To Get a Perfect “A”…Project StatementDesign ProcessBasic Theory of OscillatorsMeet the Wein BridgeWein Bridge Continued…The Wein Bridge Problem…Solution…Not So Perfect…The All Mighty AGCThe RectifierRectifier Stimulus ResponseThe VCRVCR ResponseFrequency SelectionPutting It All Together…Additional ImprovementsFinal Circuit – With ImprovementsFinal Circuit ContinuedOperation of Circuit in Real LifeFinal CommentsTo Get a Perfect To Get a Perfect “A”…“A”…An Engr. 311 Project by Corrin MeyerProject StatementProject StatementThe tuner should generate a pure and The tuner should generate a pure and perfect A. perfect A. The sine wave should oscillate to with in 5% The sine wave should oscillate to with in 5% of 440 Hz (which is a perfect tuning A).of 440 Hz (which is a perfect tuning A).The sine wave should have as little distortion The sine wave should have as little distortion as possible.as possible.The tuner should be able to drive a The tuner should be able to drive a speaker.speaker.The tuner should be portable (in concept).The tuner should be portable (in concept).Design ProcessDesign ProcessResearch sinusoidal oscillators.Research sinusoidal oscillators.Understand benefits and pitfalls of Understand benefits and pitfalls of different oscillator designs.different oscillator designs.Choose an appropriate oscillator.Choose an appropriate oscillator.Improve basic circuit design.Improve basic circuit design.Basic Theory of Basic Theory of OscillatorsOscillatorsOscillators are by Oscillators are by definition unstable.definition unstable.The basic oscillator is The basic oscillator is depicted to the right. depicted to the right. (The response is also (The response is also shown)shown)For oscillations to occurFor oscillations to occurNegative Feedback: A*B = Negative Feedback: A*B = -1-1Positive Feedback: A*B = 1Positive Feedback: A*B = 1A VinVout VoutA Vin A Vout VoutA Vin VoutA VoutA Vin Vout1 A VoutVinA1 A Meet the Wein BridgeMeet the Wein BridgeBasicsBasicsUses an Op-Amp for Uses an Op-Amp for amplificationamplificationUses positive feedback Uses positive feedback through a RC band-pass filterthrough a RC band-pass filterAdvantagesAdvantagesFew partsFew partsAble to generate very Able to generate very accurate sine waves (Used in accurate sine waves (Used in audio equipment)audio equipment)DisadvantagesDisadvantagesNot easily tuned to the Not easily tuned to the desired frequencydesired frequencyMay introduce significant May introduce significant distortions into the resulting distortions into the resulting wave without proper wave without proper amplitude control.amplitude control.R2R4R3R1C2C100+-OUTVoutPositive RC band-pass feedback filterOp-Amp amplifierWein Bridge Continued…Wein Bridge Continued…Derivation of the Loop Derivation of the Loop Gain (A*B)Gain (A*B)Positive feedback: A*B=1Positive feedback: A*B=1B is a real number when B is a real number when w=1/RCw=1/RCB=1/3 when w=1/RCB=1/3 when w=1/RCA=3 for the loop gain to A=3 for the loop gain to equal 1equal 1The oscillator will The oscillator will oscillate at the frequency oscillate at the frequency w, where w=1/RC and w, where w=1/RC and has units rad/shas units rad/sA 1R4R3If R1 = R2 and C1 = C2...RRCs 1R1CsRRCs 113 RCs1RCs13 j RC1RCGainloopA 1R3R43 j RC1RCThe Wein Bridge The Wein Bridge ProblemProblemFor oscillations to start, A must be slightly For oscillations to start, A must be slightly greater than 3.greater than 3.If A is greater than 3 , then the loop gain is If A is greater than 3 , then the loop gain is greater than 1.greater than 1.If the loop gain is greater than 1, then the If the loop gain is greater than 1, then the sine wave amplitude will tend towards sine wave amplitude will tend towards infinity.infinity.Circuit does not infinite power, so the output Circuit does not infinite power, so the output sine wave becomes severely distorted.sine wave becomes severely distorted.……Solution…Solution…Design amplitude Design amplitude limiting circuitry.limiting circuitry.There are 3 general There are 3 general solutions.solutions.Passive devices (diodes)Passive devices (diodes)Resistive lampResistive lampAutomatic Gain Control Automatic Gain Control (AGC)(AGC)A diode limited Wein A diode limited Wein Bridge is depicted to Bridge is depicted to the right.the right.R220.3kR410kR510.96kR710.96kR83kR61.2kR31.2kR13kC133nC233nD1D1N4148D2D1N41480 00U1LM741+3-2V+7V-4OUT6OS11OS25VCCVDDVCCVDDVoutAmplitude LimiterNot So Perfect…Not So Perfect…The diode limited Wein Bridge does The diode limited Wein Bridge does NOT produce a perfect sine wave.NOT produce a perfect sine wave.The amplifier gain is different when The amplifier gain is different when the diodes conduct and when they the diodes conduct and when they do not conduct.do not conduct.Result: Distorted sine wave.Result: Distorted sine wave.Solution: AGCSolution: AGCThe All Mighty AGCThe All Mighty AGCAGC stands for Automatic Gain AGC stands for Automatic Gain Control.Control.Controls the gain of the amplifier based Controls the gain of the amplifier based on the output sine wave amplitude.on the output sine wave amplitude.The AGC requires two parts…The AGC requires two parts…An AC rectifier with signal smoothing.An AC rectifier with signal smoothing.A VCR (Voltage Controlled Resistor).A VCR (Voltage Controlled Resistor).The RectifierThe RectifierDepicted below is the precision rectifier used in the Depicted below is the precision rectifier used in the final oscillator circuit.final oscillator circuit.The rectifier is designed to invert the positive peaks The rectifier is designed to invert the positive peaks of the sine so that the wave is always negative.of the sine so that the wave is always negative.Signal smoothing is not included here.Signal smoothing is not included here.R3R1 R2D2D10VinVout+-OUT +-OUTRectifier Stimulus Rectifier Stimulus ResponseResponse Time250ms 251ms 252ms 253ms 254ms 255ms 256ms 257ms 258ms 259ms 260msV(U4:OUT) V(U1:OUT)-3.0V-2.0V-1.0V-0.0V1.0V2.0V3.0VRectifier OutputRectifier InputThe VCRThe VCRVCR stands for Voltage VCR stands for Voltage Controlled
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