Operational Amplifiers part2

Input and Output Resistances

Assuming an ideal op amp with infinite open-loop gain, the input resistance of the closed-loop inverting amplifier of Fig.5.5 is simply equal to . This can be seen from Fig. 5.6(b), where

Since the amplifier input resistance forms a voltage divider with the resistance of the source that feeds the amplifiers, to avoid the loss of signal strength, voltage amplifiers are required to have high input resistance. In the case of the inverting op-amp configuration, to make high we should select a high value for .However, if the required

 gain is also high, then could become impractically large (e.g,, greater than a few mega ohms). We may conclude that the inverting configuration suffers from a low input resistance. A solution to this problem is discussed in Example 5.2 below.

Since the output of the inverting configuration is taken at the terminals of the ideal voltage source (see Fig. 5.6a), it follows that the output resistance of the closed-loop amplifier is zero.

Exercise 3

Assuming the op amp to be ideal, derive an expression for the closed-loop gain of the circuit shown in Fig. 5.8. Use this circuit to design an inverting amplifier with a gain of 100 and an input resistance of 1MΩ. Assume that for practical reasons it is required not to use resistors greater than 1 MΩ. Compare your design with that based on the inverting configuration of Fig. 5.5.

Figure 5.8 Circuit for Exercise 3. The circled numbers indicate the sequence of the steps in the analysis.

Solution

The analysis begins at the inverting input terminal of the op amp, where the voltage is

Here we have assumed that the circuit is "working" and producing a finite output voltage Knowing , we can determine the current as follows:

Since zero current flows into the inverting input terminal, all of  will flow through

, and thus

Now we can determine the voltage at node x:

Now, since an input resistance of 1MΩ is required, we select = 1MΩ. Then, with the limitation of using resistors no greater than 1MΩ, the maximum value possible for the first factor in the gain expression is 1 and is obtained by selecting = 1MΩ. To obtain a gain of -100, and must be selected, so that the second factor in the gain expression is 100. If we select the maximum allowed (in this example) value of 1 MΩ for , then the required value of can be calculated to be 10.2 kΩ.

Thus this circuit utilizes three 1MΩ resistors and a 10.2 kΩ resistor. In comparison, if the inverting configuration were used with = 1MΩ we would have required a feedback resistor of 100 MΩ, an impractically large value!

Finally, it is insightful to enquire into the mechanism by which the circuit is able to

realize a large voltage gain without using large resistances in the feedback path. For this, observe that because of the virtual ground at the inverting input terminal of the op amp, and are in effect in parallel. Thus, by making lower than

An Important Application-The Inverting Weighted Summer

A very important application of the inverting configuration is the weighted-summer circuit shown in Fig.5.10. Here we have a resistance in the negative-feedback path (as before), but we have a number of input signals applied to a corresponding resistor and which are connected to the inverting terminal of the op amp. The ideal op amp will have a virtual ground appearing at its negative input terminal. Ohm's law then tells us that the currents

That is, the output voltage is a weighted sum of the input signals  . This circuit is therefore called a weighted summer. Note that each summing coefficient may be independently adjusted by adjusting the corresponding "feed-in" resistor .This simplification of circuit adjustment, is a direct consequence of the virtual ground that exists at the inverting op-amp terminal.

The weighted summer of Fig. 5.10 has the constraint that all the summing coefficients are of the same sign. The need occasionally arises for summing signals with opposite signs. Such a function can be implemented using two op amps as shown in Fig.5.11. Assuming ideal op amps, it can be easily shown that the output voltage is given by

THE NONINVERTING CONFIGURATION

The second closed-loop configuration we shall study is shown in Fig. 5.12. Here the input signal is applied directly to the positive input terminal of the op amp while one terminal of is connected to ground.

The Closed-Loop Gain

Analysis of the non-inverting circuit to determine its closed-loop gain is illustrated in Fig. 5.13. Notice that the order of the steps in the analysis is indicated by circled numbers.

 

Figure 5.13 Analysis of the non-inverting circuit. The sequence of the steps in the analysis is indicated by the circled numbers.

