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Practical operational amplifier circuits examples

What is the Voltage Divider Basic and Rule? The operational amplifier is an integrated circuit that has two input pins and one output pin. It is used to amplify and output the voltage difference between the two input pins. Based on its characteristics, operational amplifier has different functions in different circuits. Here introduces common and fundamental op amp circuits examples with descriptions.

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WATCH RELATED VIDEO: Lab 5a Operational Amplifier inverting amplifier

Introduction to Ideal Op-Amp Circuit Characteristics

Inst Tools. A real device deviates from a perfect difference amplifier. One minus one may not be zero. It may have an offset like an analog meter which is not zeroed. The inputs may draw current. The characteristics may drift with age and temperature. Gain may be reduced at high frequencies, and phase may shift from input to output.

These imperfection may cause no noticable errors in some applications, unacceptable errors in others. In some cases these errors may be compensated for. Sometimes a higher quality, higher cost device is required. As stated before, an ideal differential amplifier only amplifies the voltage difference between its two inputs. If the two inputs of a differential amplifier were to be shorted together thus ensuring zero potential difference between them , there should be no change in output voltage for any amount of voltage applied between those two shorted inputs and ground:.

This translates to a common-mode voltage gain of zero. The operational amplifier , being a differential amplifier with high differential gain, would ideally have zero common-mode gain as well. In real life, however, this is not easily attained. The performance of a real op-amp in this regard is most commonly measured in terms of its differential voltage gain how much it amplifies the difference between two input voltages versus its common-mode voltage gain how much it amplifies a common-mode voltage.

The ratio of the former to the latter is called the common-mode rejection ratio , abbreviated as CMRR:. An ideal op-amp, with zero common-mode gain would have an infinite CMRR.

Real op-amps have high CMRRs, the ubiquitous having something around 70 dB, which works out to a little over 3, in terms of a ratio. Because the common mode rejection ratio in a typical op-amp is so high, common-mode gain is usually not a great concern in circuits where the op-amp is being used with negative feedback. If the common-mode input voltage of an amplifier circuit were to suddenly change, thus producing a corresponding change in the output due to common-mode gain, that change in output would be quickly corrected as negative feedback and differential gain being much greater than common-mode gain worked to bring the system back to equilibrium.

Sure enough, a change might be seen at the output, but it would be a lot smaller than what you might expect. A consideration to keep in mind, though, is common-mode gain in differential op-amp circuits such as instrumentation amplifiers.

We should expect to see no change in output voltage as the common-mode voltage changes:. Aside from very small deviations actually due to quirks of SPICE rather than real behavior of the circuit , the output remains stable where it should be: at 0 volts, with zero input voltage differential. Our input voltage differential is still zero volts, yet the output voltage changes significantly as the common-mode voltage is changed.

More than that, its a common-mode gain of our own making, having nothing to do with imperfections in the op-amps themselves. With a much-tempered differential gain actually equal to 3 in this particular circuit and no negative feedback outside the circuit, this common-mode gain will go unchecked in an instrument signal application. There is only one way to correct this common-mode gain, and that is to balance all the resistor values.

Suppose that all resistor values are exactly as they should be, but a common-mode gain exists due to an imperfection in one of the op-amps. With the adjustment provision, the resistance could be trimmed to compensate for this unwanted gain. One quirk of some op-amp models is that of output latch-up , usually caused by the common-mode input voltage exceeding allowable limits. In JFET-input operational amplifiers, latch-up may occur if the common-mode input voltage approaches too closely to the negative power supply rail voltage.

On the TL op-amp, for example, this occurs when the common-mode input voltage comes within about 0. Such a situation may easily occur in a single-supply circuit, where the negative power supply rail is ground 0 volts , and the input signal is free to swing to 0 volts.

Latch-up may also be triggered by the common-mode input voltage exceeding power supply rail voltages, negative or positive. As a rule, you should never allow either input voltage to rise above the positive power supply rail voltage, or sink below the negative power supply rail voltage, even if the op-amp in question is protected against latch-up as are the and op-amp models.

At worst, the kind of latch-up triggered by input voltages exceeding power supply voltages may be destructive to the op-amp. While this problem may seem easy to avoid, its possibility is more likely than you might think. Consider the case of an operational amplifier circuit during power-up. If the circuit receives full input signal voltage before its own power supply has had time enough to charge the filter capacitors, the common-mode input voltage may easily exceed the power supply rail voltages for a short time.

If the op-amp receives signal voltage from a circuit supplied by a different power source, and its own power source fails, the signal voltage s may exceed the power supply rail voltages for an indefinite amount of time!

