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Published byCollin Bates Modified over 4 years ago

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Chapter 32 Oscillators

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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 + -

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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

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

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5 Basics of Feedback Block diagram analysis ∑A B v in v out veve vfvf + -

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

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

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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

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

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10 Relaxation Oscillator Schmitt trigger

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11 Relaxation Oscillator Schmitt Trigger Relaxation Oscillator

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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

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13 Relaxation Oscillator Schmitt Trigger Relaxation Oscillator Equations

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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

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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

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16 Wien Bridge Oscillator Circuit

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17 Wien Bridge Oscillator Equations

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18 Wien Bridge Oscillator Another form of Wien Bridge

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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

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

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21 Phase-Shift Oscillator Circuit

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

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23 LC Oscillators Named after pioneer engineers –Colpitts –Hartley –Clapp –Armstrong

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24 LC Oscillators Colpitts oscillator –f s = series resonance –f p = parallel resonance –L-C network → 180° phase shift at f p

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

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26 LC Oscillators Equations

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

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

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

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

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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

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32 555 Timer IC –Internal circuit

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

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34 555 Timer Relaxation oscillator NE555 ___ __ _ 1 5 v out 3 4 V CC = +15 V 8 7 2626 RARA RBRB C 0.01 μF

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35 555 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 8 7 2626 RARA C 0.01 μF ___ __ _ Trigger

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36 Voltage Controlled Oscillator- VCO ∆f out ∆v in LM566C ___ __ _ 5 v out 1 4 V CC 6 7 8 1 nF C1C1 ___ __ _ Voltage Input 3 R1R1 Square wave Triangle wave Outputs

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