Elec and Comp Tech 62B Semiconductor Devices

1
Elec and Comp Tech 62BSemiconductor Devices

• Chapter 10
• Oscillators and Timers
• Feedback Oscillators

2
The Oscillator

• The oscillator circuit produces a periodic
  waveform output with the power supply being the
  only required input
• The output can be a sine, square, ramp, or other
  waveform
• An additional input can be used to synchronize
  the oscillation
• Two major classifications of oscillators
• Feedback oscillators
• Relaxation oscillators

3
Feedback Oscillator

• Attenuation sets loop gain at 1
• Net phase shift at oscillation frequency equals
  zero

4
Oscillator Start-Up

• Some initial stimulus is needed to start
  oscillation
• Start-up gain must be gt1 for oscillation to build
  to desired level

5
Start-Up Stimulus

• In a real oscillator circuit, noise from the
  op-amp is usually sufficient to provide the
  starting stimulus
• In Multisim, some external stimulus is required
  to start the oscillation

6
Start-Up Gain

• Gain much larger than 1
• Starts oscillation with less stimulus
• Oscillation builds to desired output voltage
  faster
• Without some method of lowering the gain after
  reaching the desired output voltage, the output
  continues to grow until reaching amplifier
  saturation

7
Fixed-Gain Oscillators

• Gain slightly greater than 1
• May not start oscillating without external
  stimulus
• Amplifier saturation is used to limit gain
• When slight distortion is acceptable, additional
  gain limiting circuitry is not needed.

8
Wein-Bridge Oscillator

• Bandpass filter
• fr 1/2pRC
• Peak Vout at frVout/Vin 1/3
• Feedback phase shift 0

9
Wein-Bridge Oscillator

• Wein bridge is created with lead-lag (bandpass)
  positive feedback and resistive negative feedback
• Positive feedback Vout/Vin1/3
• Negative feedback sets Acl3, so net gain through
  bridge 1

10
Self-Starting Wein-Bridge

• Start-up requires loop gain gt3 to build output
  level
• Continuous oscillation without saturation
  requires gain 3
• An automatic gain control (AGC) is needed

11
Back-to-Back Zener Method

• At start-up, and as output builds, R3 is in
  series with R1, so AVgt3
• When voltage across R3gtVZ0.7 V, diodes short out
  drop across R3, limiting gain to AV3

12
JFET AGC

• Q1 operates in ohmic region
• Negative peak detector D1 and C3 provide Q1 bias
  to form voltage-variable resistance
• Rf is adjustable due to variances in gm of JFETs.

13
Phase-Shift Oscillator

• 3 RC hi-pass
• fr 1/2p 6RC when R1, 2, 3 and C1, 2, 3 are
  equal

• When each individual RC phase shift (?) 60,
  total network ? 180, fed to inverting input,
  makes total ? 0

• Attenuation B (Vout/Vin) 1/29, so Rf/R329 for
  unity gain of phase-shift network

14
Twin-T Oscillator

• Hi-pass and Lo-pass T filters in parallel form a
  notch filter