 

Assuming that the op amp is ideal with infinite gain, a virtual short circuit exists between its two input terminals. Hence the difference input signal is

Thus the voltage at the inverting input terminal will be equal to that at the non-inverting input terminal, which is the applied voltage . The current through can then

be determined as . Because of the infinite input impedance of the op amp, this current will flow through as shown in Fig. 5.13. Now the output voltage can be determined from

Further insight into the operation of the noninverting configuration can be obtained by considering the following: Since the current into the op-amp inverting input is zero, the circuit composed of and acts in effect as a voltage divider feeding a fraction of the output voltage back to the inverting input terminal of the op amp; that is,

Then the infinite op-amp gain and the resulting virtual short circuit between the two input terminals of the op-amp forces this voltage to be equal to that applied at the positive input terminal; thus

which yields the gain expression given in Eq. (5.9).

Note: If increases,increases causing to increase as a result of the high (ideally infinite) gain of the op amp. However, a fraction of the increase in will be fed back to the inverting input terminal of the op amp through the voltage divider. The result of this feedback will be to counteract the increase in

, driving back to zero. This degenerative action of negative feedback gives it the alternative name degenerative feedback. Finally, note that the argument above applies equally well if decreases.

Characteristics of the Noninverting Configuration

· The gain of the noninverting configuration is positive—hence the name noninverting.

· The input impedance of this closed-loop amplifier is ideally infinite, since no current flows into the positive input terminal of the op amp.

· The output of the noninverting amplifier is taken at the terminals of the ideal voltage source A() (see the op-amp equivalent circuit in Fig. 5.3), thus the output resistance of the noninverting configuration is zero.

Effect of Finite Open-Loop Gain

Let us consider the effect of the finite op-amp open-loop gain A on the gain of the non- inverting configuration. Assuming the op amp to be ideal, except for having a finite open- loop gain A, it can be shown that the closed-loop gain of the non-inverting amplifier circuit of Fig. 5.12 is given by

Observe that the denominator is identical to that for the case of the inverting configuration, (Eq. 5.5), because it is a result of the fact that both the inverting and the non-inverting configurations have the same feedback loop, which can be readily seen if the input signal source is eliminated (i.e., short-circuited). The numerators, however, are different, for the numerator gives the ideal or nominal closed-loop gain , for the inverting configuration, and for the noninverting configuration. The gain expression in Eq. (5.11) reduces to the ideal value of for

The Voltage Follower

The property of high input impedance is a very desirable feature of the noninverting configuration. It enables using this circuit as a buffer amplifier to connect a source with a high impedance to a low-impedance load. In many applications the buffer amplifier is not required to provide any voltage gain; rather, it is used mainly as an impedance transformer or a power amplifier, In such cases we may make and to obtain the unity-gain amplifier shown in

Fig. 5.19(a), This circuit is commonly referred to as a voltage follower, since the output "follows" the input. In the ideal case,,  and , and the follower has the equivalent circuit shown in Fig. 5.14(b).

Fig 5.14 The unity gain buffer or follower amplifier and (b) Its equivalent circuit model Since in the voltage-follower circuit the entire output is fed back to the inverting input, the circuit is said to have 100% negative feedback. The infinite gain of the op amp then acts to make and hence .

Since the noninverting configuration has a gain greater than or equal to unity, depending on the choice of , some prefer to call it "a follower with gain."

DIFFERENCE AMPLIFIERS

It is one of the very important applications of Opamp. A difference amplifier is one that responds to the difference between the two signals applied at its input

and ideally rejects signals that are common to the two inputs. The representation of signals in terms of their differential and common-mode components was given in Fig.

