BASIC KNOWLEDGE Op-amp: The “Operational Amplifier”

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The world’s first operational amplifier “op-amp”: “µA741” was developed in 1968. Today most op-amps are sold in the form of integrated circuits. The article explains op-amps in detail with their symbol, working, parameters, modes, applications, advantages, and limitations.

In addition to amplification, an op-amp is a type of amplifier that can be configured to perform various mathematical operations. As a result, a popular American Engineer John R. Ragazzini coined the term “operational amplifier” or “op-amp” for the first time in 1959.

Op-amp symbol

Any amplifier type is represented by a triangular symbol in circuit diagrams and schematics. An operational amplifier has two input terminals and one output terminal. One input terminal of the op-amp is inverting and the other is non-inverting.

The “-” sign denotes the inverting terminal and the “+” sign denotes the non-inverting terminal. The positive supply voltage is necessary to drive the op-amp. The negative supply voltage is there as a reference for op-amp internal circuitry.

How does an op-amp work?

An operational amplifier appears to be a single amplifier in the form of a chip but it is a multi-stage amplifier with a complex internal circuitry. Various BJTs, FETs, and resistors are connected internally to make up amplifier stages and perform required functions.

Input stage

The input stage is the first stage of an operational amplifier. It is a type of dual input balanced output differential amplifier. Simply put, an op-amp functions like a differential amplifier. In this stage, two input terminals consist of one inverting and another non-inverting type. Overall, the input stage is responsible for frequency response, voltage gain, and high input impedance of the operational amplifier.

Intermediate stage

The intermediate stage of an op-amp is again a differential amplifier with dual-input unbalanced output. There is a direct coupling between the input and intermediate stages. The output of the input stage is used to drive the intermediate stage.

Level shifting stage

The level shifting stage is present to lower DC levels in the output signal to zero with respect to the ground. An emitter follower (collector) and constant current source are used in this stage.

Output stage

The output stage is the last stage of an operational amplifier, which is responsible for delivering the output signal. A complementary symmetry such as a push-pull amplifier is used in this stage. The output modes of the op-amp (listed below) represent the output terminals and resulting waveforms. In conclusion, the output stage lowers the output impedance and increases output voltage with the current carrying capability of the op-amp.

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Op-amp configuration

This section delves into the key parameters that define an operational amplifier's performance characteristics.

Open-loop op-amp

The op-amp in open loop configuration has no feedback from output to input terminals. Most of the operation in op-amp modes (below) is explained in an open-loop configuration. Practically, op-amps offer a very large but non-constant and unstable gain in the open-loop configuration.

Closed-loop op-amp

The op-amp in closed loop configuration consists of feedback from output to input terminals. The feedback components can be resistors and capacitors.

Positive feedback
Positive or regenerative feedback is the feedback given to the non-inverting terminal of the op-amp.

Negative feedback
Negative or degenerative feedback is the feedback given to the inverting terminal of the operational amplifier.

Op-amp parameters

This section delves into the key parameters that define an operational amplifier's performance characteristics.

Input resistance

Input resistance is the resistance measured at any one terminal, either inverting or non-inverting terminal of the operational amplifier. The other terminal must be grounded. Ideally, input resistance should be as high as possible.

Input capacitance

Input capacitance is measured at any, either inverting or non-inverting terminal of the op-amp. The other terminal must be grounded. Ideally, input capacitance should be as small as possible.

Output resistance

Output resistance is measured at the output terminal of the op-amp when input terminals are short-circuited. Ideally, output resistance should be zero.

Input bias current

The average of currents flowing into inverting and non-inverting terminals of an op-amp is called input bias current.

Input offset current

The algebraic difference between currents flowing into inverting and non-inverting terminals of an op-amp is called input offset current.

Input offset voltage

The input offset voltage is the smallest amount of voltage applied between the input terminals to make the output voltage zero.

Output offset voltage

Output offset voltage is the voltage produced at the output due to the presence of input offset voltage.

Slew rate

Slew rate is defined as the rate of change of output voltage with respect to time. Ideally, the slew rate should be as high as possible.

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Bandwidth

Op-amp bandwidth is the range of frequencies over which op-amp effectively amplifies.

Gain bandwidth product

The gain bandwidth product of an op-amp is the bandwidth at which the voltage gain is one.

Saturation voltage

Saturation voltage is the voltage at which the output voltage of an op-amp saturates.

Output voltage swing

The difference between maximum positive and negative saturation voltages is called output voltage swing.

Gains

Large signal gain is among the most important op-amp parameters. It is the ratio of the output voltage to the input differential voltage. Ideally, the large signal gain should be as high as possible.

Similarly, closed loop voltage gain is the voltage gain with external feedback and open loop voltage gain is the voltage gain without external feedback.

CMRR

The CMRR (Common Mode Rejection Ratio) is the ability of an op-amp to reject common mode signals.

PSRR

The PSRR (Power Supply Rejection Ratio) is the ability of an op-amp to suppress variations in power supply voltage.

