Operational Amplifier (Op-Amp)

Thursday, April 12th, 2018 - Analog Electronics

The operational amplifier is an extremely efficient and versatile device. Its applications span the broad electronic industry filling requirements for signal conditioning, special transfer functions, analog instrumentation, analog computation, and special systems design. The analog assets of simplicity and precision characterize circuits utilizing operational amplifiers.

Operational Amplifier Applications (Computation, Control, Instrumentation)

Originally, the term, “Operational Amplifier,” was used in the computing field to describe amplifiers that performed various mathematical operations. It was found that the application of negative feedback around a high gain DC amplifier would produce a circuit with a precise gain characteristic that depended only on the feedback used. By the proper selection of feedback components, operational amplifier circuits could be used to add, subtract, average, integrate, and differentiate.

As practical operational amplifier techniques became more widely known, it was apparent that these feedback techniques could be useful in many control and instrumentation applications. Today, the general use of operational amplifiers has been extended to include such applications as DC Amplifiers, AC Amplifiers, Comparators, Servo Valve Drivers, Deflection Yoke Drivers, Low Distortion Oscillators, AC to DC Converters, Multivibrators, and a host of others.

What the operational amplifier can do is limited only by the imagination and ingenuity of the user. With a good working knowledge of their characteristics, the user will be able to exploit more fully the useful properties of operational amplifiers.

Notation and Terminology Of Operational Amplifier

Power pins are often omitted from the schematic symbol when the power supply voltages are explicit elsewhere in the schematic. Some op amp symbols also include offset nulling pins, enable / disable pins, output voltage threshold inputs, and other specialized functions.

Operational Amplifier simbols

Operational Amplifier simbols

  1. Symbol (a) is a buffer op amp
  2. Symbol (b) is a differential input, single ended output op amp. This symbol represents the most common types of op amps, including voltage feedback, and current feedback. It is often times pictured with the non-inverting input at the top and the inverting input at the bottom.
  3. Symbol (c) is a differential input, differential output op amp. The outputs can be thought of as “inverting” and “non-inverting”, and are shown across from the opposite polarity input for easy completion of feedback loops on schematics.

Operational Amplifier Package Options

Operational Amplifier Package OptionsFigure above represents a small number of the many op amp package options.

Input Terminals Of Operational Amplifier

The “-“ pin is the “inverting input” or “summing point,” meaning a positive voltage produces a negative voltage at the output on symbol (b), and the top (non-inverting) output on symbol (c). When only one input or output terminal exists, its voltage is measured with respect to ground. This is indicated by the term, “single ended.” It is a popular ambiguity not to explain if a circuit, earth, or chassis ground is meant by this, so the use of a common line is preferred with the ground symbol used to indicate which line is the common.

When there is an inverting input, the voltage at the inverting input may be measured with respect to the non-inverting input. In use, such an amplifier responds to the difference between the voltages at the inverting and non-inverting inputs, i.e., a “differential input.

In many circuits, the non-inverting input is connected to ground. Due to the high gain of operational amplifiers, only a very small input voltage then appears at the output and the output is virtually at ground potential. For purposes of circuit analysis, it can be assumed to be ground – a “virtual ground.”

Output Terminals Of Opertional Amplifier

The relation between the inverting and non-inverting inputs and the output was stated above. For the symbol of (c) the second output voltage is approximately equal and opposite in polarity to the other output voltage, each measured with respect to ground. When the two outputs are used as the output terminals without ground reference, they are known as “differential outputs”.

Power Connections Of Operational Amplifier

Power Supply Connections Of Operational AmplifierPower is supplied to each of these units at connections as shown in figure above. Such a connection is implied in all operational amplifier circuits. The dual supply presents the same absolute value of voltage to ground from either side, while the center connection ultimately defines the common line and ground potential. The exceptions to this are AC amplifier circuits that may use a single power supply. This is accomplished by creating a floating AC ground with DC blocking capacitors. In such circuits, a source of “half-supply” creates a “virtual ground” exactly half way between the positive supply and ground potentials of operational amplifer.

The Feedback Technique Of Operational Amplifier

The precision and flexibility of the operational amplifier is a direct result of the use of negative feedback. Generally speaking, amplifiers employing feedback will have superior operating characteristics at a sacrifice of gain.

Operational Amplifier with Feedback

With enough feedback, the closed loop amplifier characteristics become a function of the feedback elements. In the typical feedback circuit, figure 1, the feedback elements are two resistors. The precision of the “closed loop” gain is set by the ratio of the two resistors and is practically independent of the “open loop” amplifier. Thus, amplification to almost any degree of precision can be achieved with ease.

Summary of Notation The Operational Amplifier

Summary of Notation IntroducedIf it is understood that the there may not be two inputs or two outputs, figure above is a concise summary of the notation introduced. The arrows denote the “direction” of the polarity at operational amplifier each pin.

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