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
___ __ _