Chapter 32 Oscillators

2 Basics of Feedback Block diagram of feedback amplifier Forward gain, A Feedback, B Summing junction, ∑ Useful for oscillators ∑A B v in vFvF v out + -

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 ∑A B v in v out veve vfvf + -

6 Basics of Feedback Inverting amplifier ∑A B v in v out veve vfvf + -

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

8 Relaxation Oscillator Schmitt Trigger –R 1 and R 2 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 R 1 and R 2 voltage divider Capacitor charges through R F V C < +V SAT then C charges toward +V SAT V C > –V SAT then C charges toward –V SAT

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 R F 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 –f s = series resonance –f p = parallel resonance –L-C network → 180° phase shift at f p

25 LC Oscillators __ _ __ _ -+-+ __ _ __ _ RFRF R in +V+V –V–V v out C2C2 C1C1 L

26 LC Oscillators Equations

27 LC Oscillators Hartley oscillator –Similar to Colpitts –L and C’s interchanged –Also have f s and f p

28 LC Oscillators ___ __ _ -+-+ ___ __ _ RFRF R in +V+V –V–V v out L1L1 C1C1 L2L2

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

30 Crystal Oscillators Electrical model –Both have parallel and series resonance Symbol –Quartz crystal –metal plates C1C1 L1L1 RFRF C0C0

31 Crystal Oscillators Impedance varies with frequency Square wave crystal oscillator circuit Choose C 1 and C 2 –Oscillation frequency between f s and f p ___ __ _ R2R2 R1R1 v out C1C1 XTAL C2C2 CMOS Inverter R2R2

Timer IC –Internal circuit

Timer Usage –Monostable timing –Astable mode = relaxation oscillator –Trigger voltage –Control voltage –Threshold voltage –R-S flip-flop

Timer Relaxation oscillator NE555 ___ __ _ 1 5 v out 3 4 V CC = +15 V RARA RBRB C 0.01 μF

Timer Monostable Circuit (one-shot) Trigger high → v out = low Trigger low → v out = high NE555 ___ __ _ 1 5 v out 3 4 V CC = +15 V RARA C 0.01 μF ___ __ _ Trigger

36 Voltage Controlled Oscillator- VCO ∆f out ∆v in LM566C ___ __ _ 5 v out 1 4 V CC nF C1C1 ___ __ _ Voltage Input 3 R1R1 Square wave Triangle wave Outputs