5.4. Although ideally the difference amplifier will amplify only the differential input signal , and reject completely the common-mode input signal

, practical circuits will have an output voltage , given by

where denotes the amplifier differential gain and denotes its common- mode gain (ideally zero). The efficacy of a differential amplifier is measured by the degree of its rejection of common-mode signals in preference to differential signals. This is usually quantified by a measure known as the common-mode rejection ratio (CMRR), defined as

 This need for difference amplifiers arises frequently in the design of electronic systems, especially those employed in instrumentation. As a common example, consider a transducer providing a small (e.g., 1 mV) signal between its two output terminals while each of the two wires leading from the transducer terminals to the measuring instrument may have a large interference signal (e.g., 1 V) relative to the circuit ground. The instrument front end obviously needs a difference amplifier.

Note: Though the op amp is itself a difference amplifier; it can’t be used as a difference amplifier, because of its very high (ideally infinite) gain which makes it impossible to use by itself.

Figure 5.15 Representing the input signals to a differential amplifier in terms of their differential and common-mode components.

Therefore, we have to devise an appropriate feedback network to connect to the op amp to create a circuit whose closed-loop gain is finite, predictable, and stable.

A Single Op-Amp Difference Amplifier

Design of a difference amplifier is motivated by the observation that the gain of the noninverting amplifier configuration is positive, (1 + R2/R1), while that of the inverting configuration is negative, (- R2/R1). Combining the two configurations together is then a step in the right direction—namely, getting the difference between two input signals. The two gain magnitudes should be made equal in order to reject common-mode signals.

Figure 5.16 A Differential Amplifier

This, however, can be easily achieved by attenuating the positive input signal to reduce the gain of the positive path from (1 + R2/R1) to ( R2/R1). The resulting circuit would then look like that shown in Fig. 5.16, where the attenuation in the positive input path is achieved by the voltage divider (R3-R4). The proper ratio of this voltage divider can be determined from

which can be put in the form,

This condition is satisfied by selecting

Since, the circuit is linear, we can use superposition.

Figure 5.17 Application of Superposition to the Differential Amplifier of Fig.5.16

To apply superposition, we first reduce to zero—that is, ground the terminal to which is applied and then find the corresponding output voltage, which will be due entirely to .We denote this output voltage and its value may be found from the circuit in Fig. 5.17(a), which we recognize as that of the inverting configuration. The existence of and does not affect the gain expression, since no current flows through either of them. Thus,

Next, we reduce to zero and evaluate the corresponding output voltage

. The circuit will now take the form shown in Fig. 5.17(b), which we recognize as the noninverting configuration with an additional voltage divider, made up of and

connected to the input The output voltage is therefore given by

where we have utilized Eq. (5.15).The superposition principle tells us that the output voltage is equal to the sum of Thus we have

Thus, as expected, the circuit acts as a difference amplifier with a differential gain of R2/ R1.

This is possible if the op amp is ideal, and furthermore on the selection of

and so that their ratio matches that of .

Let's next consider the circuit with only a common-mode signal applied at the input, as shown in Fig. 5.18. The figure also shows some of the analysis steps.

Figure 5.18 Analysis of the Difference Amplifier to determine its Common mode Gain Thus,

For the design with the resistor ratios selected according to Eq. (5.15), we obtain as expected.

Note: Any mismatch in the resistance ratios can make nonzero, and hence CMRR finite.

In addition to rejecting common-mode signals, a difference amplifier is usually required to have a high input resistance. To find the input resistance between the two

input terminals (i.e., the resistance seen by ), called the differential input resistance , consider Fig. 2.19.Here we have assumed that the resistors are selected so that and

Note :

i) If the amplifier is required to have a large differential gain (R2/R1), then R1, of necessity will be relatively small and the input resistance will be correspondingly low, a drawback of this circuit.

ii) Another drawback of the circuit is that it is not easy to vary the differential gain of the amplifier.

Both of these drawbacks are overcome in the instrumentation amplifier discussed next.

Controlled Sources

Op amps can be used in various configurations to give controlled sources, where a input voltage or current can be used to control an output voltage or current. Such sources can be used in different Instrumentation circuits.

