Transcript Document

Oscillators with LC Feedback
Circuits
• LC Feedback elements are used for the
generation of higher frequencies of oscillation.
• Because of lower unity gain frequency of most
op amps, discrete transistors are often used as
the gain elements in LC Feedback oscillators.
• Colpitts, Clapp, Hartley, Armstrong and Crystal
controlled oscillators are the examples
The Colpitts Oscillator
• LC feedback circuit provides the required
phase shift and acts as resonant filter that
passes only the desired frequency of
oscillation.
Conditions for Oscillation and
Startup
• Values of C1 and C2 determine the
attenuation as
or
But for
Oscillation
For the oscillator to be self starting,
OR
Loading Effect on Feedback Circuit
Input impedance of the amplifier acts as a load on the resonant feedback
circuit and reduces the quality factor of the circuit
When Q>10,
When Q<10, the resonant frequency is significantly reduced
To avoid this loading effect, we
can use JFET instead of BJT as
it has very high input
impedance as compared to
BJT.
The concept of Reflected Load and Impedance
Matching using Transformer Coupling
Matching Audio amplifier with an output resistance of 800 ohm to an 8
ohm speaker using transformer coupling
The Clapp Oscillator
• Clapp oscillator is a variation of Colpitts
• An additional capacitance C3 in series with
the inductor
• This new capacitance is in series with C1
and C2 within the tank circuit
• Total capacitance is given by
If C3 is much smaller than C1 and C2 ,then C3 almost entirely controls the
resonant frequency
Since one end of each of C1 and C2 is grounded, junction capacitance
and other stray capacitances appear in parallel with these altering their
values
C3 is not affected yet and hence helps to provide a more accurate and
stable frequency of oscillation
The Hartley Oscillator
OR
Loading Effect is the same as in the case of Colpitts, i.e, to reduce the
quality factor hence reducing the resonant frequency.
The Armstrong Oscillator
•Uses transformer coupling for feedback provision of the signal
voltage.
•Sometimes called tickler oscillator in reference to transformer
secondary of tickler coil.
•Less common as compared to previous ones mainly because of
transformer size and cost.
•The frequency of oscillation is set by the inductance of primary
winding, Lpri in parallel with C1
Crystal Controlled Oscillators
• Most stable and accurate type of feedback oscillators
• Exhibits a very high Q (values of several thousand are
typical)
• Piezoelectric crystal is used in the feedback loop for the
frequency control
• Quartz crystal is normally used in electronic applications
in the form of a quartz wafer
• Piezoelectric Effect: when a changing mechanical stress
is applied across the crystal to cause it to vibrate, a
voltage is developed at the frequency of mechanical
vibration
• Conversely when an ac voltage is applied across the
crystal, it vibrates at the frequency of applied voltage
• The greatest vibration occurs at the natural resonant
frequency.
• Natural resonant frequency is determined
by the physical dimensions and by the way
the crystal is cut.
Quartz wafer is mounted between two electrodes and enclosed in a
protective can
Series-Parallel RLC circuit can operate either in series or parallel
resonance
At the series resonance, the inductive reactance is cancelled by the reactance
of CS and now RS determines the impedance of the crystal
Parallel resonance occurs when the inductance reactance and reactance of Cm
are equal
Parallel resonant frequency is usually at least 1kHz higher than the series
resonant frequency
Crystal used as series resonant circuit
Maximum feedback is provided at
series resonant frequency where
impedance of the crystal is
minimum.
CC is used to fine tune the
oscillator frequency by pulling the
resonant frequency of the crystal
slightly up or down
Crystal used as parallel resonant circuit (modified Colpitts
configuration)
Impedance of the crystal is maximum
at parallel resonance and
maximum voltage is develops
across the capacitors
The voltage across the capacitor C1
is fed back to the input
Modes of Oscillation in the Crystals
There are two modes of oscillation
Fundamental Mode:
•
It is the lowest natural resonant frequency of oscillation.
•
It depends upon the crystal mechanical dimensions, type of cut and inversely
proportional to the thickness of crystal slab.
•
Upper limit on fundamental frequency is set by the thickness of crystal slab as
it cannot be cut too thin without fracturing.
•
For most crystals, this upper limit is less than 20MHz.
Overtone Mode:
•
Overtones are the approximate integer multiples of the fundamental frequency
•
Usually these are the odd multiples of the fundamental