Op-amp input and output modes

This section explores how input signals are processed and output signals are generated by operational amplifiers.

Op-amp input modes

Input modes in an operational amplifier represent the type of input signals and their effect on the amplifier's output.

Single-ended mode
In single-ended input mode, the input signal is fed to only one of the two input terminals. The other terminal is grounded. Here, the input signal is given to the inverting input terminal of the op-amp. As a result, the output of the op-amp will be an amplified inverted signal.

In the diagram below, the input signal is given to the non-inverting terminal of the op-amp. The resulting output signal is an amplified non-inverted signal.

Common mode
In the common input mode, the same voltage signal is applied to both the input terminals of the op-amp.

Due to the presence of the inverting terminal, the polarity of the two input signals is opposite. As a result, the two input signals cancel each other to produce a zero voltage output. In practical applications, instead of producing 0 Volts, operational amplifiers generate a small output voltage.

The common mode signal can be noise or interference. Ideally, an op-amp should “reject” a common mode signal to produce the non-zero output voltage. The ability of an op-amp to reject common mode signals is a figure of merit.

Differential mode
In differential input mode, two different voltages are fed to the op-amp. The two input signals are 180 degrees out of phase. The output of the op-amp is the amplified difference of the two input signals.

Please note that the difference between these two signals is not an actual waveform but a depiction of a possible solution. The differential mode is also called double-ended mode. In practical applications, the op-amp is mostly used in differential mode.

Op-amp output modes

Output modes in operational amplifiers are critical to understanding the magnitude of the output signal.

Single output mode (most common)
An op-amp always produces a single output.

All the above-mentioned modes are discussed in a single output mode.

Double output mode
A double-output mode or double-ended output mode produces two outputs. In such cases, op-amps have two output terminals instead of one. Both output signals are opposite in polarity. An op-amp in double output mode with a single-ended input is shown below.

The output V₀₁ and V₀₂ are 180 degrees out of phase. The overall output of such an op-amp is considered to be the difference between two output signals.

V_D= V₀₁-V₀₂

The overall output signal is called a floating signal because none of the two output terminals is grounded.

Op-amp IC LM741

Popularly known as “LM741”, this operational amplifier IC offers high voltage gain, reliability, and less power consumption.

The significance of pins 2, 3, 4, 6, and 7 are explained in earlier sections of the article. Pin 8 is a dummy pin that should be left unconnected. Pin 1 and 5 are there to nullify the offset voltage.

Op-amp applications

The typical usage of an operational amplifier in any system is to provide voltage changes in terms of amplitude and polarity. Op-amps are extensively used in oscillators, filters, mathematical operation circuits, analog-to-digital and digital-to-analog converters, signal processing systems, waveform shapers (clippers and clampers), and instrumentation devices.

Amplification

Op-amp, mainly, performs the function of amplification: increasing the strength of the low-voltage input signal. It can amplify AC and DC signals effectively to produce a high-voltage output. The op-amp can function as both inverting and non-inverting amplifiers. The diagram below is an output of a non-inverting operational amplifier.

Unity follower

The op-amp unity follower amplifier is similar to emitter follower (common collector) and source follower (common drain) circuits. The output has the same magnitude and polarity as the input signal. The only difference is that the op-amp unity gain amplifier provides a voltage gain of 1.

Comparator

An operational amplifier can be used to build a comparator circuit. A comparator is a special type of circuit that compares two voltage levels to generate an output. An op-amp without any feedback or an open-loop op-amp is a comparator.

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Summing amplifier

Op-amp was invented as the “summing amplifier” in 1941. A summing amplifier is the most basic operation of an op-amp. Input voltages are multiplied by a constant gain factor and algebraically summed. Op-amp integrator with a summing amplifier mode has been historically used in analog computers.

Op-amp integrator

A fixed voltage is applied to the input terminal of the integrator circuit. A capacitor is placed as the feedback component. The integrator circuit resembles the mathematical integration operation over the growth of output voltage with respect to time. The output of the integrator circuit is a ramp voltage.

Differentiator

Op-amp differentiator performs the mathematical operation of differentiation. A resistor is used as the feedback component. The differentiator circuit produces an output voltage proportional to the rate of change of input voltage with respect to time. Op-amp differentiators are used in signal processing, waveform shaping, filtering, and instrumentation amplifiers.

Advantages of op-amp

The unique characteristics of operational amplifiers make them highly versatile components.

• Operational amplifiers offer a high voltage gain of about 100,000 (open-loop gain).

• Op-amp ICs do not latch up.

• No frequency compensation is required in op-amp ICs.

• Op-amps have low power dissipation.

Disadvantages of op-amp

Op-amps have inherent limitations that restrict their performance.

• Op-amps have finite input impedance and capacitance and non-zero output impedance.

• Op-amps produce large signal distortions in lower bandwidths.

• Operational amplifiers are not useful in applications with high current ratings.

References

Boylestad, R., & Nashelesky, L. (2012). Electronic Devices and Circuits Theory (11th ed.). Pearson Education.

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