EE201 Lecture 27 P 1 Op Amp Basics Ideal Op Amps Revisit Op Amp with negative feedback Rf Ro vRs Ri v v v vo Features v v is small A is large 104 Ro is as small as possible Voltage input not current EE201 Lecture 27 P 2 Consider modifications to simplify analysis v vImplication of this assumption voltage transfer characteristic is modified as shown below vo v v Voltage gain becomes infinite Virtual short conditions v vA EE201 Lecture 27 P 3 Output resistance Ro 0 Implication of this assumption no voltage drop after dependent source ideal output behavior Voltage input minimal currents into op amp Ri Implication of this assumption i i 0 Performing KCL on op amp see p 3 lect 26 i i io ic ic 0 Using above reduces this to io ic ic EE201 Lecture 27 P 4 Ideal Op Amp conditions i v Ri A Ri i Ro 0 Ro 0 v vo v v i 0 i 0 Open circuit at input v v Now revisit inverting and noninverting op amps EE201 Lecture 27 Inverting Amplifier is 50mV 25k 100k P 5 if v o Example Find vo assuming ideal behavior Solution 1 Compute v and v v 0 grounded Using ideal op amp model v v v 0 2 Compute is From Ohm s law is vs Rs is 50 mV 25kV 2 A EE201 Lecture 27 P 6 3 Compute if From Ohm s law if vo Rf if vo 100k 4 Relate is to if From KCL i is if i s if Because i 0 Therefore is vs Rs if vo Rf vo Rf Rs vs vo 100k 25k 50 mV 200mV This is called an inverting amplifier because the output and input voltage polarity are opposite One of the input terminals in this example is grounded single ended mode which means the other is virtually grounded because of the constraints of the virtual short circuit model i e v v EE201 Lecture 27 P 7 Noninverting Amplifier Gf Example Find the voltage gain ig i A Gg i vs if vo Solution 1 Compute v and v v vs Using ideal op amp model v v v vs 2 KCL A i ig if ig if EE201 Lecture 27 P 8 3 Express if and iin in terms of voltages From Ohm s law ig Gg VA Gg vs if Gf vo vs 4 Use current relation to find voltage gain Gf vo vs Gg vs Voltage gain vo vs Gf Gg Gf vo vs 1 Gg Gf Note that gain is positive If noninverting terminal is more positive than inverting terminal the output voltage will have the same polarity as the input voltage amplifier is noninverting If inverting terminal is more positive than noninverting terminal the output voltage will have the opposite polarity as the input voltage EE201 Lecture 27 P 9 Summing Amplifier v1 G1 v2 G2 v3 G3 v4 G4 Gf vo Gs vs Analyze circuit to show properties of this amplifier EE201 Lecture 27 P 10 Approach 1 KCL v node G1 v1 v G2 v2 v Gf v vo v v1G1 v2G2 voGf G1 G2 Gf 2 KCL v node G3 v3 v G4 v4 v Gi v vs v v3G3 v4G4 vsGs G3 G4 Gs 3 Equate V to V virtual ground Gf G1 G2 Gf vo G1v1 G2v2 G1 G2 Gf G3v3 G4v4 Gsvs G3 G4 Gs Output voltage is weighted sum of input voltages See Examples 4 5 and 4 6 in text
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