Voltage Controlled Voltage Source (VCVS)

In such a source output voltage is controlled by the input voltage , represented as .An ideal VCVS is as shown below:

Fig. 5.20 An ideal VCVS

Such a circuit can be built by using the op amp in the inverting or non-inverting mode as shown below:

Figure 5.21 (a) VCVS using Inverting input.(b) VCVS using Noninverting input

For Figure 5.21 (a) the output voltage is

And for Figure 5.21 (b) using non-inverting input, the output voltage is Clearly, the output voltage constitutes a Voltage Source that is controlled by the input current source. Since the Inverting amplifier has a negative voltage series feedback, which gives very high input impedance and a very low output impedance, its equivalent circuit is as shown in Figure 5.22

Voltage Controlled Current Source (VCCS)

An ideal Voltage Controlled Current Source, whose output current is controlled by the input voltage is as shown below:

A practical circuit using an op amp, where the current through the load resistor is controlled by the input voltage can be built as shown below:

Figure 5.24 A practical VCCS Circuit using an op amp

Using the virtual short concept the voltage at the inverting terminal is same as that at the noninverting terminal. Hence the current through is equal to which also flows through the load .

Current Controlled Voltage Source (CCVS)

An ideal Current Controlled Voltage Source, whose output voltage , is controlled by the input current is as shown in Figure 5.25 below:

Figure 5.25 An ideal CCVS Circuit

Here, the current coming from the current source will completely flow through the load resistor , producing an output voltage

Figure 5.26 A practical CCCS Circuit using an op amp

Current Controlled Current Source (CCCS)

Figure 5.27 shows an ideal circuit for a current controlled current Source, whose output current is controlled by or dependent on input current .

Figure 5.27 An ideal CCCS

The same circuit can be implemented using an op amp as shown in Figure 5.28

Figure 5.28 A practical CCCS

A Superior Circuit-The Instrumentation Amplifier

The low-input-resistance problem of the difference amplifier of Fig. 5.16 can be solved by buffering the two input terminals using voltage followers; that is, a voltage follower of the type in Fig. 5.14 is connected between each input terminal and the corresponding input terminal of the difference amplifier. Also additional voltage gain can also be obtained. It is especially interesting, that we can achieve this without compromising the high input resistance simply by using followers-with-gain rather than unity-gain followers. Bulk of the Gain is achieved in this new first stage i.e the differential amplifier, which eases the burden on the difference amplifier in the second stage, leaving it to its main task of implementing the differencing function and thus rejecting common-mode signals.

The resulting circuit is shown in Fig. 5.29(a).It consists of two stages. The first stage is formed by op amps and , and their associated resistors, and the second stage is the difference amplifier formed by op amp A3 and its four associated resistors. Observe that, each of and is connected in the noninverting

configuration and thus realizes a gain of It follows that each of

Advantages:

i) Very high (ideally infinite) input resistance

ii) High differential gain.

iii) High CMRR

iv) Low Output Resistance

The circuit, however, has three major disadvantages:

1. The input common-mode signal is amplified in the first stage by a gain equal to that experienced by the differential signal . This is a very serious issue, for it could result in the signals at the outputs of and being of such large magnitudes that the op amps saturate. But even if the op amps do not saturate, the difference amplifier of the second stage will now have to deal with much larger common-mode signals, with the result that the CMRR of the overall amplifier will inevitably be reduced.

2. The two amplifier channels in the first stage have to be perfectly matched, otherwise a spurious signal may appear between their two outputs. Such a signal would get amplified by the difference amplifier in the second stage.

3. To vary the differential gain, two resistors have to be varied simultaneously, say the two resistors labeled R1. At each gain setting the two resistors have to be perfectly matched, a difficult task.

