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Chapter 32

Chapter 32. Oscillators. Basics of Feedback. Block diagram of feedback amplifier Forward gain, A Feedback, B Summing junction, ∑ Useful for oscillators. +. ∑. A. v in. v out. -. v F. B. Basics of Feedback. Op-amps Inverting & non-inverting

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Chapter 32

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  1. Chapter 32 Oscillators

  2. Basics of Feedback • Block diagram of feedback amplifier • Forward gain, A • Feedback, B • Summing junction, ∑ • Useful for oscillators + ∑ A vin vout - vF B

  3. Basics of Feedback • Op-amps • Inverting & non-inverting • Negative feedback 180°out of phase w/input • High input impedance • Low output impedance • Wide bandwidth • Stable operation

  4. Basics of Feedback • Oscillators • Positive feedback • In-phase with input • Unstable

  5. Basics of Feedback • Block diagram analysis + ve ∑ A vin vout - vf B

  6. + ve ∑ A vin vout - vf B Basics of Feedback • Inverting amplifier

  7. Relaxation Oscillator • Square wave generator • Composed of • Schmitt trigger comparator • Positive feedback • RC circuit to determine period

  8. Relaxation Oscillator • Schmitt Trigger • R1 and R2 form a voltage divider • Portion of output applied at + input • Hysteresis: output dependent on input and previous value of input

  9. Relaxation Oscillator • Schmitt Trigger • Hysteresis: upper and lower trip points • Can use a voltage follower for adjustable trip points

  10. Relaxation Oscillator • Schmitt trigger

  11. Relaxation Oscillator • Schmitt Trigger Relaxation Oscillator

  12. Relaxation Oscillator • R1 and R2 voltage divider • Capacitor charges through RF • VC < +VSAT then C charges toward +VSAT • VC > –VSAT then C charges toward –VSAT

  13. Relaxation Oscillator • Schmitt Trigger Relaxation Oscillator Equations

  14. Wien Bridge Oscillator • For a sinusoidal oscillator output • Closed loop gain ≥ 1 • Phase shift between input and output = 0° at frequency of oscillation • With these conditions a circuit • Oscillates with no external input • Positive feedback = regenerative feedback

  15. Wien Bridge Oscillator • Regenerative oscillator • Initial input is small noise voltage • Builds to steady state oscillation • Wien Bridge oscillator • Positive feedback, RC network branch • Resistor branch establish amplifier gain

  16. Wien Bridge Oscillator • Circuit

  17. Wien Bridge Oscillator • Equations

  18. Wien Bridge Oscillator • Another form of Wien Bridge

  19. Wien Bridge Oscillator • For a closed-loop gain, AB = 1 • Op-amp gain ≥ 3 • Improved circuit • Separate RF into 1 variable and 1 fixed resistor • Variable: minimize distortion • Zener Diodes: limit range of output voltage

  20. Phase-Shift Oscillator • Three-section R-C network • ≈ 60° per section • Negative FB = 180° • 180° + (60° + 60° + 60°) = 360° = Positive FB

  21. Phase-Shift Oscillator • Circuit

  22. LC Oscillators • LC circuits can produce oscillations • Used for • Test and measurement circuits • RF circuits

  23. LC Oscillators • Named after pioneer engineers • Colpitts • Hartley • Clapp • Armstrong

  24. LC Oscillators • Colpitts oscillator • fs = series resonance • fp = parallel resonance • L-C network → 180° phase shift at fp

  25. RF +V - + vout Rin –V ___ __ _ L C2 C1 ___ __ _ LC Oscillators

  26. LC Oscillators • Equations

  27. LC Oscillators • Hartley oscillator • Similar to Colpitts • L and C’s interchanged • Also have fs and fp

  28. RF +V - + vout Rin –V ___ __ _ C1 L2 L1 ___ __ _ LC Oscillators

  29. Crystal Oscillators • Quartz crystals • Mechanical device • Higher frequencies (>1 MHz) • Stability • Accuracy • Reliability • Piezoelectric effect

  30. RF C1 L1 C0 Crystal Oscillators • Electrical model • Both have parallel and series resonance • Symbol • Quartz crystal • metal plates

  31. Crystal Oscillators • Impedance varies with frequency • Square wave crystal oscillator circuit • Choose C1 and C2 • Oscillation frequency between fs and fp R2 R2 vout CMOS Inverter R1 XTAL C1 C2 ___ __ _

  32. 555 Timer • IC • Internal circuit

  33. 555 Timer • Usage • Monostable timing • Astable mode = relaxation oscillator • Trigger voltage • Control voltage • Threshold voltage • R-S flip-flop

  34. 555 Timer • Relaxation oscillator VCC= +15 V RA 8 4 7 NE555 RB 2 6 3 vout 1 5 C 0.01 μF ___ __ _

  35. 555 Timer • Monostable Circuit (one-shot) • Trigger high → vout = low • Trigger low → vout = high VCC = +15 V RA 8 4 7 NE555 2 6 3 vout C 1 5 ___ __ _ 0.01 μF Trigger ___ __ _

  36. Voltage Controlled Oscillator-VCO • ∆fout ∆vin R1 VCC Outputs 1 nF 6 8 Square wave 3 5 LM566C Voltage Input 4 Triangle wave 7 1 vout C1 ___ __ _ ___ __ _

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