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Oscillators

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Chapter 32 Oscillators Basics of Feedback Block diagram of feedback amplifier Forward gain, A Feedback, B Summing junction, Useful for oscillators Basics of ... – PowerPoint PPT presentation

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Title: Oscillators


1
Chapter 32
  • Oscillators

2
Basics of Feedback
  • Block diagram of feedback amplifier
  • Forward gain, A
  • Feedback, B
  • Summing junction, ?
  • Useful for oscillators


?
A
vin
vout
-
vF
B
3
Basics of Feedback
  • Op-amps
  • Inverting non-inverting
  • Negative feedback 180out of phase w/input
  • High input impedance
  • Low output impedance
  • Wide bandwidth
  • Stable operation

4
Basics of Feedback
  • Oscillators
  • Positive feedback
  • In-phase with input
  • Unstable

5
Basics of Feedback
  • Block diagram analysis


ve
?
A
vin
vout
-
vf
B
6
Basics of Feedback
  • Inverting amplifier

7
Relaxation Oscillator
  • Square wave generator
  • Composed of
  • Schmitt trigger comparator
  • Positive feedback
  • RC circuit to determine period

8
Relaxation Oscillator
  • Schmitt Trigger
  • R1 and R2 form a voltage divider
  • Portion of output applied at input
  • Hysteresis output dependent on input and
    previous value of input

9
Relaxation Oscillator
  • Schmitt Trigger
  • Hysteresis upper and lower trip points
  • Can use a voltage follower for adjustable trip
    points

10
Relaxation Oscillator
  • Schmitt trigger

11
Relaxation Oscillator
  • Schmitt Trigger Relaxation Oscillator

12
Relaxation Oscillator
  • R1 and R2 voltage divider
  • Capacitor charges through RF
  • VC lt VSAT then C charges toward VSAT
  • VC gt VSAT then C charges toward VSAT

13
Relaxation Oscillator
  • Schmitt Trigger Relaxation Oscillator Equations

14
Wien Bridge Oscillator
  • For a sinusoidal oscillator output
  • Closed loop gain 1
  • Phase shift between input and output 0 at
    frequency of oscillation
  • With these conditions a circuit
  • Oscillates with no external input
  • Positive feedback regenerative feedback

15
Wien Bridge Oscillator
  • Regenerative oscillator
  • Initial input is small noise voltage
  • Builds to steady state oscillation
  • Wien Bridge oscillator
  • Positive feedback, RC network branch
  • Resistor branch establish amplifier gain

16
Wien Bridge Oscillator
  • Circuit

17
Wien Bridge Oscillator
  • Equations

18
Wien Bridge Oscillator
  • Another form of Wien Bridge

19
Wien Bridge Oscillator
  • For a closed-loop gain, AB 1
  • Op-amp gain 3
  • Improved circuit
  • Separate RF into 1 variable and 1 fixed resistor
  • Variable minimize distortion
  • Zener Diodes limit range of output voltage

20
Phase-Shift Oscillator
  • Three-section R-C network
  • 60 per section
  • Negative FB 180
  • 180 (60 60 60) 360 Positive FB

21
Phase-Shift Oscillator
  • Circuit

22
LC Oscillators
  • LC circuits can produce oscillations
  • Used for
  • Test and measurement circuits
  • RF circuits

23
LC Oscillators
  • Named after pioneer engineers
  • Colpitts
  • Hartley
  • Clapp
  • Armstrong

24
LC Oscillators
  • Colpitts oscillator
  • fs series resonance
  • fp parallel resonance
  • L-C network ? 180 phase shift at fp

25
LC Oscillators
26
LC Oscillators
  • Equations

27
LC Oscillators
  • Hartley oscillator
  • Similar to Colpitts
  • L and Cs interchanged
  • Also have fs and fp

28
LC Oscillators
29
Crystal Oscillators
  • Quartz crystals
  • Mechanical device
  • Higher frequencies (gt1 MHz)
  • Stability
  • Accuracy
  • Reliability
  • Piezoelectric effect

30
Crystal Oscillators
  • Electrical model
  • Both have parallel and series resonance
  • Symbol
  • Quartz crystal
  • metal plates

31
Crystal Oscillators
  • Impedance varies with frequency
  • Square wave crystal oscillator circuit
  • Choose C1 and C2
  • Oscillation frequency between fs and fp

R2
R2
vout
CMOS Inverter
R1
XTAL
C1
C2
___ __ _
32
555 Timer
  • IC
  • Internal circuit

33
555 Timer
  • Usage
  • Monostable timing
  • Astable mode relaxation oscillator
  • Trigger voltage
  • Control voltage
  • Threshold voltage
  • R-S flip-flop

34
555 Timer
  • Relaxation oscillator

VCC 15 V
8
RA
4
7
NE555
RB
3
2 6
vout
1 5
C
0.01 µF
___ __ _
35
555 Timer
  • Monostable Circuit (one-shot)
  • Trigger high ? vout low
  • Trigger low ? vout high

VCC 15 V
8
RA
4
7
NE555
3
2 6
vout
C
1 5
0.01 µF
___ __ _
Trigger
___ __ _
36
Voltage Controlled Oscillator-VCO
  • ?fout ?vin

R1
VCC
Outputs
1 nF
6
8
Square wave
3
LM566C
5
Voltage Input
4
Triangle wave
1
7
vout
___ __ _
C1
___ __ _
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