Operational amplifier applications
A non-ideal op amp's equivalent circuit, showing a finite input impedance, non-zero output impedance, and a finite gain. This article illustrates some typical applications of operational amplifiers. A real op amp has a number of non-ideal features as shown in the diagram, but here a simplified schematic notation is used, and the reader is reminded that 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.
Practical considerations
Operational amplifiers parameter requirements
In order for a particular device to be used in an application, it must satisfy certain requirements. The operational amplifier must
- have large open-loop signal gain (voltage gain of 200,000 is obtained in early integrated circuit exemplars), and
- have input impedance large with respect to values present in the feedback network.
With these requirements satisfied, the op amp is considered ideal, and one can use the method of virtual ground to quickly and intuitively grasp the 'behavior' of any of the op amp circuits below.
Component specification
Resistors used in practical solid-state op-amp circuits are typically in the kΩ range. Resistors much greater than 1 MΩ cause excessive thermal noise and make the circuit operation susceptible to significant errors due to bias or leakage currents.
Input bias currents and input offset
Practical operational amplifiers draw a small current from each of their inputs due to bias requirements (in the case of bipolar junction transistor-based inputs) or leakage (in the case of MOSFET-based inputs).
These currents flow through the resistances connected to the inputs and produce small voltage drops across those resistances. Appropriate design of the feedback network can alleviate problems associated with input bias currents and common-mode gain, as explained below. The heuristic rule is to ensure that the impedance "looking out" of each input terminal is identical.
To the extent that the input bias currents do not match, there will be an effective input offset voltage present, which can lead to problems in circuit performance. Many commercial op amp offerings provide a method for tuning the operational amplifier to balance the inputs (e.g., "offset null" or "balance" pins that can interact with an external voltage source attached to a potentiometer). Alternatively, a tunable external voltage can be added to one of the inputs in order to balance out the offset effect. In cases where a design calls for one input to be short-circuited to ground, that short circuit can be replaced with a variable resistance that can be tuned to mitigate the offset problem.
Operational amplifiers using MOSFET-based input stages have input leakage currents that will be, in many designs, negligible.
Power supply effects
Although power supplies are not indicated in the (simplified) operational amplifier designs below, they are nonetheless present and can be critical in operational amplifier circuit design.
Supply noise
Power supply imperfections (e.g., power signal ripple, non-zero source impedance) may lead to noticeable deviations from ideal operational amplifier behavior. For example, operational amplifiers have a specified power supply rejection ratio that indicates how well the output can reject signals that appear on the power supply inputs. Power supply inputs are often noisy in large designs because the power supply is used by nearly every component in the design, and inductance effects prevent current from being instantaneously delivered to every component at once. As a consequence, when a component requires large injections of current (e.g., a digital component that is frequently switching from one state to another), nearby components can experience sagging at their connection to the power supply. This problem can be mitigated with appropriate use of bypass capacitors connected across each power supply pin and ground. When bursts of current are required by a component, the component can bypass the power supply by receiving the current directly from the nearby capacitor (which is then slowly recharged by the power supply).
Using power supply currents in the signal path
Additionally, current drawn into the operational amplifier from the power supply can be used as inputs to external circuitry that augment the capabilities of the operational amplifier. For example, an operational amplifier may not be fit for a particular high-gain application because its output would be required to generate signals outside of the safe range generated by the amplifier. In this case, an external push–pull amplifier can be controlled by the current into and out of the operational amplifier. Thus, the operational amplifier may itself operate within its factory specified bounds while still allowing the negative feedback path to include a large output signal well outside of those bounds.^{[1]}
Amplifiers
We begin these examples with that of the differential amplifier, from which many of the other applications can be derived, including the inverting, non-inverting, and summing amplifier, the voltage follower, integrator, differentiator, and gyrator.
Differential amplifier (difference amplifier)
Amplifies the difference in voltage between its inputs.
- The name "differential amplifier" must not be confused with the "differentiator," which is also shown on this page.
- The "instrumentation amplifier," which is also shown on this page, is a modification of the differential amplifier that also provides high input impedance.
The circuit shown computes the difference of two voltages, multiplied by some gain factor. The output voltage:
Or, expressed as a function of the common mode input V_{com} and difference input V_{dif}
the output voltage is
In order for this circuit to produce a signal proportional to the voltage difference of the input terminals, the coefficient of the V_{com} term (the common-mode gain) must be zero, or
With this constraint^{[nb 1]} in place, the common-mode rejection ratio of this circuit is infinitely large, and the output
where the simple expression R_{f} / R_{1} represents the closed-loop gain of the differential amplifier.
