Unit– IV

Oscillators

Index

4.1 Types of feedback(Positive and Negative)

4.2 Principle of oscillation.

4.3 Oscillators: Hartley and Colpitts

4.1 Types of feedback(Positive and Negative)

Feedback in amplifier

In amplifier circuit, when a small signal is taken from

the output and given to the input. It is called feedback in

amplifier.

• Input voltage Vi is given to an amplifier having gain A.

Output voltage is Vo.

• Feedback V f = β Vo is given to the input.

• Β is the feedback factor. Value of the β is less than 1.

• Due to feedback , input to the amplifier changes from Vi

to Vi’

• If the feedback is in phase opposition to the input signal

it is called degenerative feedback or negative feedback.

Vi’ = Vi – V f

• If the feedback is in phase with the input signal it is

called positive feedback.

Vi’ = Vi + V f

Oscillation are produced without any external signal

source. The only input power to an oscillator is the

d.c.power supply.In fact oscillator is the amplifier with

positive feedback in which no input signal is given.

Oscillator

4.2 Principle of oscillation.

An electronic device that

generates sinusoidal oscillation of

desired frequency is known as

oscillator.

Some possible output waveforms

Oscillator

Oscillators convert dc to ac.

ac outdc in

Damped oscillation in LC tuned circuit:

• A tank or oscillatory circuit is

a parallel form of inductor and

capacitor elements which

produces the electrical

oscillations of any desired

frequency.

• Both these elements are

capable of storing energy.

Whenever the potential

difference exists across a

capacitor plates, it stores

energy in its electric field.

• Similarly, whenever current flows thorough an inductor,

energy is stored in its magnetic field.

• The below figure shows a tank circuit in which inductor

L and capacitor C are connected in parallel.

• Consider that the capacitor is initially charged with a

DC source having the polarities upper plate positive and

lower plate negative as shown below.

• This represents that the upper plate has of electrons

deficiency , whereas the lower plate has excess of

electrons. Therefore, potential differences exist between

these two plates.

• Consider that this charged capacitor is connected across

the inductor through a switch S as shown in figure.

• When the switch S is closed, the conventional current

flow or electrons moves from plate A to B through the

inductor coil. Therefore the energy stored or strength of

the electric field in the capacitor decreases.

• The current flowing through the inductor induces an EMF

which opposes the electrons flow through it.

• This current flow set up a magnetic field around the

inductor thereby it starts storing the magnetic energy. When

the capacitor is fully discharged, current or electron flow

through the coil becomes zero. At this time magnetic field

has maximum value and there is no electric field.

• Once the capacitor is fully discharged, magnetic field

around the inductor starts collapsing and produces the

counter emf.

• As per the Lenz’s law, this counter emf

produces the current which begin to charge the capacitor

with opposite polarity by making plate upper plate

negative and lower plate positive as shown in figure

below.

• When the capacitor is fully charged in opposite direction,

the entire magnetic energy is converted back into the

electric energy in capacitor, i.e., magnetic energy is

collapsed.

• At this instant, capacitor starts discharging in the opposite

direction as shown in figure. Once again the capacitor is

fully discharged and this process will be continued.

• This continuous charging and discharging process results

an alternating motion of electrons which is nothing but an

oscillating current.

• But these oscillations of the capacitor are damped

because every time transferring of energy from L to C

and C to L dissipates energy in the form of heat in the

resistance of the coil and in the connecting wires in the

form of electromagnetic radiation.

• These losses decrease the amplitude of oscillating

current gradually till it ceases. These are called as

exponentially decaying oscillations or damped

oscillations.

Sustained(undamped) oscillation:

• The oscillation produced are

damped oscillation because at every

oscillation there is waste of energy

in capacitor and inductor.

• If energy equal to the loss is

supplied at every oscillation there is

no decrease in the voltage and

sustained oscillation are produced.

Amplifier with positive feedback as oscillator:

“the process of injecting a fraction of output energy of

some device back to the input is known as feedback”.

There are two types:

1) Positive feedback

2) Negative feedback

Amplifier with positive feedback as oscillator:

• An amplifier with gain A is shown in figure(a).

• Positive feedback is given through β network taking signal

from the output.

Total gain is given by

A f = A / 1-Aβ

Aβ is called the loop gain.

