It describes about oscillators and relaxation oscillators. Some of its applications are also given.

1. RELAXATION
OSCILLATOR
Oscillator:
An oscillator is a sine wave signal source of known frequency. It
produces an output signal yet it requires no external input signal. The only
external input connection to an oscillator is for the dc power source.
The total output is equals to input signal +positive feedback
Negative feedback stables the output and decreases Gain
Positive feedback unstable the output and increases the gain
We have to make sinusoidal wave for which positive
feedback is increased till input signal’s impact become zero
Here,
G =amplifier voltage gain without feedback
B= feedback factor
If feedback become zero the output become 180° out of phase with
respected input signal
If we increase the feedback by increasing the value of B ,denominator
tends to zero due to which voltage gain tends to infinity respectively
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2. For proper oscillation we have to referred BARKHAUSEN CRITERIA
according to which two conditions must achieve
1- Loop gain of an amplifier feedback network must be equal to unity
or 1
2- The total phase shift around the feedback must be equal to zero
degree
Product of G & B is known as loop gain
here are some conditions
>loop gain =1(unity) gives proper oscillation
>if
loop gain < 1
no proper oscillation
>if
loop gain > 1
distorted oscillation
The PHASE SHIFT
it must be equal to 0°, which is achieve by the two shifts of 180° . one
by amplifier and another by feed backing factor
180°(amplifier)+180°(feedback)=360°or 0°
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3. Relaxation Oscillator:
A relaxation oscillator is an oscillator based upon the behavior of a physical system's
return to equilibrium after being disturbed. That is, a dynamical system within the oscillator
continuously dissipates its internal energy. Normally the system would return to its natural
equilibrium; however, each time the system reaches some urt sufficiently close to its equilibrium,
a mechanism disturbs it with additional energy. Hence, the oscillator's behavior is characterized
by long periods of dissipation followed by short impulses. The period of the oscillations is set by
the time it takes for the system to relax from each disturbed state to the threshold that triggers
the next disturbance.
Working:
The lower part of the relaxation oscillator is same as the
Schmitt trigger.
Assume at t=0
Vc=0V , Vout=12V
Capacitor starts charging
Vc=(1-et/T ) 12V
When Vc > 6V , Vout=-12V
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4. Capacitor starts discharging
Vc=Vin + (Vf-Vinitial)(1-e-t/T)
Vc=6 + (-12-6)(1-e-t/T)
When Vc < -6V , Vout= 12V
Capacitor again starts charging in a cycle
T=2t
F=
If the values of the resistors are changed then the time period
will be changed.
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5. T=2RcC ln
F=
Triangular wave generator :
We can generate triangular wave by connecting
integrator circuit at the output
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6. 555-Timer as Relaxation Oscillator:
The very popular 555 integrated circuit has everything in it to make a relaxation oscillator.
It can detect the voltages V1 and V2, and operate a transistor switch to discharge the capacitor
when the voltage reaches V2, turning it off again when the voltage drops to V1.voltages are
detected by the voltage comparators, represented by triangles. The box is a set-reset (SR) flip-flop
with complementary outputs Q and Q bar. You have access to output Q, and can get up to 200
mA from it, if you need such a large current. There is a reset pin that forces the output low when
it is grounded and a trigger pin that forces it high. The threshold input also drives the output high,
and this is its normal job. The CV (control voltage) input gives access to the threshold node,
Types of relaxation oscillator circuits include:
Multivibrator
Ring oscillator
Delay line oscillator
Royer oscillator
Rotary traveling wave oscillator.
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7. ADVANTAGES AND DISADVANTAGES of
RELAXATION OSCILLATORS
ADVANTAGES:
The output of relaxation oscillators depends on the
time constant ( =RC). Hence it can be used in
circuits where timing is required.
As they produce non-sinusoidal output, hence they
are used extensively in digital circuits.
They have a large and linear tuning range, that is
often convenient and sometimes crucial for
Frequency Modulation (FM) with low distortion.
DISADVANTAGES:
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8. Relaxation oscillators are frequency sensitive, i.e. any
change in the amplitude of oscillation can be
translated directly into a change of frequency.
They have a low duty cycle, hence have a low on time
(ton) and are also sometimes called blocking
oscillators.
They have poor phase-noise compared to other
oscillators.
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