The special case when the closed-loop gain is unity is a differential follower, with:
Inverting amplifier
An inverting amplifier is a special case of the differential amplifier in which that circuit's non-inverting input V_{2} is grounded, and inverting input V_{1} is identified with V_{in} above. The closed-loop gain is R_{f} / R_{in}, hence
- .
The simplified circuit above is like the differential amplifier in the limit of R_{2} and R_{g} very small. In this case, though, the circuit will be susceptible to input bias current drift because of the mismatch between R_{f} and R_{in}.
To intuitively see the gain equation above, calculate the current in R_{in}:
then recall that this same current must be passing through R_{f}, therefore (because V_{−} = V_{+} = 0):
A mechanical analogy is a seesaw, with the V_{−} node (between R_{in} and R_{f}) as the fulcrum, at ground potential. V_{in} is at a length R_{in} from the fulcrum; V_{out} is at a length R_{f}. When V_{in} descends "below ground", the output V_{out} rises proportionately to balance the seesaw, and vice versa.^{[2]}
Non-inverting amplifier
A non-inverting amplifier is a special case of the differential amplifier in which that circuit's inverting input V_{1} is grounded, and non-inverting input V_{2} is identified with V_{in} above, with R_{1} ≫ R_{2}. Referring to the circuit immediately above,
- .
To intuitively see this gain equation, use the virtual ground technique to calculate the current in resistor R_{1}:
then recall that this same current must be passing through R_{2}, therefore:
Unlike the inverting amplifier, a non-inverting amplifier cannot have a gain of less than 1.
A mechanical analogy is a class-2 lever, with one terminal of R_{1} as the fulcrum, at ground potential. V_{in} is at a length R_{1} from the fulcrum; V_{out} is at a length R_{2} further along. When V_{in} ascends "above ground", the output V_{out} rises proportionately with the lever.
The input impedance of the simplified non-inverting amplifier is high, of order R_{dif} × A_{OL} times the closed-loop gain, where R_{dif} is the op amp's input impedance to differential signals, and A_{OL} is the open-loop voltage gain of the op amp; in the case of the ideal op amp, with A_{OL} infinite and R_{dif} infinite, the input impedance is infinite. In this case, though, the circuit will be susceptible to input bias current drift because of the mismatch between the impedances driving the V_{+} and V_{−} op amp inputs.
Voltage follower (unity buffer amplifier)
Used as a buffer amplifier to eliminate loading effects (e.g., connecting a device with a high source impedance to a device with a low input impedance).
- (realistically, the differential input impedance of the op-amp itself, 1 MΩ to 1 TΩ)
Due to the strong (i.e., unity gain) feedback and certain non-ideal characteristics of real operational amplifiers, this feedback system is prone to have poor stability margins. Consequently, the system may be unstable when connected to sufficiently capacitive loads. In these cases, a lag compensation network (e.g., connecting the load to the voltage follower through a resistor) can be used to restore stability. The manufacturer data sheet for the operational amplifier may provide guidance for the selection of components in external compensation networks. Alternatively, another operational amplifier can be chosen that has more appropriate internal compensation.
Summing amplifier
A summing amplifier sums several (weighted) voltages:
- When , and independent
- When
- Output is inverted
- Input impedance of the nth input is ( is a virtual ground)
Instrumentation amplifier
Combines very high input impedance, high common-mode rejection, low DC offset, and other properties used in making very accurate, low-noise measurements
- Is made by adding a non-inverting buffer to each input of the differential amplifier to increase the input impedance.
Oscillators
Wien bridge oscillator
Produces a very low distortion sine wave. Uses negative temperature compensation in the form of a light bulb or diode.
Filters
Operational amplifiers can be used in construction of active filters, providing high-pass, low-pass, band-pass, reject and delay functions. The high input impedance and gain of an op-amp allow straightforward calculation of element values, allowing accurate implementation of any desired filter topology with little concern for the loading effects of stages in the filter or of subsequent stages. However, the frequencies at which active filters can be implemented is limited; when the behavior of the amplifiers departs significantly from the ideal behavior assumed in elementary design of the filters, filter performance is degraded.
