SIGNALS AND CIRCUITS 2 OSCILLATORS COLPITTS OSCILLATOR

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SIGNALS AND CIRCUITS 2
OSCILLATORSCOLPITTS OSCILLATOR
Invented in 1920 by American engineer Edwin H.
Colpitts
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Colpitts oscillator - Oscillation frequency The
ideal frequency of oscillation for the circuits
in previous slide are given by the equation
where the series combination of C1 and C2 creates
the effective capacitance of the LC tank. Real
circuits will oscillate at a slightly lower
frequency due to junction capacitances of the
transistor and possibly other stray capacitances.
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SIGNALS AND CIRCUITS 2 OSCILLATORS

• Basic RC Oscillator Circuit

The mathematics for calculating the oscillation
frequency and oscillation criterion for these
circuits are surprisingly complex, due to each
R-C stage loading the previous ones. IF . then
Since the resistor-capacitor combination in the
RC Oscillator circuit also acts as an attenuator
producing an attenuation of
1/29th (Vo/Vi ß) the gain of
the amplifier must be sufficient to overcome the
losses and in our three mesh network above the
amplifier gain must be greater than 29
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Real phase-shift RC oscillator (Opamp ideal)
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Real phase-shift RC oscillators investigation
The loading effect of the amplifier on the
feedback network has an effect on the frequency
of oscillations and can cause the oscillator
frequency to be higher than calculated
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Wien bridge oscillator
The oscillator is based on a bridge circuit
originally developed by Max Wien in 1891.
The conditon of oscillation is is satisfied by
loops gain T1
assuming
and
and
                                   
with
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SIGNALS AND CIRCUITS 2 OSCILLATORS
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Magnitude frequency response of Wien bridge
oscillator
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Phase frequency response of Wien bridge oscillator
F, kHz 0.01 0.1 0.5 1 1.7 3 5 20 50 1000
Aß 0.02 0.2 0.73 0.97 1.02 0.9 0.72 0.24 0.01 0.002
f Aß, deg 89 78 45 18 -0.5 -28 -45 -78 -90 -150
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Non-inverting Schmitt trigger
A circuit diagram of a Schmitt trigger made using
an operational amplifier.
In this circuit, the two resistors R1 and R2 form
a parallel voltage summer.
Parallel positive feedback creates the needed
hysteresis Hysteresis from           
to         
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Inverting Schmitt trigger
Series positive feedback creates the needed
hysteresis
Trigger levels                
A Schmitt trigger is a bistable multivibrator.
Connecting a single RC integrating circuit
between the output and the input of an
inverting Schmitt trigger we can build
Relaxation oscillator
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SIGNALS AND CIRCUITS 2 OSCILLATORS
Additionally, slow negative feedback is added
with an integrating RC network.
Output automatically oscillates from VSS to VDD
as the capacitor charges from one Schmitt trigger
threshold to the other
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Relaxation square and triangle waves oscillator
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Relaxation saw-toof and sampling waves
oscillator
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I1 (Vin-V1)/Rp, power P2V22/Rload).
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