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Oscillators 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) ii)

Audio frequency (AF) oscillator-20Hz to 20KHz 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.

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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 đ?’ąin is the input to the amplifier. â‹ 1 is the initial signal. An amplifier with gain ‘A’ and feedback factor â€˜Î˛â€™

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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

f 

1 2 LC

For L-C Oscillator and f 

1 for RC Phase Shift Oscillator 2 RC 6

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.

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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.

f 

1 ………….Formula 2 RC 6

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.

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Disadvantages i) ii)

The frequency stability is excellent. i.e. frequency remains constant.

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 F

1 1  3 6 2 RC 6 6.28 10 10  0.110  2.44 1  103 6.28  2.44  0.0652 103  65.5Hz.

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

R1,C1, R2,C2 R3 and R4 is known as Wein’s bridge. Fig. shows its

simplified block diagram and the fig. its actual circuit diagram.

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In two stage amplifier the output voltage of second stage amplifier is given to the feedback circuit and the voltage across parallel combination of R2, and C2 is given to input of the first stage amplifier.

It gives in phase signal as a + ve feedback and the voltage across R4 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 C1 and C2 simultaneously by means of a gang capacitor can vary the frequency of

oscillation.

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Formula

f 

1 2 R1 R2 C1 C2

If R1  R2 and C1  C2 then f 

1 2 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

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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.

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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 splittank 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.

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Other component functions for both Hartley and Colpitt’s are summarized below in the table.

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 C1and R2

To prevent, AC RF towards DC supply from tank circuit Coupling capacitors to block DC

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.

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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=20 μf , CB=5 μf , and L=0.1mH. Find the frequency of oscillator.

Solution C f 

C ACB 20  5 100    4 f  4 106 f , l  0.1mH  0.1103 H C A  CB 25 25 1

2 LC



1 6.28 0.1103  4 106



1 6.28 104  4 106

Formula: 1 6.28 4 10

10



1 1  105 5 6.28  2 10 12.56

 0.07961105  7.961KHz

Uses of oscillator 1)

To test amplifiers or to use it as an input signal.

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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 L-C Oscillator

R-C Oscillator

1) L-C Tank circuit essential 2) Positive feedback is obtained by using tap.

3) Frequency stability is poor.

4) Suitable for high frequency (RF) generation.

5) E.g. Hartley, Colpitt Oscillator.

1) Tank circuit is not required.

2) R-C sections provide positive feedback.

3) Frequency stability is better.

4) Suitable for low frequency (AF) generation.

5) E.g. R-C phase shift, Wein bridge etc.

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.

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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:

f  f 

1 2 LC 1

2  3.14 0.110 10

12



1 6.28 106

f  0.159 106 Hz  159 103 Hz

f  159KHz 2. In a Hartley oscillator frequency of 250 KHz is required. If C = 470 pf find the required inductor L. Solution:

f 

1 2

f2

 L1  L2  C

, Where  L1  L2  L 

1 4 2 LC

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L  L

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1 4 2 LC 1

4  3.14    250 103   470 1012 2

2

 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:

f  

1 2 RC 6

1 1  106 3 6.28  4  2.45 6.28  4 10 110  6 3

 0.016 106 farad  0.016 f  16nf 4. In a Wein bridge oscillator if R1  R2  200K ohm and R1  R2  200K C1  C2  0.2nf .

Find the frequency of oscillation.

Solution:-

f 

1 1 1   3 9 2 RC 2  3.14  200 10  0.2 10 6.28  200  0.2 106  0.00398 106  3.98KHz

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