Elec and Comp Tech 62B
Semiconductor Devices
Chapter 10
Oscillators and Timers
Feedback Oscillators
9/25/04
1
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
9/25/04
62Bchap10a
Page 2
Feedback Oscillator
Attenuation
sets loop gain
at 1
Net phase shift
at oscillation
frequency
equals zero
9/25/04
62Bchap10a
Page 3
Oscillator Start-Up
Some
initial stimulus is needed to
start oscillation
Start-up gain must be >1 for
oscillation to build to desired level
9/25/04
62Bchap10a
Page 4
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
9/25/04
62Bchap10a
Page 5
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
9/25/04
62Bchap10a
Page 6
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.
9/25/04
62Bchap10a
Page 7
Wein-Bridge Oscillator
Bandpass
filter
 fr
= 1/2πRC
 Peak Vout at fr
Vout/Vin = 1/3
Feedback
phase shift = 0
9/25/04
62Bchap10a
Page 8
Wein-Bridge Oscillator
 Wein
bridge is
created with leadlag (bandpass)
positive feedback
and resistive
negative feedback
 Positive feedback
Vout/Vin=1/3
 Negative feedback
sets Acl=3, so net
gain through bridge
=1
9/25/04
62Bchap10a
Page 9
Self-Starting Wein-Bridge
 Start-up
requires
loop gain >3 to
build output level
 Continuous
oscillation without
saturation requires
gain = 3
 An automatic gain
control (AGC) is
needed
9/25/04
62Bchap10a
Page 10
Back-to-Back Zener Method
 At
start-up, and as
output builds, R3 is
in series with R1, so
AV>3
 When voltage across
R3>VZ+0.7 V, diodes
short out drop
across R3, limiting
gain to AV=3
9/25/04
62Bchap10a
Page 11
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.
9/25/04
62Bchap10a
Page 12
Phase-Shift Oscillator
3
RC hi-pass
 fr
= 1/2π 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/R3=29 for unity gain of phase-shift
network
9/25/04
62Bchap10a
Page 13
Twin-T Oscillator
Hi-pass
and Lo-pass T filters in
parallel form a notch filter
9/25/04
62Bchap10a
Page 14