Electrical Engineering

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Oscillators 122
CLASSIFICATION OF OSCILLATORS
An oscillator in general may be defined as an electronic circuit capable of generating
repetitive output waveform by using DC supply and without any input AC signal
hence it is known as DC to AC converter. Practically it draws DC energy and
converts it into bi-directional AC.
Oscillator may be classified in different ways as follows:
A) According to wave form
i) Sinusoidal ii) Non sinusoidal
B) According to design principle
i) Positive feedback
(e.g. LC oscillators, RC oscillators) (Sawtooth oscillator using UJT)
C) According to frequency range
i) Audio frequency (AF) oscillator-20Hz to 20KHz
ii) Radio frequency (RF) Oscillator-20KHz to 30MHz
iii) Video frequency Oscillator-DC to 5MHz
iv) High frequency Oscillator-1.5 MHz to 30MHz
v) Very high frequency (VHF) Oscillator-30MHz TO 300MHz
D) According to component used
i) L-C Oscillator 2)R-C Oscillator 3)Crystal Oscillator
BASIC REQUIREMENTS OF AN OSCILLATOR
An oscillator circuit normally consists of:
i) An amplifier ii) Positive feedback
iii) Amplitude limiting device iv) Frequency control circuit
Oscillator should have these four sections to get sustained oscillations.
Oscillators

Oscillators 123
1) An Amplifier
The block diagram of basic oscillator fig. shows that an amplifier provides a) a
feedback to its input b) the desired AC output. The transistor or vacuum tube is
used as an amplifier; a portion of output signal is fed to its input in proper phase.
An amplifier again amplifies this feedback signal. Each time the feedback signal is
amplified until maximum or saturation value of output current has been reached
as shown in fig.
2) Positive Feedback
When the signal from the output circuit is applied to input it is known as feedback.
If the feedback signal is in phase with input then it is called as positive feedback
and if it is180° out of phase then known as negative feedback.Positive feedback
plays an important role in oscillator circuit. The amount of feedback is very
important, it affect the amplitude of oscillation. Fig illustrates this action. Let 𝒱in is
the input to the amplifier.
⋁1 is the initial signal. An amplifier with gain ‘A’ and feedback factor ‘β’

Oscillators 124
3) Amplitude limit device
It is done by both feedback and amplifier. ‘Barkhausen’ Criterion of oscillation’
gives the condition for sustained oscillation. Fig shown the effect of A×β=Aβ on
amplitude of oscillation.
As shown in fig.
i) If Aβ<1 it product damped oscillations.
ii) If Aβ>1 it product growing oscillations.
iii) If Aβ=1 it product sustained oscillations.
4) Frequency control circuit
Frequency of an oscillator is controlled by L-C tuned circuit in LC oscillator and
by R-C component in RC oscillator because
1
2
f
LC
 For L-C Oscillator and
1
2 6
f
RC
 for RC Phase Shift Oscillator
R-C PHASE-SHIFT OSCILLATOR
An RC network can be used in the circuit to produce sine wave oscillations.
There are two basic oscillator in RC types
1) RC phase shift and
2) Wein-bridge oscillator.
An RC oscillator is most popular in generation of low frequencies, since it does not
require inductor or tank circuit. RC oscillator can be fabricated in IC form.

Oscillators 125
 BASIC PRINCIPAL
The circuit of RC phase shift oscillator is shown in fig. It is a common emitter
configuration with a three stage RC network. It is used to provide the required 180°
phase shift in addition to the phase shift of transistor or 180° Thus 360° or 0°
phase shift is produced for desired positive feedback. Since, each RC network
with proper selection of RC component gives 60° phase shift.
Circuit Action
The circuit is set in to oscillation by any random variation of the base current
suddenly jumps from zero to initial bias value. This change is amplified by the
transistor and fed back again to base through RC phase shifting network.
1
2 6
f
RC
 ………….Formula
Advantages
i) Simplest types of RC oscillator than other RC types.
ii) Very low frequency from 1 Hz is possible due to large value of R but maximum
frequency limited up to 300 KHz.
iii) Easy manufacturing in integrated circuit form.

