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Elec and Comp Tech 62B Semiconductor Devices. Chapter 10 Oscillators and Timers Feedback Oscillators. The Oscillator. The oscillator circuit produces a periodic waveform output with the power supply being the only required input The output can be a sine, square, ramp, or other waveform - PowerPoint PPT Presentation
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9/25/04 1
Elec and Comp Tech 62BElec and Comp Tech 62BSemiconductor DevicesSemiconductor Devices
Chapter 10Chapter 10
Oscillators and TimersOscillators and Timers
Feedback OscillatorsFeedback Oscillators
9/25/04 62Bchap10a Page 2
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
9/25/04 62Bchap10a Page 4
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
9/25/04 62Bchap10a Page 6
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
9/25/04 62Bchap10a Page 7
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.
9/25/04 62Bchap10a Page 8
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.
9/25/04 62Bchap10a Page 13
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
9/25/04 62Bchap10a Page 14
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