Presentation on theme: "Basic Block Diagram of Op-Amp"— Presentation transcript:
1 Basic Block Diagram of Op-Amp An Op-Amp can be conveniently divided in to four main blocksAn Input Stage or Input Diff. Amp.The Gain StageThe Level TranslatorAn Out put StageNote: It can be used to perform various mathematical operations such as Addition, Subtraction, Integration, Differentiation, log etc.V1Out put Stage (Buffer)Level ShifterI/PInput Stage(Diff. Amp.)Gain Stage (C E Amp.)VOV2Op-Amp IC
2 An IDEAL OP AMP An ideal op amp has the following characteristics: Infinite open-loop voltage gain, AV ≈ ∞.Infinite input resistance, Ri ≈ ∞.Zero output resistance, Ro ≈ 0.Infinite CMRR, ρ =∞The output voltage Vo=0; when Vd = V2-V1 = 0Change of output with respect to input, slew rate = ∞Change in out put voltage with Temp., ∂Vo/∂Vi=0
4 The Operational Amplifier +VS_i(-)InvertingROAvidOutputRivO = AdVidNoninvertingi(+)+-VSi(+), i(-) : Currents into the amplifier on the inverting and non-inverting lines respectivelyvid : The input voltage from inverting to non-inverting inputs+VS , -VS : DC source voltages, usually +15V and –15VRi : The input resistance, ideally infinityA : The gain of the amplifier. Ideally very high, in the 1x1010 range.RO: The output resistance, ideally zerovO: The output voltage; vO = AOLvid where AOL is the open-loop voltage gain
5 Operational Amplifier Model An operational amplifier circuit is designed so thatVout = Av (V1-V2) (Av is a very large gain)Input resistance (Rin) is very largeOutput resistance (Rout) is very lowV1RoutVoutRinAv(V1- V2)+-V2
6 Practical Op-Amp Circuits These Op-amp circuits are commonly used:Inverting AmplifierNoninverting AmplifierUnity FollowerSumming AmplifierIntegratorDifferentiator
12 Inverting Amplifier: Input and Output Resistances Rout is found by applying a test current (or voltage) source to the amplifier output and determining the voltage (or current) after turning off all independent sources. Hence, vs = 0But i1=i2Since v- = 0, i1=0. Therefore vo = 0 irrespective of the value of io .
13 Differential Amplifier Using Op Amp I/P Current to op amp is zero
15 The Unity-Gain Amplifier or “Buffer” This is a special case of the non-inverting amplifier, which is also called a voltage follower, with infinite R1 and zero R2.Hence Av = 1.It provides an excellent electrical isolation while maintaining the signal voltage level.The “ideal” buffer requires no input current and can drive any desired load resistance without loss of signal voltage.Such a buffer is used in many sensor and data acquisition system applications.
16 Unity-Gain Buffer Closed-loop voltage gain Used as a "line driver" that transforms a high input impedance (resistance) to a low output impedance. Can provide substantial current gain.
20 Applying KCL at the inverting input Op-Amp Differentiator Cont…Since the inverting input is at virtual groundApplying KCL at the inverting inputi1+i2 = 0Differentiators are avoided in practice as they amplify noise
21 Instrumentation Amplifier NOTECombines 2 non-inverting amplifiers with the difference amplifier to provide higher gain and higher input resistance.Gain can be varied by varying single resistor R1Ideal input resistance is infinite because input current to both op amps is zero. The CMRR is determined only by Op Amp 3.
22 Finite Open-loop Gain and Gain Error This is the “ideal” voltage gain of the amplifier. If Ab is not >>1, there will be “Gain Error”.is called the feedback factor.
23 Gain Error is given byGE = (ideal gain) - (actual gain)For the non-inverting amplifier,Gain error is also expressed as a fractional or percentage error.
24 Output Voltage and Current Limits Practical op amps have limited output voltage and current ranges.Voltage: Usually limited to a few volts less than power supply span.Current: Limited by additional circuits (to limit power dissipation or protect against accidental short circuits).The current limit is frequently specified in terms of the minimum load resistance that the amplifier can drive with a given output voltage swing. Eg:For the inverting amplifier,