The document discusses oscillators and their working principles. It begins by classifying oscillators and analyzing their circuits. It describes the conditions for oscillation using the Barkhausen criteria. It then examines tuned oscillators, crystal oscillators, and other oscillator types. Applications of oscillators in communication circuits, timers, and other devices are also overviewed.

1. SUBMITTED BY :-
POOJA VISHWAKARMA
SUBMITTED TO :-
Mr. GYAN RAO DHOTE
PHYSICS

2. Oscillator

3.  12-1 Introduction
 12-2 Classifications of Oscillators
 12-3 Circuit Analysis of a General Oscillator
 12-4 Conditions for Oscillation: Barkhausen- Criteria
 12-5 Tuned Oscillator
 12-6 Crystal Oscillator
 12-7 Applications of Oscillators

4.  In this chapter we will explore the working principle of the
oscillator. Generally speaking, the oscillator produces sinusoidal and
other waveforms.
 Beginning with a detailed circuit analysis of the oscillator, we will
proceed to discuss the conditions and frequency of oscillation.
 Following this, the different types of oscillators—Tuned
oscillator, Hartley oscillator, Colpitts oscillator, Clapp
oscillator, Phase-shift oscillator, Crystal oscillator and
Wien-bridge oscillator—will be examined with detailed
mathematical analysis and illustrations.
 The chapter ends with an overview of the applications of the
oscillator.

5.  An oscillator is an electronic system.
 It comprises active and passive circuit elements and
sinusoidal produces repetitive waveforms at the output without the
application of a direct external input signal to the circuit.
 It converts the dc power from the source to ac power in the
load. A rectifier circuit converts ac to dc power, but an oscillator converts dc
noise signal/power to its ac equivalent.
 The general form of a harmonic oscillator is an electronic
amplifier with the output attached to a narrow-band electronic filter, and the
output of the filter attached to the input of the amplifier.
 In this chapter, the oscillator analysis is done in two methods
—first by a general analysis, considering all other circuits are the special
form of a common generalized circuit and second, using the individual
circuit KVL analysis.

7.  Oscillators are classified based on the type of the output
waveform.
 If the generated waveform is sinusoidal or close to sinusoidal
(with a certain frequency) then the oscillator is said to be a Sinusoidal
Oscillator.
If the output waveform is non-sinusoidal, which refers to
square/saw-tooth waveforms, the oscillator is said to be a
Relaxation Oscillator.
 An oscillator has a positive feedback with the loop gain
infinite. Feedback-type sinusoidal oscillators can be classified as LC
(inductor-capacitor) and RC (resistor-capacitor) oscillators.

8.  The classification of various oscillators is shown in Table 12-1.

9.  This section discusses the general oscillator circuit with a simple
generalized analysis using the transistor, as shown in Fig. 12-2.
 An impedance z1 is connected between the base B and the emitter E, an
impedance z2 is connected between the collector C and emitter E. To apply a
positive feedback z3 is connected between
 the collector and the base terminal.
 All the other different oscillators can be analyzed as a special case of the
generalized analysis of oscillator.

10.  The above generalized circuit of an oscillator is considered using a simple transistor-equivalent circuit
model. The current voltage expressions are expressed as follows:

31.  Advantages of Wien-Bridge Oscillator:
 1. The frequency of oscillation can be easily varied just by changing RC
network
 2. High gain due to two-stage amplifier
 3. Stability is high
 Disadvantages of Wien-Bridge Oscillator
 The main disadvantage of the Wien-bridge oscillator is that a high
frequency of oscillation cannot be generated.

