Linear Integrated Circuits

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UNIT IINTRODUCTION TO OPERATIONAL AMPLIFIERSWhat do you mean by IC?IC Integrated Circuits. It is a complete Electronic Circuit in which bothpassiveandactivecomponentsarefabricatedonasinglechipof silicon. They are usedin the computer, Automobile andmany more industrieswheretheypermit miniaturizationandsuperior performance not possiblewithdiscretecomponents. ICsarenowbeingusedinall types of electronic equipments because of long, trouble free service they provide. In addition, they are economical because they are mass produced. Types of ICs:Integrated circuits can be classified based on many properties. (i) Based on Mode of operation(a) Digital I.C.Digital ICs are complete functioning logic networks that are equivalents of basic transistor logic circuit. They are used to form circuits such as gates, counters, multiplexers, Demultiplexers, shift registers and others.Since adigitalICisacompletepredesignedpackage,itusually requires nothingmorethanapower supply, input andoutput. These circuits are primarily concerned with only two levels of voltage (or current): High and Low. Therefore accurate control of operating region characteristics is not required in digital circuits. (b) Analog IC (Linear IC)Linear ICs are equivalents of discrete transistor networks, such as amplifiers, filters, frequency multipliers, and modulators that often require VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY1additional external components for a satisfactory operation. For example, external resistors are necessary to control the voltage gainandthe frequency response of an Op-amp. In linear circuits the output electrical signalsvaryinproportiontotheinput signalsappliedor thephysical quantities they represent. Since the electrical signals are analogous to the physical quantities, linear circuits are also referred to as Analog Circuits.(ii) Based on Fabrication(a) Monolithic I.C.It is a process in which all transistors and passive elements are fabricated on a single piece of semi conductor material. Monolithic is a Greek-basedwordmeaningOneSton. TheMonolithicprocessmakes low-cost mass production of ICs possible. Also Monolithic ICs exhibit good thermal stability because all the components are integrated on the same chip very close to each other. (b) Hybrid I.C.Here the passive components (such as resistors & capacitors) and the interconnectionsbetweenthemareformedonaninsulatingsubstrate. The substrate is used as a chassis for the integrated components. Active components such as transistors and diodes as well as monolithic ICs are then connected to form a complete circuit(iii) Based on Integration level SSI Small Scale Integration 10000 gates / cm2 About Digital I.C:Digital ICs are complete functioning logic networks that are equivalents of Basic transistor logic circuits. These ICs are primarily concerned with only Digital circuits which have only two level of voltage: `high andLow. Digital ICsareusedtoformcircuitssuchasgates, counters, multiplexers and others. About Linear I.C:Linear I.C. are equivalents of discrete transistor networks, such as VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY2amplifiers, filtersandmodulatorsthat oftenrequireadditional external components for satisfactory operation. In these linear circuits the output electrical signalsvaryinproportiontotheinput signalsappliedorthe physical quantities they represent. Since the electrical signals are analogous to the physical quantities, Linear circuits are also referred to as Analog circuits.Advantages of I.C: Reduced size (Compactness) Economical because they are mass produced. Highly reliable due to the elimination of soldered joints. Improved functional performance. Increased operating speeds. Reduced power consumption.Basic building blocks of any linear I.C.There are many blocks involved in the design of Linear ICs but the following are the common circuits used in most of the circuits. Constant current source circuits Differential amplifiers Band gap reference circuits Supply and Temperature compensation circuits. Why op-amp is considered to be the most important IC in the field of Linear IC?The OP-amp (Operational Amplifier) is a versatile device that can be used to amplify dc as well as ac input signals and was originally such as addition, subtraction, multiplication and integration. Thus the name operational amplifier.Withtheadditionof suitableexternal feedback components, the modern day op-amp can be used for a variety of applications. Suchas ac anddc signal amplification, active filters, oscillators, comparators, regulators and others. THE IDEAL