Oscillators 126
Disadvantages
i) The frequency stability is excellent. i.e. frequency remains constant.
ii) Frequency variation is not so easy like LC oscillator since three identical RC
network components.
iii) It cannot be used to generate high frequency or RF.
Examples
A R-C phase-shift oscillator has R=10KΩ and C=0.1µf. Find the frequency of oscillator.
Solution
3 6
3
3
1 1
6.28 10 10 0.1 10 2.442 6
1
10
6.28 2.44
0.0652 10
65.5 .
F
RC
Hz
 
 
    
 

 

WEIN-BRIDGE OSCILLATOR
This is in an important R-C oscillator capable of generating low frequency it has a
wide range of frequency variation 5Hz to 1MHZ but normally used for Audio range.
It is very easy to vary the frequency of oscillation than R-C phase shift oscillator.
Construction
The Wein bridge oscillator has two- stage amplifier and a lead-lag R-C network.
It used both positive feedback and negative feedback.
The bridge formed by 1, 1, 2, 2 3 4R C R C R and R is known as Wein’s bridge. Fig. shows its
simplified block diagram and the fig. its actual circuit diagram.

Oscillators 127
In two stage amplifier the output voltage of second stage amplifier is given to the
feedback circuit and the voltage across parallel combination of 2, 2R and C is given to
input of the first stage amplifier.
It gives in phase signal as a + ve feedback and the voltage across 4R is the negative
feedback for stability of the amplifier.
Working
It initiates its signal from noise and amplifier (Aβ>1) till it reached to its desire level
and then it makes Aβ=1 to get sustained oscillation. Varying the two capacitors
1 2andC C simultaneously by means of a gang capacitor can vary the frequency of
oscillation.

Oscillators 128
Formula
1 2 1 2
1
2
f
R R C C

1 2 1 2If and then
1
2
R R C C
f
RC
 

L-C OSCILLATORS
A common method to generate Radio frequency (RF) and sine wave output is by L-C
Oscillator. In L-C oscillator the heart of the circuit is L-C tuned circuit, common
named is L-C tank circuit. The L-C tank circuit is a parallel resonant network; an
inductor is in parallel with capacitor C.
The tank circuit combined with an amplifier and positive feedback form an L-C
Oscillator, which is a most popular circuit in communication circuits like radio, TV
receiver for generation of high frequency only. It is difficult to get low frequency with
this type due to limitation of class of oscillator. A number of Oscillator circuit are
possible like Hartley, Colpitt’s, Clapp etc. Here we will study Hartley and Colpitt’s
oscillator. Before that let us see the working of tank circuit as an oscillatory device.
Action of tank circuit
In L-C tank circuit oscillations are developed by repeated exchange of electrical energy
(electrons) between the capacitor and the inductor. The action of the circuit is
explained with the help of fig. Here inductor and capacitor are equivalent to tanks of
electrical energy.
While reading this explanation; observe the fig. carefully, the direction of current,
magnetic field around the inductor, total charge on the capacitor plates, and
magnitude of the current.
First half cycle
It is assumed that in fig. capacitor is already charged with polarities as shown. Now
capacitor starts to discharge through inductor fig. During this process the movement

Oscillators 129
of electrons is from the lower plate to the upper plate of capacitor, a magnetic field
starts expanding, the inductor has a property to oppose sudden increase in current
slowly increase.
Second half cycle
Now the magnetic field around the inductor has completely collapsed, the energy that
has been stored in inductor is transferred to the electrostatic field across the
capacitor. Capacitor will now start discharging, in the opposite direction, See fig. and
waveform from (c to h) the process is quite similar and finally (Fig.) thus capacitor
charges in the opposite polarity and we get a negative half cycle.

Oscillators 130
HARTLEY OSCILLATOR
This type is a most common L-C oscillator and the frequency range is beyond 100 KHz
up to few MHz fig. as shows the circuit of transistorised Heatley oscillator.
Circuit Description
Transistor is connected in common emitter configuration because of its
advantages.
a) Moderate input and output impedance.
b) Voltage, current and power gain is greater than one, phase inversion between input
and output occurs in this configuration.
Tapped coil divides the coil L1 into L1A and L1B, this type of coil is called as split-
tank inductor or tapped inductor. Split inductor serves three important functions.
1) It provides positive feedback to the active device. Observe the polarities shown in
the circuit diagram. Suppose polarity of base is (-ve), it is inverted at collector by
1800 i.e. (+ve), due to tapped inductor. Thus, the lower terminal of L1B is (-ve). This
is an phase with input (base) thus positive feedback. This feedback is fed across
base and emitter, (since tap and emitter are at ground) it is known as shunt
feedback oscillator.
2) Together with CT forms resonant tank circuit.
3) It determines the oscillator frequency.

