FEEDBACK AMPLIFIERS AND OSCILLATORS
is the necessary condition for a Wien bridge oscillator circuit to have
Then for oscillations to occur in a Wien Bridge Oscillator circuit the
following conditions must apply.
input signal the Wien Bridge Oscillator produces output oscillations.
Bridge Oscillator can produce a large range of frequencies.
Voltage gain of the amplifier must be at least 3.
network can be used with a Non-inverting amplifier.
input resistance of the amplifier must be high compared to R so that the RC
network isnot overloaded and alter the required conditions.
output resistance of the amplifier must be low so that the effect of external
loading is minimised.
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.
Differentiate oscillator from amplifier.
1. They are self-generating circuits. They
generate waveforms like sine, square and triangular waveforms of their own.
Without having input signal.
2. They are not self-generating circuits. They
need a signal at the input and they just increase the level of the input
3. It have infinite gain
2. It have finite gain
4. Oscillator uses positive feedback.
5. Amplifier uses negative feedback.
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 360o or 0o
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.
any two methods of achieving sweep linearity of a time-base waveform.
6. Draw the
equivalent circuit of a pulse transformer. Name the various elements in it.
R1=Primary winding and source resistance
R2=Total resistance reflected to primary side
C=Total effective shunt capacitance
7. What is
the function of time base circuit?
A linear time base generator produces an output
waveform, which produces a portion which exhibits a linear variation of voltage
or current with respect to time.
Define feedback. What are the types of it?
The process of combining a fraction or part of
output energy back to the input is known as feedback. The different types of
feedback, b. Negative feedback
9. What is
meant by a) positive feedback b) negative feedback? Positive feedback
If feedback signal applied is in phase with the
input signal and thus increases the input, it is called as positive feedback.
It is also known as regenerative feedback.
If the feedback signal applied to the input is
out of phase with the input signal and thus signal decrease, it is called
negative feedback. It is also known as degenerative feedback.
What are the advantages of negative feedback?
improves the stability of the circuit.
improves the frequency response of the amplifier.
improves the percentage of harmonic distortion.
improves the signal to noise ratio (SNR).
reduces the gain of the circuit.
Define the feedback factor β?
It is the ratio between the feedback voltages
to the output voltage of the amplifier.
β = Vf/ Vo
Where, β is a feedback factor (or) feedback
ratio, Vf is the feedback voltage, Vo is the output
is transresistance amplifier?
In voltage shunt feedback amplifier the sampled
signal is a voltage and the feedback signal (Which is fed in shunt) is a
Rm = Vo / Ii
(or) Vo = Rm. Ii Where, Rm =
Vo = Output voltage. Ii =
does an oscillator differ from an amplifier?
is Barkhausan criterion?
conditions for oscillator to produce oscillation are given by Barkhausan criterion. They are:
total phase shift produced by the circuit should be 360o or 0o
Magnitude of loop gain must be greater than or equal to 1
List the disadvantages of crystal Oscillator.
· It is suitable for only low power circuits
amplitude of vibrations may crack the crystal.
in frequency is only possible replacing the crystal with another one by
Write the expression for frequency of
Oscillation of a colpitts oscillator.