Relaxation oscillator


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

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Relaxation oscillator

  1. 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 1
  2. 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° 2
  3. 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 3
  4. 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. 4
  5. 5. T=2RcC ln F= Triangular wave generator : We can generate triangular wave by connecting integrator circuit at the output 5
  6. 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. 6
  7. 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: 7
  8. 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. 8