All three problems can be solved with a very simple wiring change; Simply disconnect the node between the two resistors labeled R1, node X, from ground. The circuit with this small, but functionally profound change is redrawn in Fig. 5.29(b), where we have lumped the two resistors (R1 and R1) together into a single resistor (2 R1). Analysis of the circuit in Fig. 5.29(b), assuming ideal op amps, is straightforward, as is illustrated in Fig. 5.29 (c). The key point is that the virtual short circuits at the inputs of opamps A1 and A2 cause the input voltages , and to appear at the two terminals of resistor 2R1. Thus the differential input voltage , appears across 2R1 and causes a current to flow through 2R1 and the two resistors labeled R2. This current in turn produces a voltage difference between the output terminals of and A1 and A2 given by

 The difference amplifier formed by op amp A3 and its associated resistors senses the voltage difference and provides a proportional output voltage

Thus the overall differential voltage gain is given by,

Response to Common Mode Input Signal

When the two input terminals are connected together to a common-mode input voltage , an equal voltage appears at the negative input terminals of and causing the current through 2 to be zero. Thus there will be no current flowing in the R2 resistors, and the voltages at the output terminals of and will be equal to the input (i.e., ). Thus the first stage no longer amplifies it simply propagates to its two output terminals, where they are subtracted to produce a zero common-mode output by A3. The difference amplifier in the second stage, however, now has a much improved situation at its input: The difference signal has been amplified by while the common-mode voltage remained unchanged.

Note: As seen from the expression in Eq. (5.22), the gain can be varied by changing only one resistor, 2 . Hence this circuit forms an excellent differential amplifier and is widely employed as an instrumentation amplifier; that is, as the input amplifier used in a variety of electronic instruments

Example 3:

Design the instrumentation amplifier circuit in Fig. 5.20(b) to provide a gain that can be varied over the range of 2 to 1000 utilizing a 100kΩ variable resistance (a potentiometer, or "pot" for short).

It is usually preferable to obtain all the required gain in the first stage, leaving the second stage to perform the task of taking the difference between the outputs of the first stage and thereby rejecting the common-mode signal. In other words, the second stage is usually designed for a gain of 1.

Figure 5.30: To obtain variable gain for an Instrumentation Amplifier

EFFECT OF FINITE OPEN-LOOP GAIN AND BANDWIDTH ON CIRCUIT PERFORMANCE

Generally, the analysis of opamp circuits assumed the op amps to be ideal. Although in many applications such an assumption is not a bad one, a circuit designer has to be thoroughly familiar with the characteristics of practical op amps and the effects of such characteristics on the performance of op-amp circuits. Only then will the designer be able to use the op amp intelligently, especially if the application at hand is not a straightforward one.

Note: The non-ideal properties of op amps will, of course, limit the range of operation of the circuits analyzed in the previous examples.

Let us consider some of the important non-ideal properties of the op amp. We do this by treating one parameter at a time, beginning with the most serious op-amp non-idealities, its finite gain and limited bandwidth.

Frequency Dependence of the Open-Loop Gain

The differential open-loop gain of an op amp is not infinite; rather, it is finite and decreases with frequency, Figure 5.30 shows a plot for lAI, with the numbers typical of most commercially available general-purpose op amps (such as the 741- type op amp, which is available from many semiconductor manufacturers).

at a rather low frequency (10 Hz in our example). The uniform -20dB/decade gain rolloff shown is typical of internally compensated op amps. These are units that have a network (usually a single capacitor) included within the same IC chip whose function is to cause the op-amp gain to have the single-time-constant (STC) low-pass response shown. This process of modifying the open-loop gain is termed frequency compensation, and its purpose is to ensure that op-amp circuits will be stable (as opposed to oscillatory).

By analogy to the response of low-pass STC circuits, the gain A(s) of an internally compensated op amp may be expressed as

Thus if is known (106 Hz in our example), one can easily determine the magnitude of the op-amp gain at a given frequency . Furthermore, this relationship correlates with the Bode plot in Fig. 5.22. Specifically, for , doubling (an octave increase) results in halving the gain (a 6-dB reduction). Similarly, increasing

by a factor of 10 (a decade increase) results in reducing lAI by a factor of 10 (20 dB).

Note: The production spread in the value of , between op-amp units of the same type is usually much smaller than that observed for and . For this reason is preferred as a specification parameter.

Finally, it should be mentioned that an op amp having this uniform -6-dB/octave (or equivalently -20-dB/decade) gain rolloff is said to have a single-pole model. Also, since this single pole dominates the amplifier frequency response, it is called a dominant pole. ( Appendix E-Sedra and Smith).

