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Chapter 32 Oscillators Basics of Feedback • Block diagram of feedback amplifier v • Forward gain, A • Feedback, B • Summing junction, ∑ • Useful for oscillators + in ∑ A - vout vF B 2 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 3 Basics of Feedback • Oscillators – Positive feedback – In-phase with input – Unstable 4 Basics of Feedback • Block diagram analysis v e v in v f v o u t A ( v in v f ) v f B vout vout v in + vin ∑ ve A - vout vf B A 1 AB 5 Basics of Feedback v e v in v f • Inverting amplifier v out A v in v f v f Bv out + vin ∑ ve v out A - vout v in v out vf v in A 1 AB 1 1 B 1 B A B B R1 RF 6 Relaxation Oscillator • Square wave generator • Composed of – Schmitt trigger comparator – Positive feedback – RC circuit to determine period 7 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 8 Relaxation Oscillator • Schmitt Trigger – Hysteresis: upper and lower trip points – Can use a voltage follower for adjustable trip points 9 Relaxation Oscillator • Schmitt trigger 10 Relaxation Oscillator • Schmitt Trigger Relaxation Oscillator 11 Relaxation Oscillator • R1 and R2 voltage divider V REF R2 R1 R 2 V SA T • Capacitor charges through RF • VC < +VSAT then C charges toward +VSAT • VC > –VSAT then C charges toward –VSAT 12 Relaxation Oscillator • Schmitt Trigger Relaxation Oscillator Equations RF C vC (t ) V F VO 1 e RtC 2 R2 T 2 R F C ln 1 R1 13 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 14 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 15 Wien Bridge Oscillator • Circuit 16 Wien Bridge Oscillator • Equations f0 B 1 2 O utput frequency R1 R 2 C 1 C 2 R 2 C1 R1 C 1 R 2 C 2 R 2 C 1 if R1 R 2 and C 1 C 2 then f0 1 2 R C and B 1 3 17 Wien Bridge Oscillator • Another form of Wien Bridge 18 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 19 Phase-Shift Oscillator • Three-section R-C network – ≈ 60° per section – Negative FB = 180° – 180° + (60° + 60° + 60°) = 360° = Positive FB f0 1 2 O utput frequency 6 RC A 29 R equired voltage g ain 20 Phase-Shift Oscillator • Circuit 21 LC Oscillators • LC circuits can produce oscillations • Used for – Test and measurement circuits – RF circuits 22 LC Oscillators • Named after pioneer engineers – Colpitts – Hartley – Clapp – Armstrong 23 LC Oscillators • Colpitts oscillator – fs = series resonance – fp = parallel resonance – L-C network → 180° phase shift at fp 24 LC Oscillators RF Rin __ _ __ _ L + C2 +V vout –V C1 __ _ __ _ 25 LC Oscillators • Equations 1 s LC2 2 Im pedance: Z ( s ) 2 s L C 1C 2 s (C1 C 2 ) 1 C1 C 2 1 O scillator frequency: f 0 2 L C 1C 2 C1 C 2 26 LC Oscillators • Hartley oscillator – Similar to Colpitts – L and C’s interchanged – Also have fs and fp 27 LC Oscillators RF Z (s) f0 sL1 1 s L 2 C 1 s 2 2 L1 L 2 C + Rin ___ __ _ 1 2 L1 L 2 C +V vout –V C1 L2 L1 ___ __ _ 28 Crystal Oscillators • • • • • • • Quartz crystals Mechanical device Higher frequencies (>1 MHz) Stability Accuracy Reliability Piezoelectric effect 29 Crystal Oscillators • Electrical model – Both have parallel and series resonance RF C1 L1 C0 • Symbol – Quartz crystal – metal plates 30 Crystal Oscillators • Impedance varies with frequency • Square wave crystal oscillator circuit • Choose C1 and C2 R2 vout CMOS Inverter – Oscillation frequency between fs and fp R1 XTAL C1 C2 ___ __ _ 31 555 Timer • IC – Internal circuit 32 555 Timer • Usage – Monostable timing – Astable mode = relaxation oscillator – Trigger voltage – Control voltage – Threshold voltage – R-S flip-flop 33 555 Timer • Relaxation oscillator T1 ln(2) R B C VCC = +15 V RA T 2 ln(2) R A R B C T ln(2) R A 2 R B C f 4 7 NE555 RB 2 6 1 T 8 3 1 C vout 5 0.01 μF 34 ___ __ _ 555 Timer • Monostable Circuit (one-shot) • Trigger high → vout = low R • Trigger low → vout = high VCC = +15 V 8 A 4 7 NE555 2 6 C 3 1 ___ __ _ Trigger vout 5 0.01 μF 35 ___ __ _ Voltage Controlled OscillatorVCO • ∆fout ∆vin R1 VCC Outputs 1 nF Voltage Input fO 2.4 V C C V C R1C 1V C C 6 8 LM566C 5 7 C1 ___ __ _ 1 ___ __ _ 3 Square wave 4 Triangle wave vout 36