Another practical concern for op-amp performance is voltage offset. That is, effect of having the output voltage something other than zero volts when the two input terminals are shorted together. When that input voltage difference is exactly zero volts, we would ideally expect to have exactly zero volts present on the output.

However, in the real world this rarely happens. Even if the op-amp in question has zero common-mode gain infinite CMRR , the output voltage may not be at zero when both inputs are shorted together. This deviation from zero is called offset. A perfect op-amp would output exactly zero volts with both its inputs shorted together and grounded.

However, most op-amps off the shelf will drive their outputs to a saturated level, either negative or positive. In the example shown above, the output voltage is saturated at a value of positive For this reason, offset voltage is usually expressed in terms of the equivalent amount of input voltage differential producing this effect.

In other words, we imagine that the op-amp is perfect no offset whatsoever , and a small voltage is being applied in series with one of the inputs to force the output voltage one way or the other away from zero. Offset voltage will tend to introduce slight errors in any op-amp circuit. So how do we compensate for it? Unlike common-mode gain, there are usually provisions made by the manufacturer to trim the offset of a packaged op-amp. These connection points are labeled offset null and are used in this general way:.

On single op-amps such as the and , the offset null connection points are pins 1 and 5 on the 8-pin DIP package.

Inputs on an op-amp have extremely high input impedances. We analyze the circuit as though there was absolutely zero current entering or exiting the input connections. This idyllic picture, however, is not entirely true. Op-amps, especially those op-amps with bipolar transistor inputs, have to have some amount of current through their input connections in order for their internal circuits to be properly biased.

These currents, logically, are called bias currents. Under certain conditions, op-amp bias currents may be problematic. The following circuit illustrates one of those problem conditions:. At first glance, we see no apparent problems with this circuit. In other words, this is a kind of comparator circuit , comparing the temperature between the end thermocouple junction and the reference junction near the op-amp.

The problem is this: the wire loop formed by the thermocouple does not provide a path for both input bias currents, because both bias currents are trying to go the same way either into the op-amp or out of it. In order for this circuit to work properly, we must ground one of the input wires, thus providing a path to or from ground for both currents:. Another way input bias currents may cause trouble is by dropping unwanted voltages across circuit resistances. Take this circuit for example:.

We expect a voltage follower circuit such as the one above to reproduce the input voltage precisely at the output. But what about the resistance in series with the input voltage source? But even then, what slight bias currents may remain can cause measurement errors to occur, so we have to find some way to mitigate them through good design.

One way to do so is based on the assumption that the two input bias currents will be the same. In reality, they are often close to being the same, the difference between them referred to as the input offset current. If they are the same, then we should be able to cancel out the effects of input resistance voltage drop by inserting an equal amount of resistance in series with the other input, like this:.

With the additional resistance added to the circuit, the output voltage will be closer to V in than before, even if there is some offset between the two input currents. In either case, the compensating resistor value is determined by calculating the parallel resistance value of R 1 and R 2. Why is the value equal to the parallel equivalent of R 1 and R 2?

This gives two parallel paths for bias current through R 1 and through R 2 , both to ground. A related problem, occasionally experienced by students just learning to build operational amplifier circuits, is caused by a lack of a common ground connection to the power supply. This provides a complete path for the bias currents, feedback current s , and for the load output current.

Take this circuit illustration, for instance, showing a properly grounded power supply:. The effect of doing this is profound:. Thus, no electrons flow through the ground connection to the left of R 1 , neither through the feedback loop.

This effectively renders the op-amp useless: it can neither sustain current through the feedback loop, nor through a grounded load, since there is no connection from any point of the power supply to ground. The bias currents are also stopped, because they rely on a path to the power supply and back to the input source through ground. The following diagram shows the bias currents only , as they go through the input terminals of the op-amp, through the base terminals of the input transistors, and eventually through the power supply terminal s and back to ground.

Without a ground reference on the power supply, the bias currents will have no complete path for a circuit, and they will halt. Since bipolar junction transistors are current-controlled devices, this renders the input stage of the op-amp useless as well, as both input transistors will be forced into cutoff by the complete lack of base current.

Bias currents are small in the microamp range , but large enough to cause problems in some applications. It is not enough to just have a conductive path from one input to the other. To cancel any offset voltages caused by bias current flowing through resistances, just add an equivalent resistance in series with the other op-amp input called a compensating resistor.

This corrective measure is based on the assumption that the two input bias currents will be equal. Any inequality between bias currents in an op-amp constitutes what is called an input offset current.

