Oscillators
Oscillator
Output can be sinusoidal signal or a non sinusoidal signal.
Sinusoidal Oscillators
ØAn oscillator is a system consisting of active and passive circuit
elements to produce a sinusoidal or other repetitive waveforms at
the output without the application of an external input signal.
ØIt transforms the dc power from the supply source to the ac power
in the load.
ØThe function of an oscillator is opposite to that of a rectifier that
converts ac power into dc power.
Sinusoidal Oscillators
ØApplication of oscillator is in radio and TV transmitters and receivers,
and in the laboratory test equipment.
ØDepending on the type of output waveform, oscillators are classified
as sinusoidal (or harmonic) oscillators and relaxation oscillators.
ØIf the output waveform is non-sinusoidal (such as square or sawtooth
waveform), the oscillator is termed as relaxation oscillator.
Sinusoidal Oscillators
ØWhen the current – voltage characteristic of the active device in the
oscillator has a negative slope over some portion of its operation, the
oscillator is said to be of negative resistance type.
ØA tunnel diode oscillator is a negative resistance oscillator.
ØIf the oscillator contains a positive feedback amplifier with the loop
gain adjusted for an infinite overall gain, the oscillator is called a
feedback oscillator.
Oscillator
Oscillator is an amplifier with positive feedback.
Sinusoidal Oscillators
ØBoth the negative-resistance and feedback oscillators can be
sinusoidal and relaxation types.
ØFeedback-type sinusoidal oscillators can again be classed as LC
(inductor-capacitor) and RC (resistor-capacitor) oscillators.
ØOscillators can also be grouped on the basis of frequency of the
generated signals.
Oscillator types based on the generated frequencies
Basic principle of oscillators
Basic principle of oscillators
Basic principle of oscillators
Basic principle of oscillators
If the phase shift introduced by the amplifier is zero,
then the feedback voltage is in phase with the input
voltage.
After removing the input voltage, whether we will get
the sustained oscillation or not that depends upon the
loop gain (A�) .
Basic principle of oscillators
The
oscillations
will die out if
A� < 1.
Basic principle of oscillators
The
oscillations
will build up
if A� > 1.
Basic principle of oscillators
We will get
sustained
oscillations
if A� = 1.
Basic principle of oscillators
To get sustained oscillations, two conditions are satisfied.
This is known as Barkhausen’s criterion.
1. �� =1
2. Phase angle of A� is 0 or an integral multiple of 360 degree.
Oscillators
Sometimes input is given at the begining and sometimes not given.
When input is not given in the circuit, how circuit is operating without
input???
The input is given from thermal noise which is always present in the
circuit.
Thermal noise comprises of all frequency components.
Oscillators
• In Oscillators, the feedback circuit used to be frequency selective
circuits.
• It can be made up of RL, RC components or quartz crystal.
Oscillators
• Depending upon the type of feedback circuit , oscillators can be
classified as
• RC oscillator
Sinusoidal oscillators
• LC oscillator or harmonic
oscillators
• Crystal oscillator
Oscillators
• Some other oscillators produce waveform of other kind of shapes
such as square or triangular.
• These are known as relaxation oscillators (Non sinusoidal oscillators).
• These types of relaxation oscillators are designed by op-amp or timer
IC’s like 555 timer.
Basic principle of oscillators
Feedback oscillator concepts
�
The overall gain of a feedback amplifier is �� = , where A is the
1+��
gain of the internal amplifier, � is the feedback ratio, and -A � is the
loop gain. If the feedback signal aids the externally applied input signal,
it is convenient to write the overall gain as
�
�� = …………..(1)
1−��
Where A� is the loop gain. If A� = 1, equation (1) shows �� = ∞.
The amplifier then gives an output voltage without requiring any
externally applied input voltage.
Feedback oscillator concepts
In other words, the amplifier becomes an oscillator . To see the
situation more clearly , we assume that a transient disturbance gives an
output voltage �0 although no input signal is applied externally.
A portion ��0 of the output signal is fedback to the input and appears
at the output as an enhanced signal A��0 . If this signal equals �0 , i. e
A��0 = �0 or A� = 1, the spurious output voltage regenerates itself or
the amplifier oscillates. This condition of unity loop gain , i.e A� = 1, is
called the Barkhausen criterion.
Feedback oscillator concepts
The basic conditions for oscillation in a feedback amplifier are
1. The feedback must be regenerative.
2. The loop gain must be unity.
In general, A� is a complex quantity and a function of frequency . For
positive feedback, 1 − A� < 1 and the amplifier may break out into
oscillations.
Feedback oscillator concepts
Ø An amplifier is stable if a transient disturbance gives a response that
decays with time.
ØThe system is unstable if a transient disturbance persists indefinitely
or grows with time till it is limited by nonlinearities in the circuit.
ØThe condition of instability of an amplifier is given by the Nyquist
criterion.
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