1 / 36

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

joyce
Download Presentation

Chapter 32

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 ___ __ _ ___ __ _

More Related