Chapter 18

1. Chapter 18 Oscillators

2. Oscillators • Oscillator – An ac signal generator. A circuit that converts dc to a sinusoidal (or other) waveform. • Oscillators use positive feedback to produce an output waveform. • Positive feedback – A type of feedback signal that is in phase with the circuit input signal.

3. Oscillators: The Basic Idea • The signal source produces the waveforms shown. • When the source is removed, the circuit loop continues to produce an output. • Because positive feedback generates an input signal, it is also called regenerative feedback.

4. The Barkhausen Criterion • Barkhausen criterion – The relationship between the circuit feedback factor (av) and voltage gain (Av) required for proper oscillator operation. • The Barkhausen criterion states that oscillations can be sustained only when

5. The significance of avAv • When avAv < 1, the circuit experiences damping. • Damping – The fading out of oscillations over some number of cycles. • When avAv > 1, the circuit output is driven into saturation and cutoff clipping. • When avAv = 1, the circuit output remains at a stable amplitude.

6. Phase-Shift Oscillators • Phase-shift oscillator – An oscillator that uses three RC circuits in its feedback network to produce a 180° phase shift. • The combined phase shifts of the op-amp and the feedback network is 360° (or 0°)

7. Stability • Oscillator stability – A measure of an oscillator’s ability to maintain constant output amplitude and frequency. • The phase-shift oscillator is rarely used because it is relatively unstable.

8. Wien-Bridge Oscillators • Wien-bridge oscillator – An oscillator that achieves regenerative feedback by producing no phase shift at its resonant frequency.

9. Frequency Response of the Positive Feedback Path • The positive feedback path acts as a band-pass filter.

10. Circuit Gain • The gain of the Wien-bridge oscillator is determined by the components in the negative feedback path.

11. The Colpitts Oscillator • Colpitts oscillator – An oscillator that uses a pair of tapped capacitors and an inductor to produce a 180° voltage phase shift in the feedback network.

12. Colpitts Oscillator Feedback Network • The amplifier output voltage is developed across C1. • The feedback voltage is developed across C2. • The capacitor voltages are 180° out of phase.

13. Transformer-Coupled Colpitts Oscillator

14. The Hartley Oscillator • Hartley oscillator – An oscillator that uses a pair of tapped inductors and a capacitor to produce a 180° voltage phase in its feedback network.

15. The Clapp Oscillator • Clapp oscillator – A Colpitts oscillator with an added capacitor (in series with the feedback inductor) used to reduce the effects of transistor junction capacitance.

16. The Armstrong Oscillator • Armstrong oscillator – An oscillator that uses a transformer in its feedback network to achieve the required 180° voltage phase shift.

17. Crystal-Controlled Oscillators • Conventional oscillators can experience a change in gain or frequency due to: • The active component being replaced. • A reactive component being replaced. • The circuit experiencing a change in temperature. • Crystal-controlled oscillator – An oscillator that uses a quartz crystal to produce an extremely stable output frequency.

18. Crystals • Piezoelectric effect – The tendency of a crystal to vibrate at a fixed frequency when exposed to an electric field. • The frequency of vibration depends on the physical dimensions of the crystal. • The three crystals used in oscillators are: • Roshelle salt. The best piezoelectric activity, but the most fragile. • Tourmaline. The toughest (physically), but the least stable vibration rate. • Quartz. Good piezoelectric activity and less fragile than Roshelle salt. Also the least expensive to produce.

19. Quartz Crystals • CC is the crystal capacitance • CM is the mounting capacitance • L is the crystal inductance • R is the crystal resistance

20. Crystal-Controlled Colpitts Oscillator

21. The Pierce Oscillator