Frequency Response of Closed-Loop Amplifiers

The effect of limited op-amp gain and bandwidth on the closed-loop transfer functions of the two basic configurations: the inverting circuit of Fig. 5.5 and the non-inverting circuit of Fig. 5.12 can be determined. The closed-loop gain of the inverting amplifier, assuming a finite op-amp open-loop gain A, is

which is of the same form as that for a low-pass STC network. Thus the inverting amplifier has an STC low-pass response with a dc gain of magnitude equal to  .The closed-loop gain rolls off at a uniform —20dB/decade slope with a corner frequency (3-dB frequency) given by

LARGE-SIGNAL OPERATION OF OP AMPS

In this section, we study the limitations on the performance of op-amp circuits when large output signals are present.

Output Voltage Saturation

Like all other amplifiers, op amps operate linearly over a limited range of output voltages. Specifically, the op-amp output saturates in the manner shown in Fig. 5.35 with L+ and L- within 1V or so of the positive and negative power supplies, respectively. Thus, an op amp that is operating from +15V supplies will saturate when the output voltage reaches about +13 V in the positive direction and -13 V in the negative direction. For this particular op amp the rated output voltage is said to be ±13 V. To avoid clipping off the peaks of the output waveform, and the resulting waveform distortion, the input signal must be kept correspondingly small.

Figure 5.32 (a) A noninverting amplifier with a nominal gain of 10 V/V designed using an op amp that saturates at ±13V output voltage and has ±20mA output current limits.

(b) When the input sine wave has a peak of 1.5 V, the output is clipped off at ±13 V.

Output Current Limits

Another limitation on the operation of op amps is that their output current is limited to a specified maximum. For instance, the popular 741 op amp is specified to have a maximum output current of ±20 mA. Thus, in designing closed-loop circuits utilizing the 741, the designer has to ensure that under no condition will the op amp be required to supply an output current, in either direction, exceeding 20 mA. This, of course, has to include both the current in the feedback circuit as well as the current supplied to a load resistor. If the circuit requires a larger current, the op-amp output voltage will saturate at the level corresponding to the maximum allowed output current.

Slew Rate

Another phenomenon that can cause nonlinear distortion when large output signals are present is that of slew-rate limiting. This refers to the fact that there is a specific

maximum rate of change possible at the output of a real op amp. This maximum is known as the slew rate (SR) of the op amp and is defined as

and is usually specified on the op-amp data sheet in units of V/μs. It follows that if the input signal applied to an op-amp circuit is such that it demands an output response that is faster than the specified value of SR, the op amp will not comply. Rather, its output will change at the maximum possible rate, which is equal to its SR.

As an example, consider an op amp connected in the unity-gain voltage-follower configuration shown in Fig. 5.33(a), and let the input signal be the step voltage shown in Fig. 5.33(b). The output of the op amp will not be able to rise instantaneously to the ideal value V; rather, the output will be the linear ramp of slope equal to SR, shown in Fig5.33(c). The amplifier is then said to be slewing, and its output is slew-rate limited.

Note: The SR phenomenon is distinct from the finite op amp bandwidth that limits the frequency response of closed loop amplifiers. The limited bandwidth is a linear phenomenon and does not result in a change in the shape of an input sinusoid; that is,

it does not lead to nonlinear distortion, The slew-rate limitation, on the other hand, can cause nonlinear distortion to an input sinusoidal signal when its frequency and amplitude are such that the corresponding ideal output would require to change at a rate greater than SR.

Also, if the step input voltage V is sufficiently small, the output can be the exponentially rising ramp shown in Fig. 5.33(d). Such an output would be expected from the follower if the only limitation on its dynamic performance is the finite op-amp bandwidth. Specifically, the transfer function of the follower can be found by

substituting =∞ and in Eq. (5.48) to obtain

which is a low-pass STC response with a time constant 1/. Its step response would therefore be (see Appendix D-S&S)

The initial slope of this exponentially rising function is Thus, as long as V is sufficiently small so that , the output will be as in Fig. 5.33(d).