33.  Nyquist criterion states that if this closed curve passes through or
encloses the point (1 + j0), the amplifier becomes unstable and oscillates.
 It is important to note that a positive feedback amplifier will not oscillate
unless the Nyquist criterion is satisfied.
 In the steady state condition the loop gain becomes unity and the
oscillations are sustained, the frequency of oscillations is controlled by the
frequency-determining network of the oscillator.
 The RC and a LC combination circuits are used in oscillators to serve
as the frequency-determining network.
 Let us summarize the key necessities of a feedback oscillator.
1. Amplifier with positive feedback produces a negative resistance in the
system.
2. A frequency-determining network creates oscillations at certain required
frequencies.
3. System non-linearity introduced by the devices contain the amplitude of
oscillation.

36.  The circuit diagram of a tuned oscillator is shown in Fig. 12-10(a). The emitter by pass
capacitor CE shunts the ac so that RE is omitted from the ac equivalent circuit of Fig. 12-10(b).
 The dc operating point of the transistor is determined by the resistances R1, R2 and
RE, and supply voltage. The transistor gives a phase-shift of 1800
.
Circuit Analysis of Tuned Oscillator:

39.  Crystal oscillator is most commonly used oscillator with high-frequency stability. They are
used for laboratory experiments, communication circuits and biomedical instruments. They are usually, fixed
frequency oscillators where stability and accuracy are the primary considerations.
 In order to design a stable and accurate LC oscillator for the upper HF and higher frequencies
it is absolutely necessary to have a crystal control; hence, the reason for crystal oscillators.
 Crystal oscillators are oscillators where the primary frequency determining element is a quartz
crystal. Because of the inherent characteristics of the quartz crystal the crystal oscillator may be held to
extreme accuracy of frequency stability. Temperature
 compensation may be applied to crystal oscillators to improve thermal stability of the crystal oscillator.
 The crystal size and cut determine the values of L, C, R and C'. The resistance R is the friction
of the vibrating crystal, capacitance C is the compliance, and inductance L is the equivalent mass. The
capacitance C' is the electrostatic capacitance between the mounted pair of electrodes with the crystal as
the dielectric.

43.  Oscillators are a common element of almost all electronic circuits. They are used
in various applications, and their use makes it possible for circuits and subsystems to
perform numerous useful functions.
 In oscillator circuits, oscillation usually builds up from zero when power is first
applied under linear circuit operation.
 The oscillator’s amplitude is kept from building up by limiting the amplifier
saturation and various non-linear effects.
 Oscillator design and simulation is a complicated process. It is also extremely
important and crucial to design a good and stable oscillator.
 Oscillators are commonly used in communication circuits. All the
communication circuits for different modulation techniques—AM, FM, PM—the use of an
oscillator is must.
 Oscillators are used as stable frequency sources in a variety of electronic
applications.
 Oscillator circuits are used in computer peripherals, counters, timers, calculators,
phase-locked loops, digital multi-metres, oscilloscopes, and numerous other applications.

44.  A common oscillator implementation is the voltage-controlled
oscillator (VCO) circuit, where an input tuning voltage is applied to an oscillator circuit
and the tuning voltage adjusted to set the frequency at which the circuit oscillates.
 The VCO is the most widely used oscillator circuit and it produces an
oscillatory output voltage.
 It provides a periodic signal, where the frequency of the periodic signal is
related to the level of an input voltage control signal supplied to the VCO.
 A VCO is simply an oscillator having a frequency output that is proportional
to an applied voltage.
 The centre frequency of a VCO is the frequency of the periodic output
signal formed by the VCO when the input
control voltage is set to a nominal level.

45.  The cascode crystal oscillator is composed of a Colpitts crystal
oscillator and a base-common buffer amplifier in mobile circuits.
 In the cascode crystal oscillator, a temparature-independent voltage
source biases the buffer amplifier and the bypass capaciter gets eliminated.
 GSM phones, set-top boxes and digital audio broadcasting
equipments use oscillators. and digital audio roadcasting equipment use
oscillators.

46.  1. Oscillator converts dc to ac.
 2. Oscillator has no input signal.
 3. Oscillator behaviour is opposite to that of a rectifier.
 4. The conditions and frequencies of oscillation are classified as: