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Elec and Comp Tech 62BElec and Comp Tech 62BSemiconductor DevicesSemiconductor Devices

Chapter 10Chapter 10

Oscillators and TimersOscillators and Timers

Feedback OscillatorsFeedback Oscillators

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The OscillatorThe OscillatorThe OscillatorThe OscillatorThe oscillator circuit produces a periodic The oscillator circuit produces a periodic

waveform output with the power supply waveform output with the power supply being the only required inputbeing the only required input

The output can be a sine, square, ramp, The output can be a sine, square, ramp, or other waveformor other waveform

An additional input can be used to An additional input can be used to synchronize the oscillationsynchronize the oscillation

Two major classifications of oscillatorsTwo major classifications of oscillatorsFeedback oscillatorsFeedback oscillatorsRelaxation oscillatorsRelaxation oscillators

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Feedback OscillatorFeedback OscillatorFeedback OscillatorFeedback Oscillator

Attenuation Attenuation sets loop gain sets loop gain at 1at 1

Net phase Net phase shift at shift at oscillation oscillation frequency frequency equals zeroequals zero

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Oscillator Start-UpOscillator Start-UpOscillator Start-UpOscillator Start-UpSome initial stimulus is needed to Some initial stimulus is needed to start oscillationstart oscillation

Start-up gain must be >1 for Start-up gain must be >1 for oscillation to build to desired leveloscillation to build to desired level

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Start-Up StimulusStart-Up StimulusStart-Up StimulusStart-Up Stimulus

In a real oscillator circuit, noise In a real oscillator circuit, noise from the op-amp is usually from the op-amp is usually sufficient to provide the sufficient to provide the starting stimulusstarting stimulus

In Multisim, some external In Multisim, some external stimulus is required to start the stimulus is required to start the oscillationoscillation

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Start-Up GainStart-Up GainStart-Up GainStart-Up Gain

Gain much larger than 1Gain much larger than 1Starts oscillation with less stimulusStarts oscillation with less stimulusOscillation builds to desired output Oscillation builds to desired output voltage fastervoltage faster

Without some method of lowering Without some method of lowering the gain after reaching the desired the gain after reaching the desired output voltage, the output output voltage, the output continues to grow until reaching continues to grow until reaching amplifier saturationamplifier saturation

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Fixed-Gain OscillatorsFixed-Gain OscillatorsFixed-Gain OscillatorsFixed-Gain Oscillators

Gain slightly greater than 1Gain slightly greater than 1May not start oscillating without May not start oscillating without external stimulusexternal stimulus

Amplifier saturation is used to limit Amplifier saturation is used to limit gaingain

When slight distortion is When slight distortion is acceptable, additional gain limiting acceptable, additional gain limiting circuitry is not needed.circuitry is not needed.

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Wein-Bridge OscillatorWein-Bridge OscillatorWein-Bridge OscillatorWein-Bridge Oscillator

Bandpass Bandpass filterfilter

ffrr = 1/2πRC = 1/2πRC

Peak VPeak Voutout at f at frr

VVoutout/V/Vinin = 1/3 = 1/3Feedback Feedback phase shift = 0phase shift = 0

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Wein-Bridge OscillatorWein-Bridge OscillatorWein-Bridge OscillatorWein-Bridge OscillatorWein bridge is Wein bridge is

created with lead-created with lead-lag (bandpass) lag (bandpass) positive feedback positive feedback and resistive and resistive negative feedbacknegative feedback

Positive feedback Positive feedback VVoutout/V/Vinin=1/3=1/3

Negative feedback Negative feedback sets Asets Aclcl=3, so net =3, so net gain through gain through bridge = 1bridge = 1

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Self-Starting Wein-BridgeSelf-Starting Wein-BridgeSelf-Starting Wein-BridgeSelf-Starting Wein-BridgeStart-up requires Start-up requires

loop gain >3 to loop gain >3 to build output levelbuild output level

Continuous Continuous oscillation without oscillation without saturation requires saturation requires gain = 3gain = 3

An automatic gain An automatic gain control (AGC) is control (AGC) is neededneeded

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Back-to-Back Zener Back-to-Back Zener MethodMethod

Back-to-Back Zener Back-to-Back Zener MethodMethod

At start-up, and as At start-up, and as output builds, Routput builds, R33 is is in series with Rin series with R11, so , so AAVV>3>3

When voltage When voltage across Racross R33>V>VZZ+0.7 V, +0.7 V, diodes short out diodes short out drop across Rdrop across R33, , limiting gain to Alimiting gain to AVV=3=3

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JFET AGCJFET AGCJFET AGCJFET AGCQQ11 operates in operates in

ohmic regionohmic regionNegative peak Negative peak

detector Ddetector D11 and and CC33 provide Q provide Q11 bias to form bias to form voltage-variable voltage-variable resistanceresistance

RRff is adjustable is adjustable due to variances due to variances in gin gmm of JFETs. of JFETs.

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Phase-Shift OscillatorPhase-Shift OscillatorPhase-Shift OscillatorPhase-Shift Oscillator

3 RC hi-pass3 RC hi-pass ffrr = 1/2π 6RC = 1/2π 6RC when Rwhen R1, 2, 31, 2, 3 and and CC1, 2, 31, 2, 3 are equal are equal

When each individual RC phase shift When each individual RC phase shift (() = 60°, total network ) = 60°, total network = 180°, fed to = 180°, fed to inverting input, makes total inverting input, makes total = 0° = 0°Attenuation B (VAttenuation B (Voutout/V/Vinin) = 1/29, so ) = 1/29, so RRff/R/R33=29 for unity gain of phase-shift =29 for unity gain of phase-shift networknetwork

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Twin-T OscillatorTwin-T OscillatorTwin-T OscillatorTwin-T Oscillator

Hi-pass and Lo-pass T filters in Hi-pass and Lo-pass T filters in parallel form a notch filterparallel form a notch filter