Full-Power Bandwidth

Op-amp slew-rate limiting can cause nonlinear distortion in sinusoidal waveforms. Consider the unity-gain follower with a sine wave input given by

The rate of change of this waveform is given by

with a maximum value of This maximum occurs at the zero crossings of the input sinusoid. Now if exceeds the slew rate of the op amp, the output waveform will be distorted in the manner shown in Fig. 5.34. Observe that the output cannot keep up with the large rate of change of the sinusoid at its zero crossings, and the op amp slews.

Figure 5.34 Effect of slew-rate limiting on output sinusoidal waveforms.

The op-amp data sheets usually specify a frequency called the full-power bandwidth. It is the frequency at which an output sinusoid with amplitude equal to the rated output voltage of the op amp begins to show distortion due to slew-rate limiting. If we denote the rated output voltage is related to SR as follows:

Thus,

It should be obvious that output sinusoids of amplitudes smaller than will show slew-rate distortion at frequencies higher than In fact, at a frequency higher than the maximum amplitude of the undistorted output sinusoid is given by

DC IMPERFECTIONS

1.Offset Voltage

Because op amps are direct-coupled devices with large gains at dc, they are prone to dc problems. The first such problem is the dc offset voltage. To understand this problem consider the following conceptual experiment; If the two input terminals of the op amp are tied together and connected to ground, it will be found that a finite dc voltage exists at the output. In fact, if the op amp has a high dc gain, the output will be at either the positive or negative saturation level. The op-amp output can be brought back to its ideal value of 0 V by connecting a dc voltage source of appropriate polarity and magnitude between the two input terminals of the op amp. This external source balances out the input offset voltage of the op amp. It follows that the input offset voltage must be of equal magnitude and of opposite polarity to the voltage we applied externally.

Figure 5.35 Circuit model for an op amp with input offset voltage VOS.

The input offset voltage arises as a result of the unavoidable mismatches present in the input differential stage inside the op amp. The effect of on the operation of closed-loop op-amp circuits are:

i) General-purpose op amps exhibit in the range of 1 mV to 5 mV.

ii) Also, the value of depends on temperature. The op-amp data sheets usually specify typical and maximum values for at room temperature as well as the temperature coefficient of (usually in μV/°C). They do not, however, specify the polarity of because the component mismatches that give rise to are obviously not known apriori;

iii) Different units of the same op-amp type may exhibit either a positive or a negative .

To analyze the effect of on operation of op-amp circuits, we need a circuit model for the op amp with input offset voltage. Such a model is shown in Fig. 5.35. It consists of a dc source of value placed in series with the positive input lead of an offset-free op amp. The justification for this model follows from the description above,

Analysis: The input voltage signal source is short circuited and the op amp is replaced with the model of Fig. 5.35. (Eliminating the input signal, done to simplify, is based on the principle of superposition.) Following this procedure we find that both the inverting and the noninverting amplifier configurations result in the same circuit, that shown in Fig. 5.36, from which the output dc voltage due to is found to be

 

Figure 5.36 Evaluating the output dc offset voltage due to in a closed-loop amplifier.

This output dc voltage can have a large magnitude. For instance, a noninverting amplifier with a closed-loop gain of 1000, when constructed from an op amp with a 5-mV input oftset voltage, will have a dc output voltage of 4-5 V or — 5 V (depending on the polarity of , rather than the ideal value of 0 V. Now, when an input signal is applied to the amplifier, the corresponding signal output will be superimposed on the 5-V dc. Then, the allowable signal swing at the output will be reduced. If the signal to be amplified is dc, we would not know whether the output is due to or to the signal!

Some op amps are provided with two additional terminals to which a specified circuit can be connected to trim to zero the output dc voltage due to Vos Figure 5.37 shows such an arrangement that is typically used with general-purpose op amps. A potentiometer is connected between the offset-nulling terminals with the wiper of the potentiometer connected to the op-amp negative supply. Moving the potentiometer wiper introduces an imbalance that counteracts the asymmetry present in the internal op- amp circuitry and that gives rise to Vos. It should be noted, however, that even though the dc output offset can be trimmed to zero, the problem remains of the variation (or drift) of with temperature.

