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Berk haussen condition and oscilators
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Slides taken from:
A.R. Hambley, Electronics, Prentice Hall,2/e, 2000
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Part C Oscillators
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2Figre !."# Re$etiti%e ram$ &a%eform.
Oscillators
Oscillators are circuits that generate periodicsignals
An oscillator converts DC power from the power
supply into AC signal power spontaneously without the need for an AC input source
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3Figre !."' A linear oscillator is formed by connecting an am$lifier and
a feedback net&ork in a loo$.
!inear Oscillators
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4Figre !."! (inear oscillator &it) e*ternal signal +ininected.
"arkhausen Criterion
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"arkhausen Criterion
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#ow does the oscillation get started$
%oise signals and the transients associatedwith the circuit turning on provide the initial
source signal that initiate the oscillation
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Practical Design Considerations
&sually' oscillators are designed so that the loop gain
magnitude is slightly higher than unity at the desired
fre(uency of oscillation
)his is done because if we designed for unity loop
gain magnitude a slight reduction in gain would result
in oscillations that die to *ero
)he drawback is that the oscillation will be will be
slightly distorted +the higher gain results in oscillation
that grows up to the point that will be clipped,
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8Figre !.#0 -y$ical linear oscillator.
-.ample of Oscillator circuit +/,
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9Figre !.# Feedback net&ork. ote t)at t)e in$t to t)e net&orkis on t)e rig)t)and side and t)e res$onse is on t)e left)and side.
-.ample of Oscillator circuit +0,
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10Figre !.#1 ienbridge oscillator.
1ien"ridge Oscillator
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11Figure 9.75 Example of output voltage of the oscillator.
1ien"ridge oscillator output
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2n a linear oscillator' amplitude stabili*ation below theamplifier clipping level is needed to reduce distortion
1e can reduce the amount of distortion by reducing theamplifier gain #owever' if the gain becomes too small the oscillations die
out
3everal approaches can be used to resolve the conflict )he easiest approach is to make the gain ad4ustable by
using a potentiometer in place of the resistors thatdetermine the gain
5ethods for Amplitude 3tabili*ation
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13Figre !.#" 3scillation am$litde can be stabili4ed &it)in t)e linear
range of t)e o$ am$ by sbstitting a lo&$o&er lam$ for R.
#P way to Amplitude 3tabili*ation
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14Figre !.## Am$litde stabili4ed ienbridge oscillator.
Amplitude 3tabili*ation with diodes
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Figre !.#' 3t$t %oltage of t)e oscillator of Figre !.##.
Amplitude 3tabili*ation with diodes
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