Fundamentals of Linear Electronics Integrated & Discrete by L7kjV3

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									CHAPTER 13

Sine Wave
Oscillator
 Circuits
               Objectives

Describe and Analyze:
• Feedback oscillator theory
• RC phase-shifting oscillators
• LC resonant oscillators
• Crystal oscillators
• Troubleshooting
                Introduction

• Oscillators use feedback to produce periodic AC
  output with DC power as the only input.
• In contrast, function generators produce periodic
  outputs by joining pieces of wave-forms together.
                 A Bit of Theory
• We know that with negative feedback in an op-amp, the
  equation for closed-loop gain in terms of open-loop gain is:
                       ACL = AOL / (1 + B  AOL)
  where B is the feedback ratio set by external resistors. Now, if
  there is an additional 180° phase shift in B, we can express it
  mathematically as:
                      ACL = AOL / (1 – B  AOL)
  In that case, what happens when B  AOL = 1?
• Mathematically, ACL “goes to infinity” (whatever that means).
  Physically, the circuit oscillates: it takes no Vin to get a Vout.
  The trick is to get that 180° shift in B, the feedback network.
               RC Oscillators

• As a group, RC oscillators use an RC network
  inserted into the feedback loop of an amplifier to
  produce positive feedback at exactly one frequency.
  As we just saw, that’s the recipe for oscillation.
• One type of RC oscillator is the Wien-Bridge
  oscillator. Another is simply called the Phase-Shift
  oscillator.
       Wien-Bridge Oscillator




Feedback to the (+) input > feedback to (–) input at fOSC
          Starting and Running
• A problem with the Wien-Bridge (and with all
  feedback oscillators) is that the feedback necessary
  to start oscillating is slightly more than the feedback
  to maintain a pure sine wave. If the gain is left too
  high, the sinewave amplitude will increase until it hits
  the rails and is clipped.
• The cure is to include a means for the circuit to lower
  its gain a bit once it starts oscillating. This is a type of
  negative feedback based on amplitude.
        Phase-Shift Oscillator
• The RC phase-shift oscillator is the simplest of its
  type. A minimum of three RC LPF sections are put in
  the feedback loop of an inverting amplifier. Each RC
  stage causes an amount of phase shift that changes
  with frequency. At one specific frequency, the phase
  shifts of the network add up to 180°.
• Since the inverting amplifier has a 180° phase shift,
  the total phase shift is 360°, which means it has
  become positive feedback. The circuit oscillates.
Phase-Shift Oscillator
       Oscillations in Amplifiers
    The old joke is that, when you’re testing them,
    oscillators don’t oscillate but amplifiers do. If there’s an
    accidental feedback path from the output to the input,
    then it’s a good bet a high-gain amplifier will oscillate.
    Such feedback paths can be:
•   Through the power rails.
•   Through magnetic coupling of signal leads.
•   Through capacitive coupling of adjacent components.
•   Through unshielded input cables.
•   Through putting a microphone too close to a speaker.
                    LC Oscillators




Pulsing an LC “tank circuit” will make it “ring”. But the oscillations
die off due to I2R losses in the circuit. If energy could be pumped
back into the tank as fast as it were being dissipated, it would
ring forever. That is the basic idea of an LC resonant oscillator.
Colpitts Oscillator




A common type of LC oscillator.
Hartley Oscillator




Another type of LC oscillator.
           Crystal Oscillators

• Piezoelectric crystals behave like extremely high-Q
  resonant circuits.
• A crystal’s frequency depends on its physical
  dimensions, which can be tightly controlled by
  grinding.
• LC resonant oscillators can be “crystal stabilized”.
• A crystal can take the place of an LC tank circuit.
   XTAL Oscillator Examples




Since you’re stuck with it anyway, the Pierce oscillator
     (b) utilizes stray capacitance for feedback.
             Troubleshooting
• Use a frequency counter to check frequency. You
  can use an oscilloscope, but your readings will be off
  by ±5%.
• Use a X10 probe to minimize loading (they work on
  counters too).
• At very high frequencies, you don’t have to touch the
  probe to the circuit, just get it close.
• There can be high voltage in a transmitter’s tank
  circuit even if it uses a 12 Volt DC supply.

								
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