Sine Wave Oscillator Circuits

1
CHAPTER 13

• Sine Wave Oscillator Circuits

2
Objectives

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

3
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.

4
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.

5
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, thats the recipe for
  oscillation.
• One type of RC oscillator is the Wien-Bridge
  oscillator. Another is simply called the
  Phase-Shift oscillator.

6
Wien-Bridge Oscillator

• Feedback to the () input gt feedback to () input
  at fOSC

7
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.

8
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.

9
Phase-Shift Oscillator
10
Oscillations in Amplifiers

• The old joke is that, when youre testing them,
  oscillators dont oscillate but amplifiers do. If
  theres an accidental feedback path from the
  output to the input, then its 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.

11
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.

12
Colpitts Oscillator

• A common type of LC oscillator.

13
Hartley Oscillator

• Another type of LC oscillator.

14
Crystal Oscillators

• Piezoelectric crystals behave like extremely
  high-Q resonant circuits.
• A crystals 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.

15
XTAL Oscillator Examples

• Since youre stuck with it anyway, the Pierce
  oscillator (b) utilizes stray capacitance for
  feedback.

16
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 dont have to touch
  the probe to the circuit, just get it close.
• There can be high voltage in a transmitters tank
  circuit even if it uses a 12 Volt DC supply.