-purpose example.

The symbol and one IC package are shown below. The triangle points from the two inputs towards the output. Feedback may connect the output back to an input externally, but the triangle itself does not show feedback direction.

Input and Output Terminals of an Operational Amplifier

An op amp has an inverting input, a non-inverting input and one output, plus positive and negative supply pins. The two inputs form the differential input of the operational amplifier. Within the linear operating range, raising the non-inverting voltage relative to the inverting voltage drives the output positive; raising the inverting voltage drives it negative. A closed-loop inverting amplifier has an ideal low-frequency phase inversion of 180^o, while actual phase shift changes with frequency. The result also depends on feedback, input common-mode range and output swing.

Power Supply for an Operational Amplifier

The supply pins are commonly labelled +V_CC and -V_CC, or V+ and V-. A dual-polarity supply is not required. An op amp can use a single, split or asymmetric supply when the total supply voltage stays within its rating. The +V_CC pin connects to the more positive rail and the -V_CC pin to the more negative rail. Most op amps have no ground pin, so circuit ground is an external reference. Inputs must remain inside the specified common-mode range, and the output must remain inside its load-dependent swing limits.

Working Principle of Op-Amp

Open Loop Operation of an Operational Amplifier

In its simplest model, an op amp produces an output proportional to the difference between its non-inverting and inverting input voltages. An ideal op-amp follows the relation shown below while operating linearly.Here, V_OUT is output voltage and A_OL is open-loop voltage gain. Practical open-loop gain is finite and falls with frequency. The IC 741 datasheet gives a typical low-frequency A_OL of 2 x 10⁵ under stated supply, load and output conditions, not a constant value.
V₁ is the non-inverting input voltage.
V₂ is the inverting input voltage.
(V₁ – V₂) is the differential input voltage.
In the ideal equation, V₁ = V₂ gives zero output, while V₁ ≠ V₂ produces an open-loop response. In practice, high gain usually drives the output towards one of its swing limits, and offset voltage, bias currents and noise create error.
The output cannot exceed its specified swing near the supply rails and can source or sink only limited current. The datasheet limits also cover input common-mode voltage, differential input voltage, power dissipation and temperature.

Closed Loop Operation

A closed-loop circuit returns part of the output to an input through a feedback network. Stable negative feedback makes the op amp adjust its output to reduce the differential input voltage. When loop gain is high, external components set the closed-loop gain closely, but finite open-loop gain creates gain error. The relation below is a simplified closed-loop model.Here V_OUT is output voltage and A_CL is closed-loop gain. The feedback network, op amp open-loop response and load determine A_CL. V_D = (V₁ – V₂) is the differential input voltage. Positive feedback reinforces a change and is used in comparators with hysteresis and some oscillators. Negative feedback opposes a change and is used in linear amplifiers, buffers, filters and control circuits. A negative-feedback circuit must still have adequate phase margin to remain stable.

Positive feedback ⇒ regenerative or switching behaviour
Negative feedback ⇒ linear control when the loop is stable
These relationships summarise the basic working principle of operational amplifiers.

Ideal Op-Amp Characteristics

An ideal op-amp should have the following characteristics:

1. Infinite open-loop voltage gain, so ideal negative feedback can make the differential input voltage approach zero
2. Infinite input resistance, so no current enters either input
3. Zero output resistance, so load current does not change the output voltage
4. Infinite bandwidth and slew rate
5. Zero input offset voltage, bias current and noise
6. Infinite power-supply rejection ratio (PSRR = ∞)
7. Infinite common-mode rejection ratio (CMRR = ∞)

Practical Operational Amplifier

A practical op amp has finite open-loop gain, input impedance, output current, bandwidth and slew rate. It also has non-zero output impedance, offset voltage, bias current, noise and distortion. Input common-mode range, output swing, capacitive-load stability, PSRR and CMRR vary with device, supply, load, frequency and temperature. The datasheet must therefore be checked for the intended operating conditions.

Applications of Operational Amplifier

Integrated op amps are available across a wide range of size, cost, supply current, precision, speed, noise and output drive. Common applications include an inverting amplifier, a non inverting amplifiers circuit, a unity-gain buffer, a summing amplifier, an active filter, a differentiator, an integrator, an instrumentation circuit and a Wien bridge oscillator. Device selection depends on the required accuracy, frequency range, signal range, load and stability.