Comparator
An operational amplifier can, if necessary, be forced to act as a comparator. The smallest difference between the input voltages will be amplified enormously, causing the output to swing to nearly the supply voltage. However, it is usually better to use a dedicated comparator for this purpose, as its output has a higher slew rate and can reach either power supply rail. Some op-amps have clamping diodes on the input that prevent use as a comparator.^{[3]}
Integration and differentiation
Inverting integrator
The integrator is mostly used in analog computers, analog-to-digital converters and wave-shaping circuits.
Integrates (and inverts) the input signal V_{in}(t) over a time interval t, t_{0} < t < t_{1}, yielding an output voltage at time t = t_{1} of
where V_{out}(t_{0}) represents the output voltage of the circuit at time t = t_{0}. This is the same as saying that the output voltage changes over time t_{0} < t < t_{1} by an amount proportional to the time integral of the input voltage:
This circuit can be viewed as a low-pass electronic filter, one with a single pole at DC (i.e., where ) and with gain.
In a practical application one encounters a significant difficulty: unless the capacitor C is periodically discharged, the output voltage will eventually drift outside of the operational amplifier's operating range. This can be due to any combination of:
- The input V_{in} has a non-zero DC component,
- Input bias current is non-zero,
- Input offset voltage is non-zero.^{[4]}
A slightly more complex circuit can ameliorate the second two problems, and in some cases, the first as well.
Here, the feedback resistor R_{f} provides a discharge path for capacitor C_{f}, while the series resistor at the non-inverting input R_{n}, when of the correct value, alleviates input bias current and common-mode problems. That value is the parallel resistance of R_{i} and R_{f}, or using the shorthand notation ||:
The relationship between input signal and output signal is now:
Inverting differentiator
Differentiates the (inverted) signal over time.
- This can also be viewed as a high-pass electronic filter. It is a filter with a single zero at DC (i.e., where angular frequency radians) and gain. The high-pass characteristics of a differentiating amplifier (i.e., the low-frequency zero) can lead to stability challenges when the circuit is used in an analog servo loop (e.g., in a PID controller with a significant derivative gain). In particular, as a root locus analysis would show, increasing feedback gain will drive a closed-loop pole toward marginal stability at the DC zero introduced by the differentiator.
Synthetic elements
Inductance gyrator
Simulates an inductor (i.e., provides inductance without the use of a possibly costly inductor). The circuit exploits the fact that the current flowing through a capacitor behaves through time as the voltage across an inductor. The capacitor used in this circuit is smaller than the inductor it simulates and its capacitance is less subject to changes in value due to environmental changes.
This circuit is unsuitable for applications relying on the back EMF property of an inductor as this will be limited in a gyrator circuit to the voltage supplies of the op-amp.
Negative impedance converter (NIC)
Creates a resistor having a negative value for any signal generator.
In this case, the ratio between the input voltage and the input current (thus the input resistance) is given by:
In general, the components , , and need not be resistors; they can be any component that can be described with an impedance.
Non-linear
Precision rectifier
The voltage drop V_{F} across the forward biased diode in the circuit of a passive rectifier is undesired. In this active version, the problem is solved by connecting the diode in the negative feedback loop. The op-amp compares the output voltage across the load with the input voltage and increases its own output voltage with the value of V_{F}. As a result, the voltage drop V_{F} is compensated and the circuit behaves very nearly as an ideal (super) diode with V_{F} = 0 V.
The circuit has speed limitations at high frequency because of the slow negative feedback and due to the low slew rate of many non-ideal op-amps.
Logarithmic output
- The relationship between the input voltage V_{in} and the output voltage V_{out} is given by:
- where I_{S} is the saturation current and V_{T} is the thermal voltage.
- If the operational amplifier is considered ideal, the inverting input pin is virtually grounded, so the current flowing into the resistor from the source (and thus through the diode to the output, since the op-amp inputs draw no current) is:
- where I_{D} is the current through the diode. As known, the relationship between the current and the voltage for a diode is:
- This, when the voltage is greater than zero, can be approximated by:
- Putting these two formulae together and considering that the output voltage is the negative of the voltage across the diode (V_{out} = −V_{D}), the relationship is proven.
This implementation does not consider temperature stability and other non-ideal effects.
Exponential output
- The relationship between the input voltage and the output voltage is given by:
where is the saturation current and is the thermal voltage.
- Considering the operational amplifier ideal, then the negative pin is virtually grounded, so the current through the diode is given by:
when the voltage is greater than zero, it can be approximated by:
The output voltage is given by:
Other applications
See also
Notes
- ↑ If you think of the left-hand side of the relation as the closed-loop gain of the inverting input, and the right-hand side as the gain of the non-inverting input, then matching these two quantities provides an output insensitive to the common-mode voltage of and .
