1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. Oscillators
Chapter 23

3. Sinusoidal oscillation requires both the correct phase and loop gain.
 = 0 = positive feedback A
vout B
AB = 1
AB < 1
AB > 1

4. Sinusoidal oscillators
The starting signal is thermal noise.
AB > 1 at startup (AB is the loop gain).
The feedback network determines B and the phase of the feedback.
Only one frequency arrives at the input as an in-phase signal (positive feedback).
Either A or B is eventually decreased so that AB = 1.

5. equilibrium is reached
Wien-Bridge oscillator
vout
2pRC 1
fr = RL
2R’ R’
Tungsten
lamp
Resistance of lamp
increases until
equilibrium is reached R’ C R R C
vout t

6. Colpitts oscillator +VCC C2 Amin = C1 C1 B = C2 RFC R1 vout C3 C1 LRE CE 2p LC 1
fr =

7. Colpitts CB oscillator
+VCC C1
C1 +C2
B =
C1 C2
C1 +C2
C =
RFC R1 2p LC 1
fr = C4 C1 C3 L RL C2 R2 RE C1
Amin =
C1 +C2

8. Hartley oscillator +VCC L1 Amin = L2 L2 B = L1 RFC R1 vout C L1
L = L1 + L2 R2 R3 L2 2p LC 1
fr =

9. Crystal-controlled oscillators are used
when frequency stability is important.
Quartz crystal
Slab cut from
crystal
Schematic
symbol
Electrodes
and leads

10. Crystal oscillator
+VCC
RFC R1
vout C1
Xtal C2 R2 R3 CE

11. Crystals
The fundamental frequency (series resonance) is controlled by the slab thickness.
Higher multiples of the fundamental are called overtones.
The electrode capacitance creates a parallel resonant frequency which is slightly higher.
Typical frequency accuracy is measured in parts per million (ppm).

12. Monostable operation of the 555 timer IC
+VCC T
T = trigger 8 3 2
vout T
555 1 W
vout

13. Astable operation of the 555 timer IC
+VCC 8 3
555
vout 1

14. 555 +VCC 8 Discharge 7 5 kW Threshold 6  VCC S Q Control 5 kW 3 5 R Q
UTP
 VCC S Q
Control
5 kW 3 5 R Q
LTP
 VCC
Out
Trigger 2
5 kW 1
Gnd 4
Reset

15. A discrete RS flip-flop
+VCC S R RC RC Q Q Q Q RB RB RS RR S R

16. 555 IC configured for monostable operation
+VCC 8
W = 1.1RC R 7 6 W C 3 2
 VCC 1

17. 555 IC configured for astable operation
+VCC
W = 0.693(R1 + R2)C 8 7
R1 + R2 R2
D =
R1 + 2R2 6 3 2 W C 1 T
T = 0.693(R1 + 2R2)C

18. 555 voltage-controlled oscillator
+VCC
VCC - Vcon
W = -(R1 + R2)C ln 8
VCC - 0.5Vcon 7
Vcon applied to pin 5 R2 6 5 3
W R2C 1
f = 2 C 1
+Vcon
+ Vcon

19. The output frequency is established by the input clock.
Pulse-width modulation with the 555 timer IC T
The output frequency is
established by the input clock.
+VCC 8 4 3
UTP =  VCC + vmod
W = -RC ln
1 -
UTP
VCC
( ) W R 7
PWM
out
555 A
Modulating
signal in 6 B 5 C 2 1
Clock in

20. The leading edge of each pulse is a function of the modulation.
Pulse-position modulation with the 555 timer IC
The leading edge of each pulse
is a function of the modulation. R1
+VCC
Pulse width is variable 8 4 3 7
PPM
out
555 R2
Space is constant A 6 C 5
Modulating
signal in 2 1 B
Space = 0.693R2C

21. Phase-locked loops
It is possible to phase-lock an oscillator to a signal by using a phase detector and negative feedback.
PLLs can be used to remove noise from a signal.
PLLs can be used to demodulate an FM signal.
PLLs are available as monolithic ICs.