Physics
Differential Amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two input signals while rejecting any signals that are common to both inputs. It is commonly used in electronic circuits to amplify small differential signals, such as those from sensors, and is a key component in operational amplifiers and analog integrated circuits.
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11 Key excerpts on "Differential Amplifier"
- Robert B. Northrop(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
3 Differential Amplifier3.1 INTRODUCTION
The differential or difference amplifier (DA) is a cornerstone element in the design of most analog signal conditioning systems used in biomedical engineering applications, as well as in general instrumentation applications. Nearly all instrumentation and medical isolation amplifiers are DAs; also, nearly all operational amplifiers are DAs. Why are DAs so ubiquitous? The answer lies in their inherent ability to reject unwanted DC levels, interference, and noise voltages common to both inputs. An ideal DA responds only to the so-called difference-mode signal at its two inputs. Most DAs have a single-ended, complex output voltage, V o , given by the phasor relation:whereV1is the complex (phasor) AC voltage at the DA’s noninverting input terminal, andV1′ is the complex AC voltage at the DA’s inverting input terminal.Ideally, it is desired that the complex gainswhereA1andA1′ be exactly equal over as wide a frequency range as possible. In reality, this does not happen, andVois more generally given by the relationandADis the complex difference mode gain , andACis the complex common-mode gain .From Equation 3.1 , it can easily be shown by vector summation thatClearly, ifA1=A1′, thenAC→ 0.3.2 DA CIRCUIT ARCHITECTURE
Figure 3.1 illustrates a simplified circuit of the Burr-Brown, OPA606, JFET-input op amp (OA). (The circles with arrows in them represent BJT transistor DC current sources.) Note that a pair of p- channel JFETs connected as a DA is used as a differential input headstage in the OA. The single-ended signal output from the left-hand (inverting input) JFET drives the base (input) of a BJT emitter-follower which drives a second BJT connected as a grounded-base amplifier. Its output, in turn, drives the OA’s output stage.To appreciate how the differential headstage works, consider the simple JFET DA circuit of Figure 3.2 . Note that in the op amp schematic, the resistors Rs , Rd , and Rd ′ are shown as active DC current sources and thus can be assumed to have very high Norton resistances on the order of megohms. (See Northrop 1990, Section 5.2, and Section 2.3.5 in this text for a description on active current sources and sinks used in IC DA designs.) Figure 3.3 a illustrates the midfrequency, small-signal model (MFSSM) of the JFET DA. To make the circuit bilaterally symmetric so that the bisection theorem (cf. this text, Glossary; Northrop 1990, Chapter 2 ) can be used in its analysis, we put two 2Rs resistors in parallel to replace the one Rs in the actual circuit. Note that all DC voltage sources are represented by small-signal grounds in all SSMs.
Available until 23 Dec |Learn moreThe Audio Dictionary
Third Edition, Revised and Expanded
- Glenn D. White, Gary J. Louie(Authors)
- 2011(Publication Date)
- University of Washington Press(Publisher)
Before the advent of integrated circuits , differ-ential amplifiers were made by adding a transformer to the input and not connecting either side of the primary to ground; this is still done in the case of some microphone preamplifiers . Integrated circuit Differential Amplifiers without transformers are com-monly available. They allow balanced configurations to be easily built at lower cost than using a good transformer. Differential Amplifier Differential Amplifier 107 Diffraction The bending of a sound wave around an obstacle and the reflection of a sound wave from a discontinuity in its path are called dif-fraction. It is wavelength dependent. Where the wavelength is short (rel-atively high frequencies ) compared to the obstacle, reflection will occur as well as bending of the wave front. When the wavelength is long (lower frequencies) with respect to the obstacle, little reflection will occur and the bending will be more pronounced. The diffraction of light is analogous to that of sound, although it is sel-dom seen because of the extremely short wavelengths of light. Digital See digital audio ; Appendix 7. Digital Acoustics Processor, DAP A consumer audio device that attempts to simulate the acoustics of an auditorium or other room by adding suit-able digital time delays and synthetic reverberation to recorded signals. It is the latest in a long line of analog devices that began with the stereo-phoner designed by the Viennese orchestra conductor Hermann Scherchen in the 1950s and the xophonic made by Radio Craftsmen in the U.S. in the 1960s. The Xophonic had multiple outputs for feeding surround loud-speakers and came with preprogrammed delay settings to simulate certain existing concert halls. DAPs are often used to process the stereo audio sig-nals from prerecorded video tapes and video discs to enhance the illusion of ambience. Illusion is the proper word, since DAPs do not produce very accurate simulations of actual acoustical spaces.
