Oscillators overview

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Oscillators overview

• This chapter will highlight the following
  concepts
• Conditions for oscillation, basically Nyquist and
  Barkhausen criteria for oscillation
• Detailed circuit analysis for oscillator
• The Colpits and Hartely oscillator
• The concept of negative resistance
• The pierce oscillator
• Crystal oscillators
• Voltage controlled oscillators
• Relaxation oscillators

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Oscillators conditions for oscillation

• Electronic oscillators are feedback networks
• Both negative and positive feedback oscillators
  will be considered
• A negative feedback oscillator normally can be
  built from a common emitter circuit
• A positive feedback oscillator can be built from
  a common base oscillator

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Oscillators Nyquist Criteria

• Consider the block diagram shown below
• The G(j?) is the transfer function of the gain
  block
• H(j?) is the transfer function of the feedback
  block

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Oscillators Nyquist Criteria

• If we write an expression of the output voltage
  in terms of the input, then we have
• For oscillation, the output voltage V0 is nonzero
  even If the input is zero
• This condition can be met if the forward loop
  gain is infinite (which is impractical) or
• If the denominator is

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Oscillators Nyquist Criteria

• This leads to the well-known condition for
  oscillation, Nyquist criterion, at some frequency
  ?0
• This means that the magnitude of the open loop
  transfer function is equal to 1, or
• And the phase shift is 180