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Op amp investing amplifier theory of plate

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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:. The concept of feedback is crucial to our understanding of op amp circuits. A negative feedback is achieved when the output is fed back to the inverting terminal of the op amp. When there is a feedback path from output to input of the op amp, the ratio of the output voltage to the input voltage is called the closed loop gain.

As a result of the negative feedback, it can be shown that the closed-loop gain is almost insensitive to the open-loop gain A of the op amp. For this reason, op amps are used in circuit with feedback paths. In other words, the output voltage is dependent on and is limited by the power supply voltage.

Although we shall always operate the op amp in the linear region, the possibility of saturation must be borne in mind when one design with op amps, to avoid designing op amp circuits that will not work in the laboratory. To facilitate the understanding of op amp circuits, we will assume ideal op amp. An op amp is ideal if it has the following characteristics:.

An ideal op amp is an amplifier with infinite open-loop gain, infinite input resistance and zero output resistance. Although assuming an ideal op amp provides only approximate analysis, most modern amplifiers have such large gain and input impedance that the approximate analysis is a good one.

Unless stated otherwise, we will assume from now on that every op amp is ideal. For circuit analysis, the ideal op amp is illustrated in [link] , which is derived from the nonideal model in [link]. Two important characteristics of the op amp are:. This is due to infinite input resistance. An infinite resistance between the input terminals implies that an open circuit exists there and current cannot enter the op amp. But the output circuit is not necessary zero according to [link].

The voltage across the input terminals is negligibly small; i. Thus, an ideal op amp has zero current into its two input terminals and negligibly small voltage between the two input terminals. In this and following sections, we consider some useful op amp circuits that often serve as modules for designing more complex circuits.

The first of such op amp circuits is the inverting amplifier shown in [link]. In this circuit, the noninverting circuit is grounded. Our goal is to obtain the relationship the input voltage vi and the output voltage v0. Applying KCL at node 1,. The designation of the circuit in [link] as an inverter arises from the negative sign. An inverting amplifier reverses the polarity of the input signal while amplifying it.

Note that the gain is the feedback resistance divided by the input resistance which means that the gain depends only on the external elements connected to the op amp. In view of [link] , an equivalent circuit for the inverting amplifier is shown in [link].

The inverting amplifier is used, for example, in current to voltage converter. Another important application of the op amp is the noninverting amplifier shown in [link]. We are interested in the output voltage and the voltage gain. Application of KCL at the inverting terminal gives. Thus, the output has the same polarity as the input. A noninverting amplifier is an op amp circuit designed to provide a positive voltage gain. Again we notice that the gain depends only on the external resistors.

Thus, for a voltage follower. Such circuit has very high input impedance and is therefore useful as an intermediate-stage or buffer amplifier to isolate one circuit from another as portrayed in [link]. The voltage follower minimizes interaction between the two stages and eliminates interstage loading. Besides amplification, the op amp can perform addition and subtraction. The addition is performed by the summing amplifier covered in this section; the subtraction is performed by the difference amplifier covered in the next section.

A summing amplifier is an op amp circuit that several inputs and produce an output that is the weighted sum of the inputs. The summing amplifier, shown in [link] , is variation of the inverting amplifier. It takes advantage of the fact that the inverting configuration can handle many inputs at the same time. We keep in mind that the current entering each op amp input is zero. Applying KCL at node a gives. We get. Indicating that the output voltage is a weighted sum of the inputs.

For this reason, the circuit in [link] is called a summer. Needless to say, the summer can have more than three inputs. Difference or differential amplifiers are used in various applications where there is need to amplify the difference between two input signals. They are first cousins of the instrumentation amplifier, the most useful and popular amplifier, which we will discuss in section 9.

A difference amplifier is a device that amplifies the difference between two inputs but rejects any signals common to the two inputs. Consider the op amp circuit shown in [link]. Keep in mind that zero currents enter the op amp terminals. Applying KCL to node a,. Substituting [link] into [link] yields. This property exists when. Thus, when the op amp circuit is a difference amplifier, [link] becomes.

This factor is a closed-loop differential gain. As previously stated, another important characteristic of electronic circuits is the input resistance. The differential input resistance of the differential amplifier can be determined using the circuit shown in [link].

The input resistance is then defined as. Taking into account the virtual short concept, we can write a loop equation, as follows:. The common mode input voltage is defined as. A nonzero common mode gain may be generated in actual op-amp circuits. A figure of merit for a difference amplifier is the common mode rejection ratio CMRR , which is defined as the amplitude of the ratio of differential gain to common mode gain, or. Ideally, the common mode rejection ratio is infinite.

In an actual differential amplifier, we would like the common mode rejection ratio to be as large as possible. As we know, op amp circuits are modules or building blocks for designing complex circuits. It is necessary in practical applications to connect op amp circuits in cascade i. In general, two circuits are cascaded when they are connected in tandem, one behind another in a single file. A cascade connection is a head to tail arrangement of two or more op amp circuits such that the output of one is the input of the next.

When op amp circuits are cascaded, each circuit in the string is called a stage; one original input signal is increased by the gain of the individual stage. Op amp circuits have the advantage that they can be cascaded without changing their input-output relationships.

This is due to the fact that each ideal op amp circuit has infinite input resistance and zero output resistance. Since the output of one stage is the input to the next stage, the overall gain of the cascade connection is the product of the gain of the individual op amp circuits, or.

Although the cascade connection does not effect the op amp input-output relationships, care must be exercised in the design of an actual op amp circuit to ensure that the load due to the next stage in the cascade does not saturate the op amp. The op amp is a fundamental building block in modern electronic instrumentation. It is used extensively in many devices, along with resistors and other passive elements. Its numerous practical applications include instrumentation amplifiers, digital to analog converters, analog computers, level shifters, filters, calibration circuits, inverters, summers, integrators, differentiators, substractors, logarithmic amplifiers, comparators, oscillators, rectifiers, regulators, voltage to current converters, current to voltage converters and clippers.

Some of these we have already considered. We will consider four more applications here: the digital-to-analog converter, the instrumentation amplifier the log amplifier and anti-log amplifier. The digital-to-analog converter DAC transforms digital signal into analog form.

A typical example of a four-bit DAC is illustrated in [link] a. The four-bit DAC can be realized in many ways. A simple realization is the binary weighted ladder , shown in [link] b. This is obviously an inverting summing amplifier.

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