Op-Amp Basics: What Is An Operational Amplifier?

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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.

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.

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