1. Chapter 4 Oscillators

2. Objectives Describe the basic concept of an oscillator
Discuss the basic principles of operation of an oscillator
Analyze the operation of RC and LC feedback oscillators
Describe the operation of the basic relaxation oscillator circuits
Discuss the use of a 555 timer in an oscillator circuit

3. Introduction to oscillator

4. What is an oscillator ???
Oscillators are circuits that produce a continuous signal of some type without the need of an input. These signals serve a purpose for a variety of purposes. Communications systems, digital systems (including computers), and test equipment make use of oscillators.

5. The types of Oscillator
An oscillator is a circuit that produces a repetitive signal from a dc voltage.
The feedback type oscillator which rely on a positive feedback of the output to maintain the oscillations.
The relaxation oscillator makes use of an RC timing circuit to generate a non-sinusoidal signal such as square wave.
Fig 16-1 Oscillator

6. F/b type oscillator Vs relaxation oscillator
Both use active (transistor, op-amp) and passive components (R,L,C)
F/b type oscillator
produces sine wave
f/b determines the oscillation frequency
Relaxation oscillator
produces non-sinusoidal wave (square etc)
use of an RC timing circuit to generate a non-sinusoidal signal

7. Feedback Oscillator Principles
The feedback oscillator is widely used for generation of sine wave signals. The positive (in phase) feedback arrangement maintains the oscillations. The feedback gain must be kept to unity to keep the output from distorting.
Fig 16-3 Pos. Feedback

8. Positive Feedback
In-phase portion of the output voltage of an amplifier is fed back to the input with no net phase shift.
Vf - in-phase f/b voltage is amplified to produce the output voltage.
The loop –sustains the signal-produce continuous sinusoidal output voltage. This phenomenon is called oscillation.

9. Conditions for oscillation
Phase shift around the f/b loop = 0 degree
The loop gain, Acl must equal to 1
See figure 16.4 p.g 776 (Floyd)

10. Feedback Oscillators Two types :
RC - limited to frequencies of 1 MHz/less
Wien-bridge, the phase-shift, and the twin-T.
LC - for frequencies above 1 MHz.
Colpitts, Clapp, Hartley, Armstrong, and crystal controlled oscillators

11. FEEDBACK OSCILLATORS: Oscillators With RC Feedback Circuits

12. Oscillators With RC Feedback Circuits
Since there is a loss of about 1/3 of the signal in the positive feedback loop, the voltage-divider ratio must be adjusted such that a positive feedback loop gain of 1 is produced. This requires a closed loop gain of 3. The ratio of R1 and R2 can be set to achieve this.
Fig 16-8a&b

13. Oscillators With RC Feedback Circuits
The lead-lag circuit of a Wien-bridge oscillator reduces the input signal by 1/3 and yields a response curve as shown. The frequency of resonance can be determined by the formula below.
fr = 1/2RC
Fig 16-6a&b

14. Oscillators With RC Feedback Circuits
The lead-lag circuit is in the positive feedback loop of Wien-bridge oscillator. The voltage divider limits gain. The lead lag circuit is basically a bandpass with a narrow bandwidth (high Q).
Fig. 16-7b Wien-bridge

15. Exercise 4.1 : Wien-bridge oscillator
Floyd page 781(Example 16.1)
Do it now!
Quiz / exercise?

16. Oscillators With RC Feedback Circuits
To start the oscillations, initially the close loop gain,Acl >3 must be achieved.
The back to back zener diode arrangement are one way of achieving this.
Voltage divider circuit has been modified
When dc is first applied the zeners appear as opens-R3 series with R1
Increasing the Acl.
Fig Wien-bridge w/zeners

17. Oscillators With RC Feedback Circuits
Zener arrangement for gain control is simple but produces distortion because of the non-linearity of zener diodes.
Automatic gain control (AGC) - maintain a gain of exact unity.
A JFET in the negative feedback loop can be used to precisely control the gain. After the initial startup and the output signal increases the JFET is biased such that the negative feed back keeps the gain at precisely 1.
Fig JFET AGC
Gain is controlled by the components
in the colored box

18. Oscillators With RC Feedback Circuits
The phase shift oscillator –
3 RC circuits to provide 180º phase shift + 180º of the op-amp itself provides 360º or 0º - to sustain oscillations.
The close loop gain, ACL must be at least 29 to maintain the oscillations.
Attenuation of 3 RC feedback circuit, B = 1/29=R3/Rf
The frequency of resonance for the this type is similar to any RC circuit oscillator.
fr = 1/26RC
Fig phase shift osc.

19. Oscillators With RC Feedback Circuits
The twin-T – 2 types RC filters (LPF & HPF) used in feedback loop-utilizes a bandstop/notch arrangement of RC circuits to block all but the frequency of operation in the negative feedback loop.
The only frequency allowed to effectively oscillate is the frequency of resonance, fr
Oscillation only occur at fr where positive f/b through the voltage divider exist. Negative f/b is negligible.
Fig 16-15a&b twin T

20. FEEDBACK OSCILLATORS: Oscillators With LC Feedback Circuits

21. Oscillators With LC Feedback Circuits
The Colpitts oscillator uses a tank circuit (LC) in the feedback loop-phase shift and act as a resonant filter.
The resonant frequency;
fr = 1/2LCT, CT= C1//C2
The resonant frequency with loading of feedback cct;
fr = (1/2LCT)((Q2/Q2+1)),
CT= C1//C2
For Q less than 10
Fig BJT Colpitts

22. Oscillators With LC Feedback Circuits
The Clapp is similar to the Colpitts except the additional C in the tank circuit. The same formula applies as the Colpitts.
FET is replaced with BJT-reduce the loading effect of the transistor’s input impedance
Fig Clapp osc.

