2. Need of an Oscillator
• An oscillator circuit is capable of producing ac
voltage of desired frequency and waveshape.
• To test performance of electronic circuits, it is called
signal generator.
• It can produce square, pulse, triangular, or sawtooth
waveshape.
• High frequency oscillator are used in broadcasting.
• Microwave oven uses an oscillator.
• Used for induction heating and dielectric heating.
3. Types of Oscillators
• Sinusoidal or non-sinusoidal.
• An oscillator generating square wave or a pulse
train is called multivibrator :
1. Bistable multivibrator (Flip-Flop Circuit).
2. Monostable multivibrator.
3. Astable multivibrator (Free-running).
• Depending upon type of feedback, we have
1. Tuned Circuit (LC) oscillators.
2. RC oscillators, and
3. Crystal oscillators.
4. Using Positive Feedback
• The gain with positive feedback is
given as
• By making 1 – Aβ = 0, or Aβ = 1, we get
gain as infinity.
• This condition (Aβ = 1) is known as
Barkhausen Criterion of oscillations.
• It means you get output without any
input !
5. How is it Possible ?
Connecting point x to y, feedback voltage
drives the amplifier.
6. • What happens to the output ?
• There are three possibilities.
7. (1) If Aβ < 1, we get decaying of damped
oscillations.
9. (3) If Aβ = 1, we get sustained oscillations.
In this case, the circuit supplies its own
input signal.
10. Wherefrom comes the starting voltage ?
• Each resistor is a noise generator.
• The feedback network is a resonant circuit giving
maximum feedback voltage at frequency f0,
providing phase shift of 0° only at this frequency.
• The initial loop gain Aβ > 1.
• The oscillations build up only at this frequency.
• After the desired output is reached, Aβ reduces
to unity.
11. Tank Circuit
• LC parallel circuit is called tank circuit.
• Once excited, it oscillates at
13. The energy keeps oscillating between
electric potential energy and magnetic
filed energy.
21. Hartley Oscillator
• Note that in the collector-tuned circuit, two
inductor coils are used.
• One end of these coils is grounded.
• If we make the tickler coil an integral part of
the circuit, we get Hartley Oscillator.
23. • When the tank circuit resonates, the
circulating current flows through L1 in
series with L2. Hence the equivalent
inductance is
The feedback factor is
24. Colpitts Oscillator
• An excellent circuit.
• Widely used in commercial signal generators.
• Uses two capacitors instead of the inductive
voltage divider.
33. • A phase-lead or phases-lag circuit can
A phase-lead or phases-lag circuit can
provide phase shift between 0° and 90°.
provide phase shift between 0° and 90°.
• For total phase shift 180°, we use three
For total phase shift 180°, we use three
identical sections each giving a phase shift
identical sections each giving a phase shift
of 60°.
of 60°.
&
• It means in the beginning the gain of
the FET amplifier must be greater than
29.
• Not very popular, as the frequency
cannot be adjusted over large range.
35. • The two arms on the left of the bridge make lead-
lag circuit.
• The two arms on the right, are 2Rt and Rt, making a
potential divider.
• It has both positive and negative feedback paths.
• Initially, when switched on, there is more positive
feedback than negative feedback.
• Oscillations build up.
• Negative feedback increases, making Aβ = 1.
36. • The reason why the loop gain reduces to unity :
1. Initially tungsten lamp has low resistance; giving
low negative feedback.
2. Thus, loop gain Aβ is greater than unity.
3. As oscillations are built up, the tungsten lamp heats
up increasing its resistance.
4. Negative feedback increase to make Aβ = 1.
• With sustained oscillations, the resistance of
the lamp increases to exactly Rt , so that the
gain becomes :
37. • At resonance, the voltage ratio or
feedback factor of the lead-lag circuit is 1
/3.
• Therefore, loop gain becomes unity.
• The oscillation frequency is the same as
that of the lead-lag circuit,
40. Crystal Oscillator
• Used when accuracy and stability of fo is
utmost important.
• Where do you need such high stability of
frequency of oscillations ?
• Instead of an inductor, it uses a crystal of
quartz, tourmaline, or Rochelle salt.
• Piezoelectric effect.
• The crystal is suitably cut and then mounted
between two metallic plates.
• The fundamental frequency is given as
42. • Crystals have incredibly high Q.
• For the given values, Q = 5500.
• Q as high as 100 000 can be possible.
• An LC circuit has Q not greater than 100.
• The extremely high value of Q makes fo highly
stable.
44. Series and Parallel Resonance
• First, resonance occurs at fs for the series
combination of L and Cs.
• Above fs the series branch LCsR has inductive
reactance.
• It then resonates at fp , with Cm.
• For this parallel resonance, equivalent series
capacitance is Cp.
46. • Normally, Cs is much smaller than Cm.
• Therefore, Cp is slightly less than Cs.
• Hence, the frequency fp is slightly greater than
fs.
• The crystal is inductive only between the
frequencies fs and fp.
• The frequency of oscillation must lie between
these frequencies.
• Hence the stability.