- J. David Irwin, David V. Kerns, Jr.(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
However, as sophisticated as it appears, it is really nothing more than a high-quality linear voltage ampli- fier. The details of how this circuit is designed are beyond the scope of this book, but follows basic principles described here. A linear voltage amplifier is an electronic circuit that produces an output voltage signal that is an exact copy or replica of the input voltage signal, except the output is increased in amplitude (magnitude) by a factor, A v , called the voltage gain. This is illustrated in Figure 10.2 and by the following equation. Note that in Figure 10.2, A v = 2. v o = A v v i In practice, op amps have significantly higher gains, A v , than that illustrated, like 10,000 or 100,000 or more. 304 Differential Amplifiers 305 01 02 INVERTING INPUT NONINVERTING INPUT 03 04 05 R1 1k R2 1k R3 50k R4 5k R12 50k R11 50 OFFSET NULL OFFSET NULL 06 1 5 2 3 07 08 09 010 011 R5 39k R7 4.5k R8 7.5k R10 50 v – = V SS 4 6 v + = V DD R9 25 OUTPUT C1 30pF 022 012 013 014 7 015 015 017 020 Offset Null 1 2 3 4 8 7 6 5 Inverting (–) Noninverting (–) (Power)v + (V DD ) Offset Null Output v – (Power) (V SS ) Not Connected (NC) 741 Op. Amp. – + FIGURE 10.1 Analog devices op-amp schematic and pin diagram (Analog Devices, Inc. data sheet) 2 1 –1 –2 t 2 v 0 v i 1 0 –1 –2 t v i l i v 0 A v FIGURE 10.2 Illustration of linear amplifier Differential AmplifierS A Differential Amplifier has two inputs, generally labeled as v + and v − , the noninverting and the inverting inputs, respectively; and there is one output, v o . The output voltage is related to the dif- ference in voltage between the two inputs. Power is supplied to the device via the two dc voltage sources, V DD and V SS , which are typically balanced, e.g. plus and minus 1.5–15 V. The output volt- age of the amplifier is linear, i.e. not saturated, as long as it is within these limits. See Figure 10.3. The Differential Amplifier circuit can be represented by the circuit model shown in Figure 10.4.
eBook - PDFMechatronics
A Foundation Course
- Clarence W. de Silva(Author)
- 2010(Publication Date)
- CRC Press(Publisher)
244 Mechatronics: A Foundation Course subtracted at the amplifier output. Since the noise level is almost the same for both inputs, it is canceled out. Any other noise (e.g., 60 Hz line noise) that might enter both inputs with the same intensity will also be canceled out at the output of a Differential Amplifier. A basic Differential Amplifier that uses a single op-amp is shown in Figure 4.9a. The input–output equation for this amplifier can be obtained in the usual manner. For instance, since current through an op-amp is negligible, the current balance at point B gives ( v i 2 − v B )/ R = v B / R f in which v B is the voltage at B . Similarly, current balance at point A gives ( v o − v A )/ R f = ( v A − v i 1 )/ R . Now we use the property v A = v B for an op-amp to eliminate v A and v B from the first two equations. This gives ( ) 2 1 f o i i R v v v R = − (4.13) Two things are clear from Equation 4.13. First, the amplifier output is proportional to the “difference” and not the absolute value of the two inputs v i 1 and v i 2 . Second, the voltage gain of the amplifier is R f / R . This is known as the differential gain . It is clear that the differ-ential gain can be accurately set by using high-precision resistors R and R f . The basic Differential Amplifier, shown in Figure 4.9a and discussed above, is an impor-tant component of an instrumentation amplifier. An instrumentation amplifier should possess the capability of adjustable gain as well. Furthermore, it is desirable to have a (a) v i 1 v i 1 v i 2 v i 2 Inputs Output v o R + – A R f B R R f (b) Output v o R 3 + – R 4 + δ R 4 R 3 R 4 + – + – Inputs R 1 R 1 R 2 A 1 2 B FIGURE 4.9 (a) A basic Differential Amplifier. (b) A basic instrumentation amplifier. Component Interconnection and Signal Conditioning 245 very high input impedance and a very low output impedance at each input lead. Also, it is desirable for an instrumentation amplifier to possess a high and more stable gain.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Orange Apple(Publisher)
Operational amplifiers (op-amps) An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain. Although the term is today commonly applied to integrated circuits, the original operational amplifier design was implemented with valves. Fully Differential Amplifiers (FDA) A fully Differential Amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. It is similar to the operational amplifier but it also has differential output pins. Video amplifiers These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The specification of the bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB for example) the bandwidth is measured. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented. Oscilloscope vertical amplifiers These are used to deal with video signals to drive an oscilloscope display tube and can have bandwidths of about 500 MHz. The specifications on step response, rise time, overshoot and aberrations can make the design of these amplifiers extremely difficult. One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company. Distributed amplifiers These use transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are combined in the output transmission ________________________ WORLD TECHNOLOGIES ________________________ line. This type of amplifier was commonly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope.
