EE201 Lecture 26 P 1 Op Amp Basics The operation amplifier is a high gain amplifier Traditionally used to perform math operations the op amp has many uses e g feedback Below is a typical package for an op amp 8 lead dual in line package for Op Amp 1 8 2 7 3 6 4 5 Terminal designations 1 Offset null 3 Noninverting input 5 Offset null 7 pos power supply 2 Inverting input 4 neg power supply 6 Output 8 no connection EE201 Lecture 26 P 2 Op Amp Symbol and Notation Noninverting v Inverting v Positive power supply vout Output Negative power supply Note only 5 terminal leads are important and of those we will primarily use 3 inverting noninverting and output EE201 Lecture 26 P 3 Op Amp Voltage Variables v vp v vn vCC vo vCC Note Negative and positive power supplies do not have to be equal Op Amp Current Variables i ip i in iC vCC io iC vCC EE201 Lecture 26 P 4 Because the op amp is a high gain voltage amplifier we are interested in the voltage transfer characteristics of the device as shown below The output voltage is plotted as a function of the difference in input voltage vsat vout Large gain device A 104 107 v v slope vsat III Negative Saturation region I II Linear Positive region Saturation region EE201 Lecture 26 P 5 Output voltage for regimes of operation vo vsat vcc v v vcc A A v v vcc A v v vcc A vsat vcc v v vcc A Typical input voltage differences A 104 vcc 10 V v v 1 mV Op Amp Model two resistors one VCVS i Ro v Ri i vo v v v All attributes of vo are satisfied by dependent source model EE201 Lecture 26 P 6 Redrawing figure on p 5 i Rout vRin i v v v vo Question how does the op amp know it is operating in its linear regime Negative feedback Connect the output voltage to the inverting input terminal This causes input voltage difference to decrease EE201 Lecture 26 P 7 Op Amp with negative feedback Rf Rout vRs Rin v v v vo EE201 Lecture 26 P 8 Analysis of Inverting Op Amp Rf a Rs vs Ro vRi v b v v Strategy perform KCL analysis at nodes a and b then solve for vo as a function of vs vo EE201 Lecture 26 P 9 v vs v v vo KCL a 0 Rs Ri Rf vo v vo A v KCL b 0 Rf Ro 1 1 1 1 1 v vo vs Rs Ri R f Rf Rs vo A 1 1 1 v vo 0 Ro R f R f Ro A Ro R f Rs R f 1 A Ro Ri Rs Ri 1 Ro R f vs EE201 Lecture 26 P 10 Analysis of noninverting Op Amp Rf Rs a Ri Rg v vg Ro v b v v vo Strategy perform KCL analysis at nodes a and b then solve for vo as a function of vs EE201 Lecture 26 P 11 v v v g v vo KCL a 0 Rs Rg Ri Rf vo v vo A v v KCL b 0 Rf Ro Note that v v g v v g if i 0 Rg Ri Rg Rearranging and solving for vo f vg vo R f Rs Rs Ro ARi vg Rs Ro A 1 Rs Rg Ri 1 ARi R f Rs R f Rs Ri Rg EE201 Lecture 26 P 12 Buffer Amplifier Voltage Follower if i i Vd vo vi Equivalent circuit v vi v v v EE201 Lecture 26 P 13 Analyze equivalent circuit vo A v v vi v vo vRelating vo to vi vo A vi vo vo A A 1 vi vi vo is said to follow vi This is called a voltage follower This circuit buffers or isolates vo from vi so that circuits connected to the output do not adversely affect the input circuitry A wire directly connecting outputs and inputs would not serve this function
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