Op-Amp Basics: What Is An Operational Amplifier?

By Saurabh Durgapal and Vinay Minj

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An operational amplifier or op-amp is simply a linear Integrated Circuit (IC) having multiple-terminals. The op-amp can be considered to be a voltage amplifying device that is designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. It is a high-gain electronic voltage amplifier with a differential input and usually a single-ended output. Op-amps are among the most widely used electronic devices today as they are used in a vast array of consumer, industrial and scientific devices.

Brief History

  • In 1947, the first operational amplifier developed from vaccum tubes by John R. Ragazzini of Columbia University.
  • With the development of silicon-based transistor, the concept of ICs became a reality. In the early 1960s, Robert J. Wildar of Fairchild Semiconductor fabricated opamp, the μA702.
  • In 1968, the μA741 was released, leading it to wide production.

Modern day op-amps are available in:

  1. The metal can package (TO) with 8 pins
  2. The dual-in line package (DIP) with 8/14 pins
  3. The flat package of flat pack with 10/14 pins

Construction

The inner schematic of a typical operational amplifier looks likes this:

Operational Amplifier (Op-amp Circuit)
Operational Amplifier (Op-amp Circuit)

The terminal with a (-) sign is called inverting input terminal and the terminal with (+) sign is called non-inverting input terminal.

The V+ and V− power supply terminals are connected to the positive and negative terminals of a DC voltage source respectively. The common terminal of the V+ and V− is connected to a reference point or ground, else twice the supply voltage may damage the op-amp.

Types of Op-Amps

An op-amp has countless applications and forms the basic building block of linear and non-linear analogue systems. Some of the types of op-amp include:

  • A differential amplifier, which is a circuit that amplifies the difference between two signals.
  • The instrumentation amplifier, which is usually built from three op-amps and helps amplify the output of a transducer (consisting of measured physical quantities).
  • The isolation amplifier, which is like an instrumentation amplifier, but having tolerance to common-mode voltages (that destroy an ordinary op-amp).
  • A negative-feedback amplifier, which is usually built from one or more op-amps and a resistive feedback network.
  • Power amplifiers to amplify small signals received from an input source such as microphone or antenna.

Op-Amp Operation

Ideally, an op-amp amplifies only the difference in voltage between the two, also called differential input voltage. The output voltage of the op-amp Vout is given by the equation:

Vout = AOL (V+ – V)

where AOL is the open-loop gain of the amplifier.

In a linear operational amplifier, the output signal is the amplification factor, known as the amplifier’s gain (A) multiplied by the value of the input signal.

Op-Amp Parameters

  • Open-loop gain is the gain without positive or negative feedback. Ideally, the gain should be infinite, but typical real values range from about 20,000 to 200,000 ohms.
  • Input impedance is the ratio of input voltage to input current. It is assumed to be infinite to prevent any current flowing from the source to the amplifiers.
  • The output impedance of an ideal operational amplifier is assumed to be zero. This impedance is in series with the load, thereby increasing the output available for the load.
  • The bandwidth of an ideal operational amplifier is infinite and can amplify any frequency signal from DC to the highest AC frequencies. However, typical bandwidth is limited by the Gain-Bandwidth product, which is equal to the frequency where the amplifier’s gain becomes unity.
  • The ideal output of an amplifier is zero when the voltage difference between the inverting and the non-inverting inputs is zero. Real world amplifiers do exhibit a small output offset voltage.

 

Some other important electrical parameters to consider are:

  • Input offset voltage: It is the voltage that must be applied between the input terminals of an op-amp to nullify the output.
  • Input offset current: It is the algebraic difference between the currents into the (-) input and (+) input.
  • Input bias current: It is the average of the currents entering into the (-) input and (+) input terminals of an op-amp.
  • Input resistance: It is the differential input resistance as seen at either of the input terminals with the other terminal connected to ground.
  • Input capacitance: It is the equivalent capacitance that can be measured at either of the input terminal with the other terminal connected to ground.
  • Slew rate: It is defined as the maximum rate of change of output voltage caused by a step input voltage. The slew rate improves with higher closed loop gain and DC supply voltage. It is also a function to temperature and generally decreases with an increase in temperature.

Note:- Although an ideal op-amp draws no current from the source and its response is independent of temperature, a real op-amp does not work this way.

An op-amp only responds to the difference between the two voltages irrespective of the individual values at the inputs. External resistors or capacitors are often connected to the op-amp in many ways to form basic circuits including Inverting, Non-Inverting, Voltage Follower, Summing, Differential, Integrator and Differentiator type amplifiers. An op-amp is easily available in IC packaging, the most common being the μA-741.

Op-amp Applications

An op-amp has countless applications and forms the basic building block of linear and non-linear analogue systems.

In linear circuits, the output signal varies with the input signal in a linear manner. Some of the linear applications are:

  1. Adder
  2. Subtractor
  3. Voltage to Current Converter (Transconductance Amplifier)
  4. Current to Voltage Converter (Transresistance Amplifier)
  5. Instrumentation amplifier
  6. Power amplifier

Another class of circuits with highly non-linear input to output characteristics are:

  1. Rectifier
  2. Peak detector
  3. Clipper
  4. Clamper
  5. Sample and hold circuit
  6. Log and antilog amplifier
  7. Multiplier and divider
  8. Comparator

Thanks to op-amps and associated circuits, they have become an integral part audio amplifiers, waveform generators, voltage regulators, active filters, 555 timers, A-D and D-A converters.

Op amp(operational amplifier) from Kausik das

You can view the video tutorial on Operational Amplifier is here.


More interesting tutorials available at the learning corner.

This article was first published on 17 July 2017 and was updated on 20 October 2020.

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