- Lazo M. Manojlovic(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
Differential Amplifier also suppress the common-mode input voltage, which is typically in the form of a DC level hum, or other type of interferences . The most generic form of a Differential Amplifier is presented in Figure Analog Signal Conditioning in Instrumentation 151 6.1, where the Differential Amplifier (DA) can be considered as the four-port device (V i , V’ i , V o , and V’ o ). However, most practical Differential Amplifiers have a single-ended output and thus they can be considered as the three-port circuits. Figure 6.1. Most generic form of a Differential Amplifier. At the beginning of analyze of a Differential Amplifier, it is convenient to define two input and output voltage signals, i.e., the differential-mode voltages and the common-mode voltages as: 2 2 2 2 o o oCM o o oDM i i iCM i i iDM V V V V V V V V V V V V ′ + = ′ − = ′ + = ′ − = . (6.1) The general input-output relationship of a Differential Amplifier can be expressed as following: , (6.2) where A DD , A DC , A CD , and A CC represents the corresponding gain factors. In the case of the single-ended outputs it is easy to show that the following is valid: . (6.3) For the single-ended output, V o the following can be written in terms of the actual inputs of a Differential Amplifier: Innovative Instrument Design and Applications 152 . (6.4) Typically, the single-ended output, V o of the Differential Amplifier is pre -sented in the form: , (6.5) where A DM and A CM are given by the corresponding terms in equation (6.3). For an ideal Differential Amplifier the following is valid: A CC = A DC = A CD = 0. So, in this case, we have fulfilled A C = 0 and A D = A DD . 6.1.1. Common-Mode Rejection Ratio The common-mode rejection ratio (CMRR) is an important parameter of any Differential Amplifier. The value of CMRR is typically expressed in decibels and it describes how well a real Differential Amplifier characteristics approach that of an ideal one.
eBook - PDF- Joseph J. Carr(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
ADDITIONAL Differential AmplifierS T h e simple D C Differential Amplifier circuits shown in this chapter are useful for low-gain applications and for those applications where a low to moderate input impedance is permissible (e.g., 3 0 0 to 2 0 0 , 0 0 0 Ω). Where a higher gain is required, we must resort to a more complex circuit called the operational amplifier instrumentation amplifier, or IA. In the next chapter we will examine the classical three-device LA circuit, as well as several integrated circuit instrumentation amplifiers (IC IA) that offer the advantages of the IA in a single small IC package. Some of those devices are now among the most commonly used in many instrumentation applications,- they will be considered below. 326 14. DC Differential Operational Amplifier Circuits INSTRUMENTATION AMPLIFIERS T h e simple D C Differential Amplifier discussed earlier suffers from several important drawbacks. First, there is a limit to the input impedance ( Z i n is approximately equal to the sum of the two input resistors). Second, there is a practical limitation on the gain available from the simple single-device D C Differential Amplifier. If high gain is attempted, then either the input bias current tends to cause large output offset voltages, or the input impedance becomes too low. In this section we will demonstrate a solution to these problems in the form of the instrumentation amplifer (IA). All of these amplifiers are differ-ential amplifiers, but they offer superior performance over the simple D C Differential Amplifiers of the last section. The instrumentation amplifier can offer higher input impedance, higher gain, and better common mode rejection than the single-device D C differential ampli-T h e simplest form of instrumentation amplifier circuit is shown in Fig.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Research World(Publisher)
1. Differential Amplifier – provides low noise amplification, high input impedance, usually a differential output. 2. Voltage amplifier – provides high voltage gain, a single-pole frequency roll-off, usually single-ended output. 3. Output amplifier – provides high current driving capability, low output impedance, current limiting and short circuit protection circuitry. ________________________ WORLD TECHNOLOGIES ________________________ Input stage Constant-current stabilization system The input stage DC conditions are stabilized by a high-gain negative feedback system whose main parts are the two current mirrors on the left of the figure, outlined in red. The main purpose of this negative feedback system—to supply the differential input stage with a stable constant current—is realized as follows. The current through the 39 kΩ resistor acts as a current reference for the ot her bias currents used in the chip. The voltage across the resistor is equal to the voltage across the supply rails ( ) minus two transistor diode drops (i.e., from Q11 and Q12), and so the current has value . The Widlar current mirror built by Q10, Q11, and the 5 kΩ resistor produces a very small fraction of I ref at the Q10 collector. This small constant current through Q10's collector supplies the base currents for Q3 and Q4 as well as the Q9 collector current. The Q8/Q9 current mirror tries to make Q9's collector current the same as the Q3 and Q4 collector currents. Thus Q3 and Q4's combined base currents (which are of the same order as the overall chip's input currents) will be a small fraction of the already small Q10 current. So, if the input stage current increases for any reason, the Q8/Q9 current mirror will draw current away from the bases of Q3 and Q4, which reduces the input stage current, and vice versa.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
____________________ WORLD TECHNOLOGIES ____________________ Chapter- 1 Operational Amplifier A Signetics μa741 operational amplifier, one of the most successful op -amps. An operational amplifier (op-amp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single -ended output. An op-amp produces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals. Operational amplifiers are important building blocks for a wide range of electronic circuits. They had their origins in analog computers where th ey were used in many linear, non-linear and frequency-dependent circuits. Their popularity in circuit design largely stems from the fact the characteristics of the final elements (such as their gain) are set by external components with little dependence on temperature changes and manufacturing variations in the op-amp itself. ____________________ WORLD TECHNOLOGIES ____________________ Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op -amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost over $100 US in small quantities. Op-amps may be packaged as components, or used as elements of more complex integrated circuits. The op-amp is one type of Differential Amplifier. Other types of Differential Amplifier include the fully Differential Amplifier (similar to the op-amp, but with two outputs), the instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to the instrumentation amplifier, but with tolerance to common -mode voltages that would destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps and a resistive feedback network).