Figure 5.37 The output dc offset voltage of an op amp can be trimmed to zero by connecting a potentiometer to the two offset-nulling terminals. The wiper of the potentiometer is connected to the negative supply of the op amp.

One way to overcome the dc offset problem is by capacitively coupling the amplifier. This will be possible only in applications where the closed-loop amplifier is not required to amplify dc or very low frequency signals. Figure 5.38(a) shows a capacitively coupled amplifier. Because of its infinite impedance at dc, the coupling capacitor will cause the gain to be zero at dc. As a result the equivalent circuit for determining the dc output voltage resulting from the op-amp input offset voltage will be that shown in Fig, 5.38(b).

Figure 5.38 (a) A capacitively coupled inverting amplifier. (b) The equivalent circuit for determining its dc output offset voltage VO.

Thus sees in effect a unity-gain voltage follower, and the dc output voltage will be equal to rather than which is the case without the coupling capacitor.

As far as input signals are concerned, the coupling capacitor C forms together with an STC high-pass circuit with a corner frequency of Thus the gain of the capacitively coupled amplifier will fall off at the low-frequency end from a

magnitude of at high frequencies, and will be 3 dB down at

Input Bias and Offset Currents

The second dc problem encountered in op amps is illustrated in Fig. 5.32. In order for the op amp to operate, its two input terminals have to be supplied with dc currents, termed the input bias currents. In Fig. 5.39 these two currents are represented by two current sources, and , connected to the two input terminals. It should be emphasized that the input bias currents are independent of the fact that a real op amp has finite though large input resistance . The op-amp manufacturer usually specifies the average value of and as well as their expected difference. The average value is called the input bias current, and the difference is called the input offset current and is given by Typical values for general-purpose op amps that use bipolar transistors are = 100 nA and 10 nA. Op amps that utilize field-effect transistors in the input stage have a much smaller input bias current (of the order of picoamperes).

Figure 5.39 The op-amp input bias currents represented by two current sources IB1 and

IB2.

To find the dc output voltage of the closed-loop amplifier due to the input bias currents, we ground the signal source and obtain the circuit shown in Fig. 5.40 for both the inverting and noninverting configurations.

Figure 5.40 Analysis of the closed-loop amplifier, taking into account the input bias currents.

As shown in Fig. 5.40, the output dc voltage is given by,

This obviously places an upper limit on the value of .Fortunately, however, a technique exists for reducing the value of the output dc voltage due to the input bias currents. The method consists of introducing a resistance in series with the noninverting input lead, as shown in Fig. 5.41. From a signal point of view, has a negligible effect (ideally no effect).

Figure 5.41 Reducing the effect of the input bias currents by introducing a resistor R3.

The appropriate value for can be determined by analyzing the circuit in Fig. 5.41, where the output voltage is given by  Consider first the case , which results in Thus we can reduce to zero by selecting such that That is, should be made equal to the parallel equivalent of and .

Having selected as above, let us evaluate the effect of a finite offset current . Let and substitute in Eq. (5.58).The result is

which is usually about an order of magnitude smaller than the value obtained without Concluding, to minimize the effect of the input bias currents one should place in the positive lead resistance equal to the dc resistance seen by the inverting input terminal.

This is only for dc operation. If the amplifier is ac coupled, w e should select as shown in Fig. 5.42.

Figure 5.42 In an ac-coupled amplifier the dc resistance seen by the inverting terminal is

R2; hence R3 is chosen equal to R2.

Note: For ac-coupled amplifiers, one must always provide a continuous dc path between each of the input terminals of the op amp and ground. For this reason the ac-coupled noninverting amplifier of Fig. 5.42 will not work without the resistance to ground. Unfortunately, including lowers considerably the input resistance of the closed-loop amplifier.

Figure 5.43 Illustrating the need for a continuous dc path for each of the op-amp input terminals. Specifically, note that the amplifier will not work without resistor R3.