OP-AMP:Operational Amplifiers, orOp-ampsastheyaremorecommonlycalled, are one of the basic building blocks of Analogue Electronic Circuits. It is a linear devicethat has all theproperties requiredfor nearlyideal DC VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY3amplification and is used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation. AnidealOperational Amplifieris basicallya3-terminal device that consists of two high impedance inputs, one an Inverting input markedwithanegativesign, ("-") andtheotheraNon-invertinginput marked with a positive plus sign ("+"). The amplified output signal of an Operational Amplifier is thedifferencebetweenthetwosignals being applied to the two inputs. The following is a summary of the different types of Operational Amplifiers and their configurations discussed in this tutorial section. TheOperational Amplifier, orOp-ampasit ismost commonly called, isanideal amplifierwithinfinite Gain and Bandwidth when usedintheOpen-loopmodewithtypical d.c. gainsof 100,000, or 100dB. The basic construction is of a 3-terminal device, 2-inputs and 1-output. An Operational Amplifier operates from a dual positive (+V) and an corresponding negative (-V) supply but they can also operate from a single DC supply voltage. It has Infinite Input impedance, (Z) resultingin"Nocurrent flowing into either of its two inputs" and zero input offset voltage "V1 = V2". It also has Zero Output impedance, (Z=0). Op-amps sense the difference between the voltage signals applied to the two input terminals and then multiply it by some pre-determined Gain, (A). ThisGain, (A)isoftenreferredtoastheamplifiers"Open-loop Gain". Op-amps can be connected into two basic circuits,Invertingand Non-inverting. Equivalent Circuit for Ideal Operational AmplifiersVEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY4 IDEALIZED CHARACTERISTICS:PARAMETER IDEALIZED CHARACTERISTICVoltage Gain, (A) Infinite- The main function of an operational amplifier is to amplify the input signal and the more open loop gain it has the better, so for an ideal amplifier the gain will be infinite. Input impedance, (Zin) Infinite- Input impedanceisassumedtobe infinitetoprevent anycurrent flowingfrom the source supply intothe amplifiers input circuitry.Output impedance, (Zout) Zero- The output impedance of the ideal operational amplifier is assumed to be zero so that it can supply as much current as necessary to the load. Bandwidth, (BW) Infinite - An ideal operational amplifier has an infinite Frequency Response and can amplify any frequency signal so it is assumed to have an infinite bandwidth. Offset Voltage, (Vio) Zero - The amplifiers output will be zero when thevoltagedifferencebetweentheinverting and non-inverting inputs is zero. Fromthese"idealized" characteristics above, wecan see that the input resistanceis infinite, sonocurrent flows intoeither input terminal(thecurrent rule) andthat thedifferential input offset VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY5voltage is zero (the voltage rule). It is important to remember these two properties as they help understand the workings of the amplifier with regards to analysis and design of operational amplifier circuits. However, realOperational Amplifierssuch as the commonly available uA741, for example do not have infinite gain or bandwidth but have a typical "Open Loop Gain" which is defined as the amplifiers output amplification without any external feedback signalsconnectedtoit andfor atypical operational amplifier is about100dBatDC(zeroHz). Thisoutputgaindecreaseslinearly with frequency down to "Unity Gain" or 1, at about 1MHz and this is shown in the following open loop gain response curve.BLOCK DIAGRAM REPRESENTATION OF OP-AMP:Anoperational amplifier isadirect coupledhighgainamplifier. It is available as a single integrated circuit package. The operational amplifier is a versatile device that can be used to amplify dc as well as ac input signals and was originally designed for performing mathematical operationssuchasaddition, subtraction, multiplicationandintegration. Thus the name operational amplifier stems from its original use for these mathematical operations and is abbreviated to Op-amp. With the addition of suitable external feedback components, the modern day Op-amp can be used for a variety of applications, such as ac and dc signal amplification, active filters, oscillators, comparators, regulators and others. Since an operations amplifier is a multistage amplifier, it can be represented by a block diagram as shown below. Block diagram representation of op-amp:VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY6InputStageInter