N_Oscillators_N17

N_Oscillators_N17 - INTRODUCTION TO OSCILLATOR CIRCUITS...

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1 Oscillators INTRODUCTION TO OSCILLATOR CIRCUITS (17) 2.1   The oscillators 2.2   Feedback oscillator principle 2.3   Oscillators with RC feedback circuit 2.4   Oscillators with LC feedback circuit 2.5   Relaxation oscillators 2.6   555 timer as an oscillator
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2 2.1   The oscillators Oscillator is a circuit that produce a  continuous signal/waveform on its  output with only the dc supply  voltage as an input.  The output voltage can be either  sinusoidal or non sinusoidal  depending on the type of oscillator.
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3 2.2   Feedback oscillator principle The feedback oscillator is widely used for  generation of sine wave signals.  Positive feedback  is the condition wherein a  portion of the output voltage of an amplifier is  fed back to the input with no net phase shift,  resulting in a reinforcement of the output  signal.  
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4 2.2   Feedback oscillator principle Conditions for Oscillation Phase shift around the feedback loop must be 0 o Voltage gain, A cl , around the closed feedback loop (loop gain) must equal 1 (unity) – The voltage gain around the closed feedback loop (A cl ) is the product of amplifier gain (A v ) and the attenuation (B) of the feedback circuit A cl  = A B Start-Up Conditions For oscillation to begin, A cl around the positive feedback loop must be greater than 1 so that the output voltage can build up to a desired level. Then A cl decrease to 1 and maintains the desired magnitude
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5 2.3   Oscillators with RC feedback circ u it Three types of RC oscillators that  produce sinusoidal outputs will be  discussed :   1. Wien-bridge oscillator  2. phase-shift oscillator   3. twin-T oscillator. Generally  RC oscillators are used for  frequencies up to about 1 MHz Wien-bridge oscillator is most widely  used for this range of frequencies
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6 Wien-Bridge Oscillator 2.3   Oscillators with RC feedback circ u it  Fundamental part : Lead-Lag circuit Lag circuit  : R 1 1           Lead circuit  : R 2 2 At resonant frequency, f r , phase shift through the circuit is 0 o   and the attenuation is 1/3 Below f r  the lead circuit dominates and the output leads the  input Above f r , the lag circuit dominates and output lags the input
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7 Wien-Bridge Oscillator (con’t. .) 2.3   Oscillators with RC feedback circ u it Resonant Frequency : At Resonant Frequency  : R 1  = R 2   and X C1  = X C2                      :
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8 Wien-Bridge Oscillator (con’t. .) 2.3   Oscillators with RC feedback circ u it The lead-lag circuit is in the  positive feedback loop of  Wien-bridge oscillator.  The voltage divider limits  gain.  The lead lag circuit is  basically a bandpass with  a narrow bandwidth (high  Q).
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This note was uploaded on 01/29/2011 for the course EE 203 taught by Professor Gp.(r)muzaffarali during the Fall '10 term at College of E&ME, NUST.

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N_Oscillators_N17 - INTRODUCTION TO OSCILLATOR CIRCUITS...

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