1	Chapter 32 Oscillators
2	Basics of Feedback Block diagram of feedback amplifier Forward gain, A Feedback, B Summing junction, ∑ Useful for oscillators
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
6	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 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
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
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
31	Crystal Oscillators Impedance varies with frequency Square wave crystal oscillator circuit Choose C₁ and C₂ Oscillation frequency between f_s and f_p
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
35	555 Timer Monostable Circuit (one-shot) Trigger high → v_out = low Trigger low → v_out = high
36	Voltage Controlled Oscillator-VCO ∆f_out ∆v_in