No longer available |Learn more- (Author)
- 2014(Publication Date)
- The English Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 4 Operational Amplifier A Signetics μa741 operational amplifier, one of the most successful op -amps. An Operational amplifier (op-amp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. An op-amp pro-duces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals. Operational amplifiers are important building blocks for a wide range of electronic circuits. They had their origins in analog computers where they were used in many linear, non-linear and frequency-dependent circuits. Their popularity in circuit design largely stems from the fact the characteristics of the final elements (such as their gain) are set by ________________________ WORLD TECHNOLOGIES ________________________ external components with little dependence on temperature changes and manufacturing variations in the op-amp itself. Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid opera-tional amplifiers with special performance specifications may cost over $100 US in small quantities. Op-amps may be packaged as components, or used as elements of more complex integrated circuits. The op-amp is one type of Differential Amplifier. Other types of Differential Amplifier include the fully Differential Amplifier (similar to the op-amp, but with two outputs), the instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to the instrumentation amplifier, but with tolerance to common-mode voltages that would destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps and a resistive feedback network).
eBook - PDFAnalogue Computing Methods
The Commonwealth and International Library: Applied Electricity and Electronics Division
- D. Welbourne, P. Hammond(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
chapter 2 ELECTRONIC ANALOGUE EQUIPMENT THE AMPLIFIER THE D.C. AMPLIFIER The d.c. amplifier is the basic building block of the electronic analogue computer. It can be made to sum, multiply by con-stants, and integrate voltages, and to drive such things as multi-pliers, voltmeters and recorders in associated equipment. Much design effort has gone into the perfection of low noise, stable, drift-free amplifiers with band widths of up to 100 kc/s. Such an amplifier is a precision piece of equipment, very reliable, and useful in many fields other than analogue computation. Consider Fig. 5a; the round-backed triangle is the conventional symbol for a d.c. amplifier. We assume for the moment that it draws no current at its input A, which will in practice be the grid of its first valve, that it has an effectively infinite negative gain, and that it does not drift. If a small voltage V g exists at A, a very large negative voltage will be produced at the output B, and a current will flow along AR 0 B until V g becomes sensibly zero. This input point A is, therefore, called the virtual earth ; for a practical computing amplifier with a gain of perhaps one million, it will never be more than about 0-1 mV from earth, which will, there-fore, give an output of 100 V. The amplifier in Fig. 5a is fed by a voltage V x through a 26 ELECTRONIC ANALOGUE EQUIPMENT 21 Ro I V W 1 V, ï V W i-^-i j>—iS^Vo (a) Ri Ro V, ï— VW f V W 1 V 2 ï— VW < > ß ï Vo or Vo = - (ãé V + 72V2 + 73 V 3 ) Ko - - (j-i Ki + Y 2 V 2 + 71 (>) R Ë ï— VW M ^>—*—0V0 Ri c í, — v w 1 II 1 R 2 Ë V 2 — V W «é —ß J>— 4—ο v 3 — V W 1 1 (Vi V 2 V Ë v ° --& R l + T 2 + Tj or Vo = -X - ( ãé Vi + yi V 2 + y 3 Vj) v — T T -v 3 o — ^ — L / ^ 1 Vo -- - (7i Ki + y 2 K 2 + y 3 Vi) (c) F I G . 5 . 28 ANALOGUE COMPUTING METHODS resistor R l9 and has a feedback resistor R 0 from the output volt-age V 0 . Writing down a current balance for the virtual earth A 9 we have from Kirchhofes law, ^ = 0.
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