Analog Circuit Design: Operational Amplifiers, ...

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An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output.[1] In this configuration, an op amp produces an output potential (relative to circuit ground) that is typically 100,000 times larger than the potential difference between its input terminals. The operational amplifier traces its origin and name to analog computers, where they were used to perform mathematical operations in linear, non-linear, and frequency-dependent circuits.

The popularity of the op amp as a building block in analog circuits is due to its versatility. By using negative feedback, the characteristics of an op-amp circuit, its gain, input and output impedance, bandwidth etc. are determined by external components and have little dependence on temperature coefficients or engineering tolerance in the op amp itself.

Op amps are used widely in electronic devices today, including a vast array of consumer, industrial, and scientific devices. Many standard integrated circuit op amps cost only a few cents; however, some integrated or hybrid operational amplifiers with special performance specifications may cost over US$100 in small quantities.[2] Op amps may be packaged as components or used as elements of more complex integrated circuits.

The response of the op-amp circuit with its input, output, and feedback circuits to an input is characterized mathematically by a transfer function; designing an op-amp circuit to have a desired transfer function is in the realm of electrical engineering. The transfer functions are important in most applications of op amps, such as in analog computers.

Sourced by many manufacturers, and in multiple similar products, an example of a bipolar transistor operational amplifier is the 741 integrated circuit designed in 1968 by David Fullagar at Fairchild Semiconductor after Bob Widlar's LM301 integrated circuit design.[12] In this discussion, we use the parameters of the hybrid-pi model to characterize the small-signal, grounded emitter characteristics of a transistor. In this model, the current gain of a transistor is denoted hfe, more commonly called the β.[13]

The non-inverting input of the operational amplifier needs a path for DC to ground; if the signal source does not supply a DC path, or if that source requires a given load impedance, then the circuit will require another resistor from the non-inverting input to ground. When the operational amplifier's input bias currents are significant, then the DC source resistances driving the inputs should be balanced.[16] The ideal value for the feedback resistors (to give minimal offset voltage) will be such that the two resistances in parallel roughly equal the resistance to ground at the non-inverting input pin. That ideal value assumes the bias currents are well matched, which may not be true for all op amps.[17]

1962: An op amp in a potted module. By 1962, several companies were producing modular potted packages that could be plugged into printed circuit boards.[citation needed] These packages were crucially important as they made the operational amplifier into a single black box which could be easily treated as a component in a larger circuit.

Recent trends. Recently[when?] supply voltages in analog circuits have decreased (as they have in digital logic) and low-voltage op amps have been introduced reflecting this. Supplies of 5 V and increasingly 3.3 V (sometimes as low as 1.8 V) are common. To maximize the signal range modern op amps commonly have rail-to-rail output (the output signal can range from the lowest supply voltage to the highest) and sometimes rail-to-rail inputs.[9]

The limitations to using operational amplifiers include the fact they are analog circuits, and require a designer that understands analog fundamentals such as loading, frequency response, and stability. It is not uncommon to design a seemingly simple op amp circuit, only to turn it on and find that it is oscillating. Due to some of the key parameters discussed earlier, the designer must understand how those parameters play into their design, which typically means the designer must have a moderate to high level of analog design experience.

The most basic operational amplifier circuit is a voltage follower (see Figure 4). This circuit does not generally require external components, and provides high input impedance and low output impedance, which makes it a useful buffer. Because the voltage input and output are equal, changes to the input produce equivalent changes to the output voltage.

In inverting operational amplifiers, the op amp forces the negative terminal to equal the positive terminal, which is commonly ground. Therefore, the input current is determined by the VIN / R1 ratio (see Figure 5).

Operational amplifiers are widely used in many analog and power applications. The benefits of using an op amp are that they are generally widely understood, well-documented and supported, and are fairly easy to use and implement. Op amps are useful for many applications, such as voltage buffers, creating analog filters, and threshold detectors. With a greater understanding of key parameters and common topologies related to operational amplifiers, you can begin implementing them in your circuits.

