UNIT-2
OSCILLATOR AND SWITCHING CIRCUITS
1. Define oscillator. K1
An oscillator is a circuit which is basically acts as a generator, generating the output signal
which oscillate with constant amplitude and desired frequency
2. Differentiate oscillator with amplifier. K1
S.No AMPLIFIER OSCILLATOR
1 It uses negative feedback It uses positive feedback
2 Input is required Input is not required
3 It produces amplified output It produces constant frequency constant amplitude
output
4 Phase shift depends on the Total phase shift has to be 360degree around a
configuration loop
5 Amplifier has required bandwidth Bandwidth is ideally zero as operation is at single
depending upon the application. frequency.
6 Used generally in audio and video Used in signal generators, heterodyne receiver,
circuits. clocks etc…
3. Give the Barkhausen's criterion for oscillators K1
The Barkhausen’s criterion state:
The total phase shift around a loop, as the proceed from input through amplifier, feedback network
back to input gain, completing a loop, precisely 0degree or 360degree, or of course an integral
multiple of 2 π radians.
The magnitude of the product of the open loop gain of the amplifier (A) and the feedback factor β is
unity i.e. |Aβ|=1.
4. Define frequency stability of oscillator K1
Measures the ability of an oscillator to maintain the desired frequency as precisely as possible for as
long time as possible is called frequency stability of an oscillator.
5. Give the classification of oscillators K1
1. Based on the output waveform (tringular,sinusoidal,rectangular)
2. Based on circuit component (R&C, C&L, crystal)
3. Based on range of frequency (R-C(low frequency 20hz-200hz),L-C (high frequency more than
300khz).
4. Based on type of feedback (UJT relaxation oscillator in which feedback in not used).
6. What are the advantages of colpitt’s oscillator? K1
The advantages of colpitt’s oscillator are:
Pure output waveform.
Good stability at high frequency.
Wide range of frequency, from 1-60MHz.
Simple construction hence simple to design.
Improved performance at high frequency.
7. Write the advantages, disadvantages and applications of Hartley oscillator. K1
Advantages:
o The frequency is simply varied by the net value of C in the tank circuit.
o The output amplitude remains constant when tuned over the frequency range.
o The feedback ratio of L1 to L2 remains constant.
Disadvantages:
o The output is rich in harmonic content and therefore not suitable where a pure sine wave is
required
APPLICATIONS:
It is used for generation of sinusoidal output signal with frequencies. The combination of this
oscillator with some devices (instead of tank circuit) can be used to achieve great temperature
stability and frequency.
8. State the expression for frequency of clapp oscillator. K1
The working of this circuit is almost identical to that of the Colpitt’s, where the feedback ratio governs
the generation and sustained of the oscillations. However the frequency of oscillation in the case of
Clapp oscillator is given by
When C3 is chosen to be smaller in comparison with C1 and C2, the net capacitance governing the
circuit will be more dependent on it. Thus the equation for the frequency can be approximated as
9. What is meant by frequency stability analysis? K1
For an oscillator the frequency of the oscillations must remain constant. The analysis of the dependence
of the oscillating frequency on the various factors like stray capacitance, temperature etc. is called as
the frequency stability analysis
10. Write the features of crystal oscillator. K1
It produces extremely stable output. Due to replacement of transistors and capacitors in
conventional oscillators such a change is not possible.
Temperature has no effect on the frequency.
It has very much reduced phase noise.
The crystal frequency depends on its physical dimensions hence proper frequency rating can be
achieved by cutting crystal to proper dimension.
Aging rates of the crystal are 2*10^-8 per year, for a quartz crystal. This is also very small.
In all, the frequency stability of crystal oscillators is very good.
11. How does the crystal oscillator maintain stable frequency? K1
The frequency stability of the oscillator depends on the dϴ/ dω i.e. rate of change of phase with respect
to frequency. Higher the values of dϴ/dω, better is the stability of the oscillator. In crystal oscillator,
the crystal oscillates at resonant frequency at which dϴ/ dω is extremely high. Hence the frequency
becomes independent of circuit features. Similarly effect of temperature on crystal are 2*10^-8 per year
which are negligibly small. Hence the frequency oscillator maintains a stable frequency.
12. What are the advantages of colpitts oscillator compared to a phase shift oscillator? K1
Pure output waveform.
Good stability at high frequency.
Wide range of frequency, from 1-60MHz.
Simple construction hence simple to design.
Improved performance at high frequency.
13. Why is it difficult to have a variable frequency operation with an RC Phase shift oscillator? K1
By changing the values of R and C, the frequency of the oscillator can be changed. But the values of
R&C of all three section of RC phase shift oscillator must be changed simultaneously to satisfy the
oscillating condition. The extra changes in R&C values of all three sections are partially impossible.
Hence it is difficult to have a variable frequency operation with an RC phase-shift oscillator.
14. Which are the various types of multivibrators? K1
1. Astable multivibrator
2. Bistable multivibrator
3. Monostable multivibrator
15. What is astable multivibrator? K1
1. A multivibrator which generates square wave without any external triggering pulse is
called astable multivibrator.
2. It has both the states as quasi-stable states. None of the states is stable
3. Due to this, the multivibrator automatically makes the successive transitions from one
quasi-stable state to other, without any external triggering pulse. So it called Free-running
multivibrator.
4. The rate of transition from one quasi-stable state to other is determined by the discharging
of a capacitive circuit.
16. What is the difference between open loop and closed loop gain of the circuit? K1
17. Explain the concept of positive feedback. K1
The feedback is a property which allows to feedback the part of the output, to the same circuit as its
input. Such a feedback is said to be positive whenever the part tf the output that is fed back to the amplifier as
its input, is in phase with the original input signal applied to the amplifier.
18. From where starting voltage for the oscillator is derived? K1
Every resistance has some free electrons. Under the influence of room temperature, these free
electrons move randomly in various directions. In such a movement of the free electrons generate a voltage
called noise voltage, across the resistance. Such noise voltage provides the starting voltage for the oscillator.
19. Why in practice A β is kept greater than unity. K1
To amplify small noise voltage present, so that oscillations can start, A β is kept initially greater than
unity.
20. What are the frequency sensitive arms? K1
The arms which decide the frequency of oscillations i.e., R1-C1 and R2-C2 are the frequency sensitive
arms.
21. What is the gain requirement in the wein bridge oscillator? K1
The gain requirement for wein bridge oscillator is minimum 3.
22. Give the comparison between RC and LC oscillators. K1
23. Write down the general applications of oscillators. K1
a) As a local oscillator in radio receivers.
b) In T.V receivers.
c) In signal generators.
d) As clock generation for logic circuits.
e) AM and FM transmitters.
f) In phase lock loops.
24. Write down the comparison of RC oscillators K1
25. What is Turn-off time (tOFF) in transistor? K1
The sum of the storage time (ts) and the fall time (tf) is called the turn-OFF time (tOFF).
(tOFF) = (ts) + (tf)