• There are three possibilities.

Amplifier with positive feedback as oscillator:

1) Aβ<1

If the value of loop gain Aβ is less than i.e. Aβ<1, the output

voltage will be decreasing with the passage of time.(fig.(b)).

2) Aβ>1

If Aβ>1, the voltage will be increasing with time.(fig(c))

3) Aβ=1

If Aβ=1, A f = 1/0 = ∞ means there is output even if there is no

input. Value of the output voltage remains constant and sustained

oscillations are produced. Thus amplifier works as oscillator.

This condition is called “ Barkhausen criterian ”

Requirement of oscillator:

• Active device: It works as amplifier. for this transistor

or FET are used.

• power supply: Power supply is necessary for biasing the

active device and to compensate for energy

loss.

• Frequency determining network: It determines the

frequency of oscillation. In LC oscillator

frequency of oscillation depends upon the

tuned circuit.

• Positive feedback: Positive type feedback is essential.

Hartley oscillator

• The Hartley oscillator is designed for generation of

sinusoidal oscillations in the R.F range (20 KHz - 30 MHz).

• It is very popular and used in radio receivers as a local

oscillator.

• The circuit diagram of Hartley oscillator (parallel or shunt-

fed) using BJT is shown in Figure.

• It consists of an R-C coupled amplifier using an n-p-n

transistor in CE configuration.

• R1 and R2 are two resistors which form a voltage divider

bias to the transistor.

• A resistor RE is connected in the circuit which stabilizes the

circuit against temperature variations.

• A capacitor CE is connected in parallel with RE, acts as a

bypass capacitor and provides a low reactive path to the

amplified ac signal.

• The coupling capacitor CC blocks dc and provides an ac

path from the collector to the tank circuit.

• The L-C feedback network (tank circuit) consists of two

inductors L1, and L2 (in series) which are placed across a

common capacitor C1 and the centre of the two inductors

is tapped as shown in fig.

• The feedback network (L1, L2 and C1) determines the

frequency of oscillation of the oscillator.

Colpitt oscillator

• Colpitts oscillator is generally used in RF applications

and the typical operating range is 20KHz to 300MHz.

• In Colpitts oscillator, the capacitive voltage divider setup

in the tank circuit works as the feed back source and this

arrangement gives better frequency stability when

compared to the Hartley oscillator which uses an

inductive voltage divider setup for feedback.

• The circuit diagram of a typical Colpitts oscillator using

transistor is shown in the figure as shown.

• In the circuit diagram resistors R1 and R2 gives a

voltage divider biasing to the transistor.

• Resistor R4 limits the collector current of the transistor.

Cin is the input DC decoupling capacitor while Cout is the

output decoupling capacitor.

• Re is the emitter resistor and its meant for thermal

stability. Ce is the emitter by-pass capacitor.

• Job of the emitter by-pass capacitor is to by-pass the

amplified AC signals from dropping across Re.

• The the emitter by-pass capacitor is not there, the

amplified AC signal will drop across Re and it will alter

the DC biasing conditions of the transistor and the result

will be reduced gain.

• Capacitors C1, C2 and inductor L1 forms the tank circuit.

Feedback to the base of transistor is taken from the

junction of Capacitor C2 and inductor L1 in the tank

circuit.

Uses of LC oscillator:

• Radio Frequency generator

• As carrier in radio and T.V. Transmitters and receivers

• Radio telephony

• Radio telegraphy

• Carrier telephony : Several signal are transmitted over asingle pair of wire

• Radio & TV receiver

• Induction heating: to harden the surface of the shaft,gears

etc.

• Di electric heating: heating of insulating material.

• Remote control

• Clock in digital signal

• Generation of ultrasonic wave

Examples (based on above two oscillator):

For Hartley oscillator c=250 pF , l1=1.5mH , l2=1.5mH

.determine the operating frequency.

For Hartley oscillator c=20 pF , l1=1000uH , l2=100uH

.determine the operating frequency.

For colpitt oscillator c1=750 pF , c2=2500pF , l=40uH

.determine the operating frequency.

For colpitt oscillator c1=0.1uF , c2=1uF , l=470uH

.determine the operating frequency.

Any Question

?

Thank You