Oscillators 131
Other component functions for both Hartley and Colpitt’s are summarized below in
the table.
COLPITT’S OSCILLATOR
The Colpitt’s oscillator is quite similar to Hartley oscillator circuit it is also a shunt fed
type; it only differs in the feedback technique, which is obtained from split capacitor
(two capacitors connected in series CA and CB instead of split inductor).
Fig. illustrate the transistorised Colpitt’s oscillator using a common- emitter
configuration, voltage across CB is taken as feedback. It is a positive feedback; observe
the polarities. All the circuit serve similar function as given in the table of Hartley
oscillator.
Component Function
R1 and R2 Voltage divider to provide base bias
R3 Swamping resistor for stability
CE To bypass AC across R3.
R4 Collector load for amplification action.
RFC (RF choke)
L3
To prevent, AC RF towards DC supply from tank circuit
C1and R2 Coupling capacitors to block DC

Oscillators 132
Advantages
The Colpitt’s oscillator has good frequency stability and hence it is useful in many
applications. This has another advantage over Hartley that tank coil does not require
any physical tap so that two connections are required for the coil.
 Example
In a Colpitt’s oscillator the tank circuit consists of CA=20μf , CB=5μf , and L=0.1mH.
Find the frequency of oscillator.
Solution
6 320 5 100
4 4 10 , 0.1 0.1 10
25 25
A B
A B
C C
C f f l mH H
C C
  
        

3 6 4 6
1 1 1
2 6.28 0.1 10 4 10 6.28 10 4 10
f
LC    
  
    
Formula:
5
510
1 1 1
10
6.28 2 10 12.566.28 4 10

  
 
5
0.07961 10 
7.961KHz
Uses of oscillator
1) To test amplifiers or to use it as an input signal.

Oscillators 133
2) In radio receivers to generate sine wave as local oscillator frequency.
3) Supply for R.F. heating
4) Sawtooth signal as time base single for CRO
5) For transmitters; to generate carrier frequency.
Comparison
SAWTOOTH OSCILLATOR
It is a negative resistance oscillator in which UJT is used as an electronic self operated
switch (refer chapter transistor and topic Uni- junction transistor). This oscillator
generates non- sinusoidal oscillations. Its waveform is just like the shape of hacksaw
blade hence name Sawtooth. A R-C circuit is used to generate sawtooth by means of
charging and discharging of capacitor it is controlled by UJT.
L-C Oscillator R-C Oscillator
1) L-C Tank circuit essential 1) Tank circuit is not required.
2) Positive feedback is obtained
by using tap.
2) R-C sections provide positive
feedback.
3) Frequency stability is poor. 3) Frequency stability is better.
4) Suitable for high frequency
(RF) generation.
4) Suitable for low frequency (AF)
generation.
5) E.g. Hartley, Colpitt Oscillator. 5) E.g. R-C phase shift, Wein bridge
etc.

Oscillators 134
Working
Initially UJT is in cut off because its emitter voltage is less than the voltage across its
internal resistance RB; its internal P-N diode is in reverse bias. UJT allows capacitor to
charge through resistance till it voltage teaches to peak emitter voltage.
When it reaches to peak-emitter voltage its internal P-N diode becomes forward biased
and hence the capacitor gets path to discharge through UJT. At this voltage UJT gets
fired its internal resistance RB1 drops to a very low value due to its negative resistance
characteristics. It allows capacitor to charge up to minimum voltage. When it reaches
to its minimum value internal P-N diode becomes reverse biased and again UJT allows
capacitor to charge. In this way UJT generates sawtooth is normally used as a time
base generate in CRO to deflect electron Beam from left to right repetitively.
SOLVED PROBLEMS
1. In A Simple IC oscillator circuit, inductance =0.1 Henry and capacitance =10pf.
Find frequency of oscillation.
Solution:
1
2
f
LC

612
1 1
6.28 102 3.14 0.1 10 10
f 
 
  
6 3
0.159 10 159 10f Hz Hz   
159f KHz
2. In a Hartley oscillator frequency of 250 KHz is required. If C = 470 pf find the
required inductor L.
Solution:
 1 2
1
2
f
L L C


, Where 1 2L L L 
2
2
1
4
f
LC


Oscillators 135
2
1
4
L
LC
 
   
22 3 12
1
4 3.14 250 10 470 10
L


   
863.2 H
3. In a RC phase shift oscillator if R=4K Ohm it. It generates a sine wave of 1 KHz find
the capacitance.
Solution:
1
2 6
f
RC

6
3 3
1 1
10
6.28 4 2.456.28 4 10 1 10 6

  
     
6
0.016 10 0.016 16farad f nf
   
4. In a Wein bridge oscillator if 1 2 200R R K  ohm and 1 2 200R R K  1 2 0.2C C nf  .
Find the frequency of oscillation.
Solution:-
3 9 6
1 1 1
2 2 3.14 200 10 0.2 10 6.28 200 0.2 10
f
RC  
  
       
6
0.00398 10 3.98KHz  

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