23. Oscillators With LC Feedback Circuits
The Hartley oscillator similar to the Clapp and Colpitts.
The tank circuit has two inductors and one capacitor.
The calculation of the resonant frequency is the same.
Fig Hartley osc.

24. Oscillators With LC Feedback Circuits
The Armstrong uses transformer coupling in the feedback loop. For this reason the Armstrong is not as popular.
Fig Armstrong osc.

25. Oscillators With LC Feedback Circuits
The crystal-controlled oscillator is the most stable and accurate of all oscillators. A crystal has a natural frequency of resonance. Quartz material can be cut or shaped to have a certain frequency. We can better understand the use of a crystal in the operation of an oscillator by viewing it’s electrical equivalent.
Fig abc&d crystal

26. Oscillators With LC Feedback Circuits
Since crystal have natural resonant frequencies of 20 MHz or less, generation of higher frequencies are attained by operating the crystal in what is called the overtone mode. Overtones are usually odd multiples of a crystal’s fundamental.
Fig 16-26a xtal osc.

27. Relaxation Oscillators
Relaxation oscillators make use of an RC timing and a device that changes states to generate a periodic waveform.
Used to produce nonsinusoidal waveforms
Relaxation Oscillators
Triangular wave oscillator
VCO
Square wave
Fig op-amp gen
Fig output waveforms

28. Relaxation Oscillators
Basic idea of Triangular wave oscillator
Dual polarity switched input (not for practical)
Position 1, negative voltage, positive going ramp
Position 2, Positive voltage, negative going ramp

29. Relaxation Oscillators
Practical circuit:
This triangular-wave oscillator makes use of a comparator and integrator to actually produce both a triangle-wave and square-wave.

30. Relaxation Oscillators
UTP-upper trigger point-comparator at max positive level
LTP-lower trigger point- comparator at max positive level
Output levels are set by the ratio of R2 and R3 times the maximum output of the comparator. The frequency of output can be determined by the formula below.
fr = 1/4R1C(R2/R3)
Fig op-amp gen

31. Relaxation Oscillators
The voltage-controlled sawtooth oscillator’s frequency can be changed by a varied by a given dc control voltage. One possible type uses a programmable unijunction transistor (PUT).
Can be either sinus or non-sinusoidal O/p
Consists of :-
Op-amp integrator
Switching device, PUT parallel with f/b Capacitor
(used to terminate the each ramp)

32. Relaxation Oscillators
Anode voltage exceeds VG=0.7V, PUT turn on, FWD biased diode
If anode voltage less than VG=0.7V, PUT turn off.
The forward voltage of the PUT (VF) determines the frequency of the output. The formula below shows the relationship.
Fig 16-31a&b VCO sawtooth
f = \VIN\/RiC(1/Vp-VF)

33. Exercise 4.3 : Sawtooth VCO
Floyd page 796(Example 16.5)
Do it now!

34. Relaxation Oscillators
A square wave oscillator relaxation oscillator use the charging and discharging of the capacitor to cause the op-amp to switch states rapidly and produce a square wave. The RC time constant determines the frequency.
Fig basic square wave
Fig waveforms

35. The 555 Timer As An Oscillator
The 555 timer is an integrated circuit that can be used in many applications. We will discuss it’s operation as a square wave oscillator. The frequency of output is determined by the external components R1, R2, and C. The formula below shows the relationship.
fr = 1.44/(R1 + 2R2)C
Fig timer
Detailed operation is described within the text.

36. The 555 Timer As An Oscillator
Duty cycles can be adjusted by values of R1 and R2. The duty cycle is limited to 50% with this arrangement. To have duty cycles less than 50%, a diode is placed across R2. The two formulas show the relationship. (see following slide)
Duty Cycle >50 % = R1 + R2/R1 + 2R2 x 100%
Duty Cycle <50 % w/diode = R1/R1 + R2 x 100%

37. The 555 Timer As An Oscillator
Fig w/diode

38. The 555 Timer As An Oscillator
The 555 timer by be operated as a VCO with a control voltage applied to the CONT input (pin 5).
Fig VCO

39. Summary Sinusoidal oscillators operate with positive feedback.
Two conditions for oscillation are 0º feed back phase shift and feedback loop gain of 1.
The initial startup requires the gain to be momentarily greater than 1
RC oscillators include the Wien-bridge, phase shift, and twin T.
LC oscillators include the Colpitts, Clapp, Hartley, Armstrong, and crystal.

40. Summary
The crystal actually uses a crystal as the LC tank circuit and is very stable and accurate.
A voltage controlled oscillator’s (VCO) frequency is controlled by a dc control voltage.
A 555 timer is a versatile integrated circuit which can be used a square wave or pulse generator.