Lecture 38 Oscillators

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Lecture 38Oscillators

• Amit Kumar Mishra
• ECE, IIT G

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Amplitude Stabilization

• Loop gain of oscillator changes due to power
  supply voltage, component value or temperature
  changes.
• If loop gain is too small, desired oscillation
  decays and if it is too large, waveform is
  distorted.
• Amplitude stabilization or gain control is used
  to automatically control loop gain and place
  poles exactly on jw axis.
• At power on, loop gain is larger than that
  required for oscillation.As oscillation builds
  up, gain is reduced to minimum required to
  sustain oscillations.

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Amplitude Stabilization in RC Oscillators Method
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R1 is replaced by a lamp. Small-signal resistance
of lamp depends on temperature of bulb
filament. If amplitude is large, current is
large, resistance of lamp increases, gain is
reduced. If amplitude is small, lamp cools,
resistance decreases, loop gain increases.
Thermal time constant of bulb averages signal
current and amplitude is stabilized.
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Active LC oscillator

• Higher range
• Higher Q factor (gt ??)

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Hartely (b) and Colpitt (a) oscillators
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LC Oscillators Colpitts Oscillator
D0, collect real and imaginary parts and set
them to zero.
At w0
Generally more gain is used to ensure oscillation
with amplitude stabilization.
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LC Oscillators Hartley Oscillator
D0, collect real and imaginary parts and set
them to zero.
At w0
G-S and G-D capacitances are neglected, assume no
mutual coupling between inductors.
Generally more gain is used to ensure oscillation
with amplitude stabilization.
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Another practical Colpitt Osc.
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Crystal oscillator

• In its heart is a piezoelectric crystal
• Pizo crystal have opposite faces plated with
  electrodes.
• 3 major advantages
• Very high Q (10s to 100s of thousands)
• Stable with temp. and time
• Can give freq. upto several MHz
• Q and res. Freq. depends on the size,
  orientation of faces, and mount

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Crystal Oscillators
Crystal A piezoelectric device that vibrates is
response to electrical stimulus, can be modeled
electrically by a very high Q (gt100,000) resonant
circuit. L, CS, R represent intrinsic series
resonance path through crystal. CP is package
capacitance. Equivalent impedance has series
resonance where CS resonates with L and parallel
resonance where L resonates with series
combination of CS and CP.
Below wS and above wP, crystal appears
capacitive, between wS and wP it exhibits
inductive reactance. Used to replace L in Colpitt
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Crystal
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Crystal Oscillators Example

• Problem Find equivalent circuit elements for
  crystal with given parameters.
• Given data fS5 MHz, Q20,000 R 50 W, CP 5 pF

Analysis
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Pierce crystal oscillator configuration
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Crystal Oscillators Topologies
Colpitts Crystal Oscillator
Crystal Oscillator using BJT
Crystal Oscillator using CMOS inverter as gain
element.
Crystal Oscillator using JFET
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The classic 555 timer circuit

• Since 1972 (by Signetics Co.) called IC Time
  Machine!
• Numerous clones available
• Low-cost, accurate and easy to design with (gt1B
  units per year)
• 23 Transistors 2 diodes 16 resistors (DIP-8)
• Can work in monostable, astable and bistable
  configurations

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From the SE555 datasheet
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Schemtics
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Block diagram

• SR0 QQ
• S1R0 Q1
• S0 R1 Q0
• Vcc 5V
• Vth 2/3Vcc
• Vtl 1/3Vcc
• Why 555?
• Transistor switch

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Monostable configuration
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Astable configuration
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Many Thanks