1. Feedback and Oscillator Circuits
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

2. I. Feedback Concepts Slide 1
The effect of negative feedback on an amplifier:
Disadvantage:
• Lower gain
Advantages:
• Higher input impedance
• More stable gain
• Improved frequency response
• Lower output impedance
• Reduced noise
• More linear operation
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

3. Feedback Connection Types
Slide 2
1. Voltage-Series Feedback
2. Voltage-Shunt Feedback
3. Current-Series Feedback
4. Current-Shunt Feedback
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

4. 1. Voltage-Series Feedback
Slide 3
The output voltage is fed back in series to the input.
Example of a FET circuit:
Feedback gain (Af):
[Formula 17.14]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

5. 2. Voltage-Shunt Feedback
Slide 4
The output voltage is fed back in parallel with the input.
Example of an op-Amp circuit:
Feedback gain (Af):
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

6. 3. Current-Series Feedback
Slide 5
Some of the output current is directed back in series with the input.
Example using a BJT:
Feedback gain (Af):
[Formula 17.20]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

7. 4. Current-Shunt Feedback
Slide 6
Some of the output current is directed back in parallel with the input.
Diagram:
Feedback gain (Af):
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

8. Gain with Feedback Slide 7
Summary of each type of feedback and the effects on gain.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

9. Impedance with Feedback
Slide 8
Summary of each type of feedback and the effects on impedance.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

10. Frequency Distortion with Feedback
Slide 9
If the feedback network is purely resistive, then the gain with feedback will be less dependent on frequency variations. In some cases the resistive feedback removes all dependence on frequency variations.
If the feedback includes frequency dependent components (capacitors and inductors), then the frequency response of the amplifier will be affected.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

11. Noise and Nonlinear Distortion with Feedback
Slide 10
The feedback network reduces noise by cancellation. The signal fed-back is often in the opposite phase from the input.
Nonlinear distortion is also reduced simply because the gain is reduced. The amplifier is operating in midrange and not at the extremes.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

12. Bandwidth with Feedback
Slide 11
Feedback increases the bandwidth of an amplifier.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

13. Gain Stability with Feedback
Slide 12
Gain calculations with feedback are often based on external resistive elements in the circuit. By removing gain calculations away from internal variations of  and gm, the gain becomes more stable.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

14. Phase and Frequency Considerations with Feedback
Slide 13
At higher frequencies the feedback signal may no longer be out of phase with the input. Therefore the feedback signal becomes a positive feedback that will add to the input signal. This may cause the amplifier to self oscillate, thus making it unstable.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

15. II. Oscillator Operation
Slide 14
For self-sustaining oscillations:
• the feedback signal must positive
• the overall gain must be equal to one (unity gain)
If the feedback signal is not positive or the gain is less than one, then the oscillations will dampen out.
If the overall gain is greater than one, then the oscillator will eventually saturate.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

16. Types of Oscillator Circuits
Slide 15
A. Phase-Shift Oscillator
B. Wien Bridge Oscillator
C. Tuned Oscillator Circuits
D. Crystal Oscillators
E. Unijunction Oscillator
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

17. A. Phase-Shift Oscillator
Slide 16
Frequency of the oscillator: [Formula 17.33]
The amplifier must supply enough gain to compensate for losses. The overall gain must be unity.
The RC networks provide the necessary phase shift for a positive feedback. They also determine the frequency of oscillation.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

18. Example of a Phase-Shift Oscillator
Slide 17
FET Phase-Shift Oscillator
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

19. B. Wien Bridge Oscillator
Slide 18
Frequency of oscillations: [Formula 17.41]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

20. C. Tuned Oscillator Circuits
Slide 19
Tuned Oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations.
There are two common types:
• Colpitts – The resonant circuit is an inductor and two capacitors.
• Hartley – The resonant circuit is a tapped inductor or two inductors and one capacitor.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

21. Colpitts Tuned Oscillator Circuit
Slide 20
FET based Colpitts Oscillator
Frequency of oscillations: [Formula 17.44]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

22. Hartley Tuned Oscillator Circuit
Slide 21
BJT based Hartley Oscillator
Frequency of oscillations: [Formula 17.46]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

23. D. Crystal Oscillators Slide 22
The crystal appears to the rest of the circuit as a resonant circuit.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

24. Crystal Resonant Frequencies
Slide 23
The crystal has two resonant frequencies:
Series resonant: RLC determine the resonant frequency. The crystal has a low impedance.
Parallel resonant: RL and CM determine the resonant frequency. The crystal has a high impedance.
The series and parallel resonant frequencies are very close, within 1% of each other.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

25. Series Resonant Crystal Oscillator
Slide 24
FET Crystal Oscillator
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

26. Parallel Resonant Crystal Oscillator
Slide 25
BJT Crystal Oscillator
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

27. E. Unijunction Oscillator
Slide 26
Frequency of oscillations: [Formula 17.48]
for  = 0.4 to [Formula 17.49]
 is a rating of the unijunction transistor.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

28. Unijunction Oscillator Waveforms
Slide 27
The unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.