References
- ↑ Paul Horowitz and Winfield Hill, The Art of Electronics. 2nd ed. Cambridge University Press, Cambridge, 1989 ISBN 0-521-37095-7
- ↑ Basic Electronics Theory, Delton T. Horn, 4th ed. McGraw-Hill Professional, 1994, p. 342–343.
- ↑ http://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/03/14/op-amps-used-as-comparators-is-it-okay.aspx
- ↑ "AN1177 Op Amp Precision Design: DC Errors" (PDF). Microchip. 2 January 2008. Archived from the original on 2013-01-11. Retrieved 26 December 2012.
Further reading
- Design with Operational Amplifiers and Analog Integrated Circuits; 4th Ed; Sergio Franco; McGraw Hill; 672 pages; 2014; ISBN 978-0078028168.
- Op Amps For Everyone; 4th Ed; Ron Mancini; Newnes; 304 pages; 2013; ISBN 978-0123914958. (3 MB PDF of older edition)
- Operational Amplifiers - Theory and Design; 2nd Ed; Johan Huijsing; Springer; 430 pages; 2011; ISBN 978-9400705951. (7 MB PDF)
- Small Signal Audio Design; 1st Ed; Douglas Self; Focal Press; 556 pages; 2010; ISBN 978-0240521770.
- Lessons in Electric Circuits - Volume III - Semiconductors; 2009. (Chapter 8 is 59 pages) (4 MB PDF)
- Linear Circuit Design Handbook; 1st Ed; Hank Zumbahlen; Newnes; 960 pages; 2008; ISBN 978-0750687034. (35 MB PDF)
- Op Amp Applications Handbook; 1st Ed; Walter Jung; Newnes; 896 pages; 2004; ISBN 978-0750678445. (17 MB PDF)
- Op Amps For Everyone; 1st Ed; Ron Mancini; 464 pages; 2002; Texas Instruments SLOD006B. (2 MB PDF)
- Design with Operational Amplifiers and Analog Integrated Circuits; 3rd Ed; Sergio Franco; 672 pages; 2002; ISBN 978-0072320848.
- Op Amps and Linear Integrated Circuits; 1st Ed; James Fiore; Cengage Learning; 616 pages; 2000; ISBN 978-0766817937.
- Operational Amplifiers and Linear Integrated Circuits; 6th Ed; Robert Coughlin; Prentice Hall; 529 pages; 2000; ISBN 978-0130149916.
- Op-Amps and Linear Integrated Circuits; 4th Ed; Ram Gayakwad; Prentice Hall; 543 pages; 1999; ISBN 978-0132808682.
- Basic Operational Amplifiers and Linear Integrated Circuits; 2nd Ed; Thomas Floyd and David Buchla; Prentice Hall; 593 pages; 1998; ISBN 978-0130829870.
- Troubleshooting Analog Circuits; 1st Ed; Bob Pease; Newnes; 217 pages; 1991; ISBN 978-0750694995.
- IC Op-Amp Cookbook; 3rd Ed; Walt Jung; Prentice Hall; 433 pages; 1986; ISBN 978-0138896010.
- Engineer's Mini-Notebook – OpAmp IC Circuits; Forrest Mims III; Radio Shack; 49 pages; 1985; ASIN B000DZG196. (4 MB PDF)
- Analog Applications Manual; Signetics; 418 pages; 1979. (Chapter 3 is 32 pages) (32 MB PDF)
External links
The Wikibook Electronics has a page on the topic of: Op-Amps |
- "Single supply op-amp circuit collection" (PDF). (163 KiB)
- "Op-amp circuit collection" (PDF). (962 KiB)
- "A Collection of Amp Applications" (PDF). (1.06 MiB) – Analog Devices Application note
- "Basic OpAmp Applications" (PDF). (173 KiB)
- "Handbook of operational amplifier applications" (PDF). (2.00 MiB) – Texas Instruments Application note
- Low Side Current Sensing Using Operational Amplifiers
- "Log/anti-log generators, cube generator, multiply/divide amp" (PDF). (165 KiB)
- Logarithmically variable gain from a linear variable component
- Impedance and admittance transformations using operational amplifiers by D. H. Sheingold
- High Speed Amplifier Techniques very practical and readable – with photos and real waveforms
- Single supply op-amp circuit collection
- Properly terminating an unused op amp