The operational amplifier (op amp) is the basic building block of analog circuits. It is used to allow negative feedback to be applied to define the desired transfer function. The op amp should have differential inputs and should have a gain large enough to make the open-loop transfer function much greater than unity. This module will concentrate on the design of the operational amplifier. The first section will discuss the compensation of op amps and this information will be used to develop design procedures for the design of the two-stage and folded-cascode op amps. This module concludes with the consideration of how to simulate and measure the performance of an op ampLectures

The open-loop op-amp comparator is an analogue circuit that operates in its non-linear region as changes in the two analogue inputs, V+ and V- causes it to behave like a digital bistable device as triggering causes it to have two possible output states, +Vcc or -Vcc. Then we can say that the voltage comparator is essentially a 1-bit analogue to digital converter, as the input signal is analogue but the output behaves digitally.

A basic op-amp comparator circuit can be used to detect either a positive or a negative going input voltage depending upon which input of the operational amplifier we connect the fixed reference voltage source and the input voltage too. In the examples above we have used the inverting input to set the reference voltage with the input voltage connected to the non-inverting input.

An operational amplifier is to be used with positive feedback to produce a Schmitt trigger circuit. If resistor, R1 = 10kΩ and resistor, R2 = 90kΩ, what will be the values of the upper and lower switching points of the reference voltage and the width of the hysteresis if the op-amp is connected to a dual ±10v power supply.

Although we can use operational amplifiers such as the 741 as a basic comparator circuit, the problem with this is that op-amps are only optimised for linear operation. That is where the input terminals are at virtually the same voltage level and its output stage is designed to produce a linear output voltage that is not saturated for long periods of time. Also standard operational amplifiers are designed to be used in closed-loop applications with negative feedback from its output to its inverting input.

In this tutorial about the Op-amp Comparator we have seen that a comparator circuit is basically an operational amplifier without feedback, that is, the op-amp is used in its open-loop configuration, and when the input voltage, VIN exceeds a preset reference voltage, VREF, the output changes state.

Due to the very high open-loop gain of the operational amplifier, using it with positive feedback or even with no feedback at all causes the output to saturate to its supply rail producing one of two distinct output voltages depending on the relative values of its two inputs. This bistable behaviour is non-linear and forms the basis of op-amp comparator and Schmitt trigger circuits.

The output stages of dedicated comparators, such as the single LM311, the dual LM393 or the quad LM339 are designed to operate in their saturation regions allowing these voltage comparator circuits to be widely used in analogue-to-digital converter applications and for various types of voltage level detection circuits.

We know from previous tutorials that an operational amplifier is an analogue device with a differential analogue input and an analogue output and if operated in its open-loop configuration its output acts like a comparator output. But dedicated voltage comparators (LM311, LM393, LM339) are widely available which will perform much better than a standard op-amp comparator.

Outcome 2 (Students will demonstrate the ability to identify and formulate advanced problems and apply knowledge of mathematics and science to solve those problems):1.Analyze & quantify the operational limits of the analog building blocks listed in 1.1.

In this tutorial, we will learn about one of the important circuits in analog circuit design: A Differential Amplifier. It is essentially an electronic amplifier, which has two inputs and amplifies the difference between those two inputs. We will see the working of a Differential Amplifier, calculate its gain and CMRR, list out some important characteristics and also see an example and an application.

EE 3112 - Analog Electronics II3 lecture hours 2 lab hours 4 credits Course Description This course focuses on design with operational amplifiers. Linear and non-linear amplifiers, active filters, and signal generators are covered. The concepts of stability of operational amplifier circuits are introduced. Static and dynamic limitations are covered. Emphasis is placed on the design of different kinds of operational amplifier circuits and their applications. (prereq: EE 3102 or EE 3002B ) Course Learning OutcomesUpon successful completion of this course, the student will be able to:Describe the fundamentals of operational amplifiersDesign operational amplifier circuits with resistive feedbackDesign simple active filtersDescribe static and dynamic limitations of operational amplifiersDetermine stability of operational amplifier circuitsDesign non-linear operation amplifier circuitsPrerequisites by Topic Transfer functionsBode plotsTransient first and second order circuit analysisBJT and FET device operationSingle stage transistor amplifier analysisCourse Topics Operational amplifier fundamentalsOperational amplifiers with resistive feedbackActive filtersStatic op amp limitationsDynamic op amp limitationsStabilityNon-linear circuitsSignal generatorsLaboratory Topics Linear amplifier design, simulation, and implementationInstrumentation amplifier design, simulation, and implementationFirst-order active filter design, simulation, and implementationSecond-order active filter design, simulation, and implementationApproximate filter designMultiple linear amplifier and active filter design, slew-rate determinationFrequency compensationSchmitt trigger designCoordinator Dr. Donovan Brocker 781b155fdc