Lect_11 - ECE 442 ECE 442 SolidState Devices & Circuits...

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CE 442 ECE 442 Solid State Devices & Circuits 11. Operational Amplifiers Jose E. Schutt-Aine lectrical & Computer Engineering Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu ECE 442–Jose Schutt Aine 1
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Operational Amplifiers Universal importance (e.g. amplification from microphone to loudspeakers) General terminal configuration with bias ECE 442–Jose Schutt Aine 2
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Operational Amplifiers ) A v v Gain= A 21 () out vA Common configuration with bias implied but not shown ignaling Signaling 1. Differential input stage ifference between input is amplified ECE 442–Jose Schutt Aine 3 2. Difference between input is amplified
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Operational Amplifiers Ideal Op Amp 1. Infinite input impedance 2. Zero output impedance finite open op gain A f 3. Infinite open-loop gain A inf 4. Infinite CMRR or zero common-mode gain finite bandwidth 5. Infinite bandwidth Also, op amps are dc (or direct coupled) amplifiers since they are xpected to amplify signals with frequency as low as DC expected to amplify signals with frequency as low as DC. ECE 442–Jose Schutt Aine 4
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Differential & Common-Mode Signals - Differential input signal v ID =v 2 -v 1 - Common-mode input signal v m =0.5(v +v ) ICm 1 2 1 2 ID ICm v vv  2 2 ID ICm v  ECE 442–Jose Schutt Aine 5
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Operational Amplifiers Ideally, v ICM should be zero to achieve high CMRR. Amplifier will amplify the difference between the two input signals ECE 442–Jose Schutt Aine 6
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Practical Considerations The output voltage swing of an op amp is limited by the DC power supply. Since op amp can exhibit high gain, power supply voltage fluctuations must be minimized use decoupling capacitors from power supply ECE 442–Jose Schutt Aine 7
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Inverting Configuration erminal 2 is tied Terminal 2 is tied to ground We introduce R F (or R 2 ) to reduce gain (from inf) When R F is connected to terminal 1, we talk about negative feedback. If R F is tied to terminal 2, we have positive feedback ECE 442–Jose Schutt Aine 8 ,p
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Inverting Configuration Need to evaluate v o /v I ssume ideal Op mp Assume ideal Op-Amp 1 () 0 o v vv  Since gain is infinite: 21 A 2 , 0 Thus v v  Note: A is open-loop gain v 1 is virtual ground 12 1 1 II v i R ECE 442–Jose Schutt Aine 9 11 RR
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Inverting Configuration Since input impedance of OP amp is infinite, current through R F is i 1 1 I v ii 1 R 0 Io o I vv v v  1 FF i
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Lect_11 - ECE 442 ECE 442 SolidState Devices & Circuits...

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