mediateStageLevelShiftingStageOutputStageDial upD.ASuch as Enter forever using Current SaverPush-PullAmplifierI/PThe purpose of Input stage in an Op-amp:The Input stage of any operational amplifier is to provide the most of the voltage gain and also to provide the higher input impedance. Most timetheinput stageisdesignedwithaDual input Balancedoutput differential amplifier.The requirement of Intermediate stage in an Op-amp:The intermediate stage is usually another differential amplifier. Which is driven by the output of the first stage. The intermediate stage provides the extra amplification required at times. In most amplifiers the intermediate stage is a dual input, unbalanced output differential amplifier. Level shifting stage in Op-amp:In Intermediate stage we use a direct coupling because of which dc voltage at the output of this stage is well above ground potential. Hence the level shifter (translator) circuit is used after intermediate stage to shift the dc level downward to zero volts with respect to ground. The purpose of output stage:Thefinal stageisusuallyapushpull complementaryamplifier outputstage.Theoutput stageincreasesthe outputvoltageswingand raisesthecurrent supplyingcapabilityof theCo-amp. Awell designed output stage also provides the low output resistance. VOLTAGE TRANSFER CURVE OF OP-AMP:This is the basic op-amp equation in which the output offset voltage isassumedtobezero. Thegraphicrepresentationof thisequationis shown;wherethe output voltage, Vo isplotted against inputdifference voltage Vid, keeping gain A constant. The output voltage cannot exceed thepositiveandnegativesaturationvoltage. Thesesaturationvoltages are specified by an output voltage swing ratings of an op-amp for given values of supply voltages. The output voltage is directly proportionalto the input difference voltage untilit reaches the saturation voltages and thereafter the output voltage remains constant.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY7AC-DC CHARACTERISTICS OF AN OP-AMP:Opamps are easy to design with as long as you follow some basic rules andunderstandhowthey operate. I won't gointo all the theory of opamps, justthebasics. ThereareotherwebsitessuchasWilliamson Labsthatwill coverthefundamentals. Thebasiccharacteristicsareas follows:1) Assume the output impedance as almost zero.2) Assume the - input impedance as almost zero.3) Assume the + input impedance as almost infinite.4) The gain is programmable by the feedback resistor, the resistor from the output pin to the negative input pin, from zero to infinite depending on the device.5) An opamp can amplify AC or DC voltages, or both.Most opamp designs require a bipolar power supply (plus and minus voltage) unless you offset the input to think it is operating on a bipolar supply. This will be explained later.Let's take the circuit in Figure 17. It is a basic inverting DC opamp configuration using a positive and negative power supply. (This circuit will also respond to AC also. AC circuits can contain DC blocking capacitors to keeptheopampoperatingwithinDCparameters but will roll off low frequencies depending on values and will not respond to DC. AC amplifiers will bediscussedlater.) Forthepurposeof thisexamplelet'ssaythe power supply voltages are +/- 15 volts. A basic circuit uses two resistors. An input resistor (Rin) and a feedback resistor (Rf). In this example both resistors will be 10K ohms.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY8If you were to build this circuit and ground the input (0 volts), you will see 0 volts on the output. If you place +1 volts on the input, you will get -1 volt on the output. If you place -1 volts on the input, the output will be +1 volt. Why? Look at the formula in Figure 18 below.Voltage gain (inverting) equals the value of the feedback resistor divided by the value of the input resistor for a gain of one, or unity. No gain, but an identical inverse of the input. If you varied the input voltage from +1 to -1 volts you will see the output swing from -1 to +1 volts. This could also be referred to as a linear amplifier because it tracks whatever voltage is on the input to the output in a linear fashion, only inverted. So what? Well, you have a unity gain inverting amplifier. What could this be used for? You have an input impedance of 10K ohms and an output impedance of almost zero ohms. Enough to provide drive for the next circuit or circuits. This could also be called an inverting buffer. How do you get 10K ohms input impedance? Remember, the inverting input, pin 2 is virtually zero ohms, so the input resistor (resistance) becomes the input impedance. What kind of gainwouldyouhaveif