It is essential for proper op-amp operation that there be a ground reference on some terminal of the power supply, to form complete paths for bias currents, feedback current s , and load current. Being semiconductor devices, op-amps are subject to slight changes in behavior with changes in operating temperature. Any changes in op-amp performance with temperature fall under the category of op-amp drift.

Lessons In Electric Circuits -- Volume III

This article illustrates some typical operational amplifier applications. A non-ideal operational amplifier's equivalent circuit has a finite input impedance, a non-zero output impedance, and a finite gain. A real op-amp has a number of non-ideal features as shown in the diagram, but here a simplified schematic notation is used, many details such as device selection and power supply connections are not shown. Operational amplifiers are optimised for use with negative feedback, and this article discusses only negative-feedback applications. When positive feedback is required, a comparator is usually more appropriate. See Comparator applications for further information. In order for a particular device to be used in an application, it must satisfy certain requirements.

Thus, in a practical scenario, the mathematical expression for the output of the subtractor amplifier can be given as: Where AC is called the.

Practical Considerations of Op-Amp

A device which is integrated into circuits to produce an amplification or gain to the voltage. Electronics Amplifier circuit symbol. Example of an op-amp used as a comparator. Measurement Techniques 2. Kinematics 3. Dynamics 4. Deformation of Solids 7. Waves 8. Superposition 9.

What is an Inverting Op Amp : Working & Its Applications

practical operational amplifier circuits examples

Op-amp Tutorial Includes: Introduction Circuits summary Inverting amplifier Summing amplifier Non-inverting amplifier Variable gain amplifier High pass active filter Low pass active filter Bandpass filter Notch filter Comparator Schmitt trigger Multivibrator Bistable Integrator Differentiator Wien bridge oscillator Phase shift oscillator Operational amplifiers are particularly versatile circuit blocks. They find applications in a host of different circuits where their attributes of high gain, high input impedance low output impedance and a differential input enable them to provide a high performance circuit with a minimum of components. By using negative, and sometimes positive feedback around the op amp chip they can be used in many applications and circuits to provide a variety of different functions from amplifiers and filters to oscillators, integrators and many other functions. There are many op amp circuits that cover most of the main analogue functions that are needed. As a result of this, operational amplifiers have become the workhorse of the analogue electronics designer.

Practical Op-amps approximate their ideal counterparts but differ in some important respects.

Differential Amplifier

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Operational Amplifier Circuits & Applications

In this module, the operational amplifier, more commonly called the op amp is introduced and used as a circuit element. While the op amp is not a fundamental circuit element, practical op amps display nearly ideal behaviour, thereby allowing one to construct many useful circuits with relative ease. Consequently, there are two motivations for studying the ideal op amp and common op amp circuits; Firstly, op amp circuits provide practical examples to practice the many circuit analysis techniques learned in the other modules. Secondly, op amps provide an easy way to implement many useful circuits. This module aims to cover the op amp at a level compatible with these two goals. With careful study, by the end of this module, one should be able to;.

Practical Op-Amp Circuits. Logarithmic Amplifier Example 1: Change the circuit below, in order to eliminate the e ect of input o set.

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Practical Considerations

Inst Tools. A real device deviates from a perfect difference amplifier. One minus one may not be zero. It may have an offset like an analog meter which is not zeroed. The inputs may draw current.

Study-Unit Description. Implementation technologies, design, construction and manufacture.


In different configurations with a few other components, op-amps can be used to process and manipulate an analog voltage signal in many different ways. This includes many kinds of filters low-pass, high-pass, band-pass, integrator, differentiator , amplifiers buffer , inverting , non-inverting , differential, summing, instrumentation , oscillators, comparators, sources voltage, current , converters voltage-to-current, current-to-voltage , and even some nonlinear applications. Today, an op-amp is an integrated circuit IC containing a few dozen individual transistors and passive components. But now that IC op-amps have only a few pins and cost just a few pennies, it usually makes sense to take advantage of their enormous potential for making analog designs simpler. Most op-amps aspire to perform like the ideal op-amp , a theoretical model that both works well in simulation and makes it easy to solve circuits by hand. They can also help you choose the correct op-amp to implement your design. Ideal Op-Amp Symbol.

Basics Of Operational Amplifier

In our first post about OpAmps we stated the basic principles of operation of this popular device. Until now we had just analysed the opamp as an open loop device, where the signals were connected directly to the inputs with no extra electronic components. This is not practical as the open loop gain is way too high, so to build electronic amplifiers we need some kind of negative feedback that controls this gain in a predictable way. This is known as a closed loop opamp design.

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