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Lect_11 - ECE442 SolidStateDevices&Circuits 11. Jose E...

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ECE 342–Jose Schutt Aine 1 ECE 442 Solid State Devices & Circuits 11. Operational Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected]
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ECE 342–Jose Schutt Aine 2 Operational Amplifiers General terminal configuration with bias Universal importance (e.g. amplification from microphone to loudspeakers)
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ECE 342–Jose Schutt Aine 3 Common configuration with bias implied but not shown Operational Amplifiers 21 () out vA v v = Gain= A Signaling 1. Differential input stage 2. Difference between input is amplified
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ECE 342–Jose Schutt Aine 4 Ideal Op Amp 1. Infinite input impedance 2. Zero output impedance 3. Infinite open-loop gain A Î inf 4. Infinite CMRR or zero common-mode gain 5. Infinite bandwidth Operational Amplifiers Also, op amps are dc (or direct coupled) amplifiers since they are expected to amplify signals with frequency as low as DC.
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ECE 342–Jose Schutt Aine 5 Differential & Common-Mode Signals - Differential input signal v ID =v 2 -v 1 - Common-mode input signal v ICm =0.5(v 1 +v 2 ) 1 2 ID ICm v vv =− 2 2 ID ICm v =+
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ECE 342–Jose Schutt Aine 6 Ideally, v ICM should be zero to achieve high CMRR. Amplifier will amplify the difference between the two input signals Operational Amplifiers
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ECE 342–Jose Schutt Aine 7 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 Î use decoupling capacitors from power supply
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ECE 342–Jose Schutt Aine 8 Inverting Configuration 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 Terminal 2 is tied to ground
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ECE 342–Jose Schutt Aine 9 Inverting Configuration Need to evaluate v o /v I Assume ideal Op-Amp 21 () 0 o v vv A = −= Since gain is infinite: v 1 is virtual ground 12 ,0 Thus v v == 1 1 11 II v i R R = = Note: A is open-loop gain
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Lect_11 - ECE442 SolidStateDevices&Circuits 11. Jose E...

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