boththefeedbackandinput resistorswere 220K? The same gain, only you have an input impedance of 220K ohms. Muchlessof aloadsoyour input circuit won't "swamp" or loadthe preceding circuit.Ok, let's make this thing actually amplify. Change the feedback resistor to 100K ohms while leaving the input resistor at 10K. Vary the input voltage to +1 volts. Now the output swings to -10 volts. Sweep the voltage all the way to -1 volts and you will see the output track all the way to +10 volts. This smooth output transition from -10 to +10 from a +1 to -1 input is why VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY9this circuit is also called a linear amplifier, because the output reflects the input times 10 only inverted. The formula for seeing a gain of ten is in Figure 19.Sofromthis basic circuit youcanseehowyoucanselect different resistorstoget different amountsof gain. Justremember, your output swing will never exceed the power supply voltages (or rails) and actually a volt or two less than each rail for a basic opamp in this demonstration. Raise the input voltage to +2 volts and measure the output and you will see that it rests somewhere around -12 to -13 volts. This is the "rail" or the lowest possible output voltage for the given +/- 15 volts. It also may be referred to as saturation of the opamp. Some opamps are called rail-to-rail opampsandtheycomeclosertotheactual powersupplyvoltage. Almost 0.3 volts from the power supply voltage. Next we willstudy the non-inverting opamp circuit. Figure 21 shows a simple non-inverting DC opamp circuit. For purposes of demonstration, the feedback resistor will be 100K ohms and the input resistors are 10K ohms. Putting +1.0 volts on the input will give +11 volts on the output. Putting -1.0 volts on the input will give -11 volts on the output.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY10Using the formula in Figure 22, we get a gain of 11 or eleven times the input. This can be used as a DC or AC amplifier. What if we wanted a gainof ten? Changethefeedbackresistor to90Kohms. What if we wanted it to be exactly unity gain? Remove the resistor to ground. The feedback resistor can be any value.How about a gain of 100? Use a 990k feedback resistor. However, you are asking for a lot of gain from the opamp. Don't forget, that's a lot of amplification. 1 volt on the input in theory will give you 100 volts on the output but that cannot happen because the power supply rails only supply 15 volts. So, you will "saturate" the opamp and it will slam up against the rail at somewhere around 13 volts. But, if you are amplifying a very small voltagesuchas 10millivolts (0.01volts), youwill get 1volt output. Remember, the gain number is the same as the multiplication factor. 0.01 X 100 = 1.What other advantages does this circuit have? It is a true buffer. A buffer is a circuit that will duplicate the input voltage but provides drive. This is important if you have a high impedance circuit and you need to drive a low impedance load or many loads. Remember, the + input to the opampisalreadyahighimpedanceinput. Thus, theinput resistor is almost irrelevant with the exception of possible protection of the input to the opamp. Remember also that the output impedance is very low. Theoretically zero ohms but usually around 47 ohms depending upon the device you are using.A buffer can also be called a "follower". It mimics the input with a low impedance output drive. The following circuit in Figure 23 is what is referred to as a follower.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY11The input resistor can be almost any value since the input impedance is almost infinite or can sometimes be omitted. What ever the input sees, it will duplicate it onthe output. Thiscanprovideextra drivefor another circuit without loading the input circuit. The feedback in this circuit is a short. It can be a resistor of almost any value as long as there isn't any other resistor on the minus input.Let's talk about AC opamp designs.AC circuits are typically used in audio amplification. Also mixing of signals, distributing audio, and processing audio. You don't really want to use a DC amplifier unless you know exactly what you are doing. DC circuits have their place in highquality audio amplifications but you must remember that the output will follow the input precisely. This is not an advantage for most applications. Input circuitsof unknownDCpotential canandwill offset theamplifier andpossiblerender it useless or drawingexcess current. Especially if driving a speaker or headphones.Figure 24 is a typical AC coupled opamp circuit used to amplify a weak signal.