3. Oscillation means that an effect that repeatedly and regularly fluctuates
about the mean value.
Oscillator is
1) An electronic circuit that generates a periodic waveform on its
output without an external signal source.
2) It converts dc noise/signal power to its ac equivalent.
3) Oscillator requires a positive feedback(regenerative feedback).
Sine wave
Square wave
Sawtooth wave
4. 1)
INPUT OUTPUT NO INPUT OUTPUT
BIAS BIAS
2) In amplifiers Negative feedback employed, while in oscillators positive
feedback employed.
Amplifier Oscillator
5. Gain of oscillators with feedback :
A(f)=
𝐀
𝟏−𝐀𝛃
Where A is forward path gain
β is feedback gain
Aβ is loop gain
For the sustained oscillations the necessary and sufficient conditions are :
1) The loop gain (Aβ) should be greater then or equal to unity (Aβ>1)
2) The net phase shift around the loop must be 2ᴫ or integer multiple of
2ᴫ.
These two condition are known as Barkhausen criterion.
6. Oscillators are classified as follows:-
1) According to output waveform
2) According to components used
3) According to frequency range
According to output waveform oscillators are classified into two ways
I. Sinusoidal Oscillators: If the generated waveform is sinusoidal or
close to sinusoidal (with a certain frequency) then the oscillator is
said to be a Sinusoidal Oscillator. Example: Hartley Oscillator,
Colpitts oscillator, Wein Bridge Oscillator etc.
II. Non Sinusoidal Oscillators ( Relaxation Oscillators): If the output
waveform is non-sinusoidal, which refers to square/saw-tooth
waveforms, the oscillator is said to be a Relaxation Oscillator.
Example: Triangular Wave Generator, Square Wave Generator etc.
7. According to Components used
I. LC Oscillators
II. RC Oscillators
According to frequency range
8. A TUNED COLLECTOR diagram as shown below:
In this oscillator 2ᴫ phase shift is achieved by transistor and transformer.
It is called tuned oscillator because the LC tuned circuit is connected at
the collector.
The oscillation frequency is given
by:
f=
1
2𝜋√𝐿𝐶
9. The Hartley oscillator is an electronic oscillator circuit in which the
oscillation frequency is determined by a tank circuit consisting
of capacitors and inductors, that is, an LC oscillator.
In Hartley oscillator 2𝜋 phase shift can be achieved by transistor which
gives 180° phase shift while remaining 180° phase shift is achieved by
tapped inductors.
The frequency of oscillation is given
by:
f˳=
1
2𝜋 𝐿(𝑒𝑞)𝐶
where L(eq) =L1+L2+2M
If mutual inductance(M) is neglected
then L(eq) = L1+L2
10. The Colpitts oscillator is a type of oscillator that uses an LC circuit in
the feedback loop.
The feedback network is made up of a pair of tapped capacitors (C1 and
C2) and an inductor L to produce a feedback necessary for oscillations.
In Colpitts oscillator 2𝜋 phase shift can be achieved by transistor which
gives 180° phase shift while remaining 180° phase shift is achieved by
tapped capacitors.
The frequency of oscillation is given
by:
f˳=
𝟏
𝟐𝝅√𝑪(𝒆𝒒)𝑳
where C(eq)=
𝐶1×𝐶2
𝐶1+𝐶2
11. The L-C oscillators are not employed at low frequencies because at low
frequencies the inductors become large and bulky, hence R-C oscillators
are used.
The feedback network is made up of a R-C section to produce a
feedback necessary for oscillations.
In R-C phase shift oscillator 2𝜋 phase shift can be achieved by transistor
which gives 180° phase shift while remaining 180° phase shift is
achieved by a R-C phase shift network consist three R-C section.
The frequency of oscillation is given
by:
f˳=
1
2𝜋𝑅𝐶√6+4(
𝑅(𝐿)
𝑅)
where R(L)= Load or output resistance
12. Wein Bridge oscillator consists a balanced wein bridge for the feedback
network.
It also consists two stage amplifier because feedback network provide 0°
phase shift.
In Wein Bridge oscillator 2ᴫ phase shift is achieved by two stage
amplifier.
The frequency of oscillation is
given by:
f˳=
1
2𝜋√𝑅1𝑅2𝐶1𝐶2
If R1=R2=R and C1=C2=C then
f˳=
1
2𝜋√𝑅𝐶
13. Certain crystals like Quartz, Rochelle salt etc. are used for the
construction of crystal oscillator.
Its operation is based on the piezoelectric effect i.e. when an a.c. voltage
is applied to such crystal its mechanically vibrated ( compressed or
stretched) and vice versa.
14. Electrical Equivalent of Crystal Oscillator:
From the fig. it is clear that there
is two resonance
a) Series Resonance
b) Parallel Resonance
a) Series Resonance:
Occurs due to series capacitor and
series inductor.
The series resonance frequency is given as
f˳=
1
2𝜋√𝐿𝐶
15. b) Parallel Resonance:
Occurs due to series branch(L1-C1-R1) and parallel capacitor (C0).
The parallel resonance frequency is given by:
fp=
1
2𝜋√
1
𝐿
1
𝑐
+
1
𝐶𝑜
The parallel resonance frequency is always greater than series resonane
frequency.
16. The Crystal oscillator with transistor with impedance curve is shown as:
The crystal oscillator gives maximum feedback voltage at parallel
resonance frequency because at this frequency its impedance is
maximum.
17. 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.
Oscillator circuits are used in computer peripherals, counters,
timers, calculators, phase-locked loops, digital multimetres,
oscilloscopes, and numerous other applications.
