Electronic Circuits - Oscillators - Important Short Questions and Answers: Oscillators

__OSCILLATORS__

**1.
Mention two essential conditions for a circuit to maintain oscillations[**

The conditions for oscillator to produce
oscillation are given by Barkhausan criterion.
They are:

i)
The
total phase shift produced by the circuit should be 360^{o} or 0^{o}

ii)
The
Magnitude of loop gain must be greater than or equal to 1 (ie)|Aβ|≥1

**2.
****What is the major disadvantage of a Twin-T
oscillator? **

Twin –T oscillator is operated only at one
frequency.

**3.
Differentiate oscillator from amplifier.**

**Oscillators**

1. They are self-generating circuits. They
generate waveforms like sine, square and triangular waveforms of their own.
Without having input signal.

2. It have infinite gain

3. Oscillator uses positive feedback.

**Amplifiers**

1. They are not self-generating circuits. They
need a signal at the input and they just increase the level of the input
waveform.

2. It have finite gain

3. Amplifier uses negative feedback.

**4. State
Barkhausen criterion for sustained oscillation. What will happen to the
oscillation if the magnitude of the loop gain is greater than unity?**

The conditions for oscillator to produce
oscillation are given by Barkhausan criterion.
They are :

i)
The
total phase shift produced by the circuit should be 360^{o} or 0^{o}

ii)
The
Magnitude of loop gain must be greater than or equal to 1 (ie)|Aβ|≥1

In practice loop gain is kept slightily greater
than unity to ensure that oscillator work even if there is a slight change in
the circuit parameters.

**5.
****Why an LC tank circuit does not produce
sustained oscillations. How can this can be overcome?**

We know that the inductor coil has some
resistance and dielectric material of the capacitor has some leakage.so small
part of the originally imparted energy is used to overcome these losses. As a
result, the amplitude of oscillating current goes on decreasing and becomes
zero when all energy is consumed as losses. So a LC tank circuit does not
produce sustained oscillations.

To maintain sustained oscillations, energy must
be supplied to the circuit at the same rate at which it is dissipated. In an
oscillator, the function of transistor and power supply source is to feed
energy to the circuit to overcome the losses at right time.

**6. Draw
the electrical equivalent circuit of crystal.
and mention its series and parallel resonance frequency.**

The crystal actually behaves as aseries RLC
circuit in parallel with C_{M.} . Because of presence of C_{M}
, the crystal has two resonant frequencies.

One of these is the series resonant frequency f_{s.}In
this case impedance is very low.

·
The
other is parallel resonance frequency f_{p}. In this case impedance is
very high.

**7. What
are the advantages and disadvantages of RC phase shift oscillators?**

** Advantages:**

i.
It is
best suited for generating fixed frequency signals in the audio frequency
range.

ii.
It
requires no transformer or inductor, hence less bulky.

iii.
Simple
Circuit.

*Pure sine wave output is possible.

__Disadvantages:__

i)
It
requires a high β transistor to overcome losses in the network.

ii)
These
oscillators are not suitable for high frequency operation.

iii)
Frequency
of oscillation can not be changed easily. To change the frequency of
oscillation, the three capacitor or resistors should be changed simultaneously.
This is inconvenient.

**8. What
is the necessary condition for a Wien bridge oscillator circuit to have sustained
oscillations? **

Then for oscillations to occur in a **Wien Bridge Oscillator** circuit the
following conditions must apply.

ü With no input signal the Wien Bridge Oscillator
produces output oscillations.

ü The Wien Bridge Oscillator can produce a large
range of frequencies.

ü The Voltage gain of the amplifier must be at
least 3.

ü The network can be used with a Non-inverting
amplifier.

ü The input resistance of the amplifier must be
high compared to R so that the RC network is not overloaded and alter the required
conditions.

ü The output resistance of the amplifier must be
low so that the effect of external loading is minimised.

**9.
****Define piezoelectric effect.**

The piezo electric Crystals exhibit a property
that if a mechanical stress is applied across one face the electric potential
is developed across opposite face and viceversa. This phenomenon is called
piezo electric effect.

**10. What
is the principle behind operation of a crystal oscillator?**

The principle behind the operation of crystal
is piezoelectric effect. According to this effect, if a mechanical stress is
applied across one face the electric potential is developed across opposite
face and viceversa.

**11. Draw
an oscillator circuit with feedback network given below.**

**12. What
are the advantages and disadvantages of wein bridge oscillators? Advantages:**

1.
Provides
a stable low distortion sinusoidal output over a wide range of frequency.

2.
The
frequency range can be selected simply by using decade resistance boxes.

3.
The
frequency of oscillation can be easily varied by varying capacitances C_{1}
and C_{2} simultaneously. The overall gain is high because of two
transistors.

__Disadvantages :__

1.
The
circuit needs two transistors and a large number of other components.

2.
The
maximum frequency output is limited because of amplitude and the phase-shift
characteristics of amplifier.

**13.
****A weinbridge oscillator is used for operations
at 9KHz. If the value of resistance R is 100KΩ, what is the value of C
required?**

**14. A
weinbridge oscillator is used for operations at 10KHz. If the value of
resistance R is 100KΩ, what is the value of C required?**

**15.
****A tuned collector oscillator in a radio
receiver has a fixed inductance of 60µH and has to be tunable over the
frequency band of 400KHz to 1200KHz. Find the range of variable capacitor to be
used.**

**16.
****Draw the feedback circuit of a colpitts
oscillator. Obtain the value of the equivalent series capacitance required if
it uses a L of 100mH and is to oscillate at 40KHz.**

**17.
****In a Hartley oscillator if L _{1}=0.2mH,L_{2}=0.3mH
and C=0.003µF. calculate the frequency of its oscillations.**

**18.
****In an RC phase shift oscillator, if its
frequency of oscillation is 955Hz and R _{1}=R_{2}=R_{3}=680KΩ.Find
the value of capacitors.**

**19.
****In an RC phase shift oscillator, if R _{1}=R_{2}=R_{3}=200KΩ
and C_{1}=C_{2}=C_{3}=100pF. Find the frequency of the
oscillator.**

**20.
****A crystal has the following parameters L=0.5H,
C=0.05pF and mounting capacitance is 2pF. Calculate its series and parallel
resonating frequencies.**

**21.
****Calculate the frequency of oscillation for the
clap oscillator with C _{1}=0.1µF, C _{2}=1µF, C_{1}=100pF
and L=470µH.**

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