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY12 The above circuit has a gain of 100.With an AC signalof 5 millivolts(0.05volts), youwill seeaoutput swingof +/- 5volts. This circuit istypicallyusedintheaudiofrequencyrange. Thedetermining factor is the device itself which can amplify up to 1 megahertz but the gain will decrease at higher frequencies. You will have to study the data sheets for the device you will be using in order to determine high frequency gain. The capacitors will determine the low frequency cut-off. In some circumstances you could leave the capacitors out and the circuit will function down to DC. As you can see, the two capacitors are polarized. The positive leads are shown to be pointing towards the device. Since this uses a bi-polar power supply, the voltage potential is sitting at zero volts. Because of this, the capacitors can be placed either way as long as the input and output are referenced to ground. Also, R4 is not needed in most applications usingabi-polar power supply. The +input canbe tied directly to ground.Hint: When troubleshooting the opamp, placing a voltmeter on any inputs or outputs will show zero volts if there is no signal present. Any voltages other than zero means the circuit is not connected right or the opamp is defective. Since bi-polar power supplies aren't always available, you can operatethissamecircuitfromasingleendedpowersupply. All thatis necessary is that the -V pin is tied to ground and the + input is "biased" to half the power supply voltage. Of course you must realize that in order to keep the same headroom, you will have to double the positive supply. In mostcasesthisisn'tnecessaryespeciallyifworkingwithsmall signals. Also the two capacitors, C1 and C2, must be present in order to keep the opamp stable and operating in its linear region. Figure 26 shows how to connect the same circuit with a single power supply.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY13You can bias the + input by building a voltage divider, filtering it, andapplyingthe1/2voltagetothe+input. C3isnotacritical value. Neither is R4 and R5. They can be as high as 100KHint: When troubleshooting this circuit, both inputs and the output will be idling at 1/2 the power supply voltage with no signal present. Any voltage other than 1/2 the supply means the circuit is not wired properly or the opamp is defective. Example. If you are using a 12 volt supply, the DC voltage on pins 1, 2, and 3 must be 6 volts.Frequencyresponse-theusablebandwidthof thecircuit. Thisis usually the 3dbpoint. Anexample is inFigure 26, R2andC1. The impedanceis10Kohmsandthecapacitanceis1uf. Usingtheformula f=1/(2Pi*(R*C)), the usable low frequency is down to16 HZ. An opamp can also be used as a bias generator (or reference) for providing 1/2 the power supply voltage for other opamps on the same board using a single ended (uni-polar) power supply. Figure 28 shows the circuit.VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY14Since the + input is a high impedance input, the values of R1 and R2 can be almost anyvalue as long as they are the same value. Thearbitrary values of47Kwillnotprovideanoticeabledrainon thepower supply.Depending on noise and stability, it may be necessary to place a small capacitor(U10)fromthe+inputtogroundbutitisnotmandatory. In Figure 28, if the +V is 24 volts, the output will be +12 volts. If the +V is 5 volts, theoutput will be+2.5volts. Toturnthiscircuit intoapower regulator, replace R2 with a zener and R1 with an appropriate value, and the output of the opamp will provide a buffered regulated voltage sourceVEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY15FREQUENCY RESPONSE OF NON-COMPENSATED OP-AMP:Theopen-loopresponsegives yousomepowerful insight intotheop amps performance. Open-loop means NO feedback; the response of the naked op amp. Two important features are 1.DC Gain, Aol the open-loop gain at DC. This gain is provided by the voltagecontrolledvoltage sourceEGAIN.A higher gainmeansa higher bandwidth and more gain accuracy when you close the loop with feedback resistors.2.First-PoleFrequency, fp1thefrequencywheretheopen-loopgain begins to fall. A simple RC creates this low-pass filter.fp1 = 1/( 2 * * RP1 * CP1)3. Unity-Gain Frequency, fu - the frequency where the open-loop gain falls to1V/V. Thebigger thefu, thefaster your opampresponds. What determines this frequency? Its a function of the DC Gain Aol and fp1.fu = Aol x fp1VEL TECH Dr. RR & Dr. SR TECHNICAL UNIVERSITY16Open-loop Frequency Response CurveCOMPENSATING NETWORKS:DOMINANT POLE COMPENSATION:Theessential ideaof frequencycompensationappliedtoop-ampisto reshape the magnitude and plot. So that |AV |