1. Oscillators
Electronic Engineering

2. Group members

3. Oscillator
Oscillators are circuits that produce
specific, periodic waveforms such as
square, triangular, saw tooth, and
sinusoidal. They use some form of
active device and passive device like
resistor capacitor and inductor.

4. Oscillation
Oscillation is the repetitive
variation, typically in time,
of some measure about a central
value (often a point of equilibrium)
or between two or more different
states.

5. Classification of Oscillator
There are two main classes of oscillators.
 Harmonic Oscillator
 Relaxation Oscillator

6. Sinusoidal Oscillator
The oscillator which gives a sinusoidal
wave form is called a sinusoidal oscillator.

7. Basic Principle
A sinusoidal oscillator generally
consists of an amplifier having
part of its output returned to
the input by means of a feedback
loop.

8. Barkhausen Criterion
For oscillation the Barkhausen
Criterion must be equal to 1.
Aβ = 1
we can write a complex phasor Aβ(jω)
‹
which can be also written as |Aβ| θ
where |Aβ| is the loop gain magnitude and θ
is the phase shift.

9. Barkhausen criterion requires that
|Aβ|= 1
and
θ=±360⁰n where n is any integer.
So in polar and rectangular form , the
Barkhausen criterion is expressed as
Aβ(jω)=1‹±360⁰n = 1+j0

10. Harmonic Oscillator
Harmonic Oscillators generate complex
waveforms, such as square, rectangular
and saw tooth.

11. Basic Principle
It contains an energy storing
element(capacitor or an inductor) and a
nonlinear trigger circuit that periodically
charges and discharges the energy stored
in the storage element thus causing abrupt
changes in the output waveform.

12. The Wien Bridge Oscillator is so
called:
 because the circuit is based on a frequency-
selective form of the Whetstone bridge circuit.
 The Wien Bridge oscillator is a two-stage RC
coupled amplifier circuit that has good stability
at its resonant frequency, low distortion and is
very easy to tune making it a popular circuit as
an audio frequency oscillator.

18. It can be seen that at very low frequencies the phase angle between the
input and output signals is "Positive" (Phase Advanced), while at very high
frequencies the phase angle becomes "Negative" (Phase Delay). In the
middle of these two points the circuit is at its resonant frequency, (ƒr) with
the two signals being "in-phase" or 0o.

19. Wien Bridge Oscillator Summary
Then for oscillations to occur in a Wien Bridge Oscillator circuit the
following conditions must apply.
1. With no input signal the Wien Bridge Oscillator produces output
oscillations.
2. The Wien Bridge Oscillator can produce a large range of frequencies.
3. The Voltage gain of the amplifier must be at least 3.
4. The network can be used with a Non-inverting amplifier.
5. The input resistance of the amplifier must be high compared to R so
that the RC network is not overloaded and alter the required conditions.

20. The RC Oscillator
The phase angle between the input and output signals of a
network or system is called phase shift.
In a RC Oscillator the input is shifted 180o through the
amplifier stage and 180o again through a second inverting
stage giving us "180o + 180o = 360o" of phase shift which is the
same as 0o thereby giving us the required positive feedback. In
other words, the phase shift of the feedback loop should be
"0".

21. In a Resistance-Capacitance Oscillator or simply an RC
Oscillator, we make use of the fact that a phase shift
occurs between the input to a RC network and the output
from the same network by using RC elements in the
feedback branch, for example.

23. Then by connecting together three such RC networks in
series we can produce a total phase shift in the circuit of
180o at the chosen frequency and this forms the bases of a
"phase shift oscillator" otherwise known as a RC Oscillator
circuit.

24. The Quartz Crystal Oscillators
One of the most important features of any
oscillator is its frequency stability, or in other words its
ability to provide a constant frequency output under
varying load conditions. Some of the factors that affect
the frequency stability of an oscillator include:
temperature, variations in the load and changes in the
DC power supply. Frequency stability of the output
signal can be improved by the proper selection of the
components used for the resonant feedback circuit
including the amplifier but there is a limit to the stability
that can be obtained from normal LC and RC tank
circuits. To obtain a very high level of oscillator stability a
Quartz Crystal is generally used as the frequency
determining device to produce another types of oscillator
circuit known generally as a Quartz Crystal
Oscillator, (XO).

25. The quartz crystal used in a Quartz Crystal Oscillator is a very small, thin
piece or wafer of cut quartz with the two parallel surfaces metallised to make
the required electrical connections. The physical size and thickness of a piece
of quartz crystal is tightly controlled since it affects the final frequency of
oscillations and is called the crystals "characteristic frequency". Then once cut
and shaped, the crystal can not be used at any other frequency. In other
words, its size and shape determines its frequency.
The crystals characteristic or resonant frequency is inversely proportional to
its physical thickness between the two metallised surfaces. A mechanically
vibrating crystal can be represented by an equivalent electrical circuit
consisting of low resistance, large inductance and small capacitance as
shown below.

26. l

27. The equivalent circuit for the quartz crystal shows an RLC series
circuit, which represents the mechanical vibrations of the crystal, in
parallel with a capacitance, Cp which represents the electrical
connections to the crystal. Quartz crystal oscillators operate at "parallel
resonance", and the equivalent impedance of the crystal has a series
resonance where Cs resonates with inductance, L and a parallel
resonance where L resonates with the series combination of Cs and Cp
as shown.
Crystal reactance:

28. The slope of the reactance against frequency above, shows that the series
reactance at frequency ƒs is inversely proportional to Cs because below ƒs
and above ƒp the crystal appears capacitive, i.e. dX/dƒ, where X is the
reactance. Between frequencies ƒs and ƒp, the crystal appears inductive as
the two parallel capacitances cancel out. The point where the reactance
values of the capacitances and inductance cancel each other out Xc = XL is
the fundamental frequency of the crystal.

29. Relaxation oscillator
• The oscillator which shows square
wave, saw tooth wave, triangular
wave and pulse wave in his o/p instead
of sine wave is called relaxation oscillator.

31. Shockley diode relaxation oscillator
• The oscillator circuit which consists of
Shockley diode and we have sawtooth
wave in its o/p is called Shockley diode
relaxation oscillator.
• As shown in Fig A and B

32. Circuit diagram of Shockley diode
relaxation oscillator
A B

33. Circuit diagram with wave shape
A
B

34. Working of Shockley diode relaxation
oscillator
 Switch close
 Capacitor charging
 Voltage reached VBR(F)
 Diode conducting mode
 Capacitor discharging
 Holding current
 Diode off
 Capacitor charging
 Capacitor charging time A-B
 Discharging time B-C

36. Charging time
Tch=RC Loge(v/v-vs)
Frequency
F=1/Tch=1/RC Loge(v/v-vs)
R’s Max & Min values
Rmax = v-vH/IH
Rmin = V-Vs/Is

40. Group members