| Oscillators |
| Block diagram of feedback amplifier | |
| Forward gain, A | |
| Feedback, B | |
| Summing junction, ∑ | |
| Useful for oscillators |
| Op-amps | ||
| Inverting & non-inverting | ||
| Negative feedback 180°out of phase w/input | ||
| High input impedance | ||
| Low output impedance | ||
| Wide bandwidth | ||
| Stable operation | ||
| Oscillators | ||
| Positive feedback | ||
| In-phase with input | ||
| Unstable | ||
| Block diagram analysis |
| Inverting amplifier |
| Square wave generator | ||
| Composed of | ||
| Schmitt trigger comparator | ||
| Positive feedback | ||
| RC circuit to determine period | ||
| 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 | ||
| Schmitt Trigger | ||
| Hysteresis: upper and lower trip points | ||
| Can use a voltage follower for adjustable trip points | ||
| Schmitt trigger | ||
| Schmitt Trigger 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 |
| Schmitt Trigger Relaxation Oscillator Equations |
| 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 | ||
| 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 | ||
| Circuit |
| Equations |
| Another form of Wien Bridge |
| 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 | ||
| Three-section R-C network | ||
| ≈ 60° per section | ||
| Negative FB = 180° | ||
| 180° + (60° + 60° + 60°) = 360° = Positive FB | ||
| Circuit |
| LC circuits can produce oscillations | ||
| Used for | ||
| Test and measurement circuits | ||
| RF circuits | ||
| Named after pioneer engineers | ||
| Colpitts | ||
| Hartley | ||
| Clapp | ||
| Armstrong | ||
| Colpitts oscillator | ||
| fs = series resonance | ||
| fp = parallel resonance | ||
| L-C network → 180° phase shift at fp | ||
| Equations |
| Hartley oscillator | ||
| Similar to Colpitts | ||
| L and C’s interchanged | ||
| Also have fs and fp | ||
| Quartz crystals | |
| Mechanical device | |
| Higher frequencies (>1 MHz) | |
| Stability | |
| Accuracy | |
| Reliability | |
| Piezoelectric effect |
| Electrical model | ||
| Both have parallel and series resonance | ||
| Symbol | ||
| Quartz crystal | ||
| metal plates | ||
| Impedance varies with frequency | ||
| Square wave crystal oscillator circuit | ||
| Choose C1 and C2 | ||
| Oscillation frequency between fs and fp | ||
| IC | ||
| Internal circuit | ||
| Usage | ||
| Monostable timing | ||
| Astable mode = relaxation oscillator | ||
| Trigger voltage | ||
| Control voltage | ||
| Threshold voltage | ||
| R-S flip-flop | ||
| Relaxation oscillator |
| Monostable Circuit (one-shot) | |
| Trigger high → vout = low | |
| Trigger low → vout = high |
Voltage Controlled Oscillator-VCO
| ∆fout ∆vin |