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Slides taken from:

A.R. Hambley, Electronics, Prentice Hall,2/e, 2000

Feedback:

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