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Chapter 4
Oscillators
Objectives
 Describe the basic concept of an oscillator
 Discuss the basic principles of operation of an oscillator
 Analyze the operation of RC and LC feedback oscillators
 Describe the operation of the basic relaxation oscillator
circuits
 Discuss the use of a 555 timer in an oscillator circuit
Introduction
to
oscillator
What is an oscillator ???
Oscillators are circuits that produce a continuous
signal of some type without the need of an input.
These signals serve a purpose for a variety of
purposes. Communications systems, digital systems
(including computers), and test equipment make
use of oscillators.
The types of Oscillator
An oscillator is a circuit that produces a repetitive signal from
a dc voltage.
The feedback type oscillator which rely on a positive
feedback of the output to maintain the oscillations.
The relaxation oscillator makes use of an RC timing circuit
to generate a non-sinusoidal signal such as square wave.
F/b type oscillator Vs relaxation oscillator

Both use active (transistor, op-amp) and
passive components (R,L,C)

F/b type oscillator



produces sine wave
f/b determines the oscillation frequency
Relaxation oscillator


produces non-sinusoidal wave (square etc)
use of an RC timing circuit to generate a nonsinusoidal signal
Feedback Oscillator Principles
The feedback oscillator is widely used for generation of
sine wave signals. The positive (in phase) feedback
arrangement maintains the oscillations. The feedback gain
must be kept to unity to keep the output from distorting.
Positive Feedback

In-phase portion of the output voltage of an amplifier is fed back to
the input with no net phase shift.

Vf - in-phase f/b voltage is amplified to produce the output voltage.

The loop –sustains the signal-produce continuous sinusoidal output
voltage. This phenomenon is called oscillation.
Conditions for oscillation

2 conditions

Phase shift around the f/b loop = 0 degree

The loop gain, Acl must equal to 1

See figure 16.4 p.g 776 (Floyd)
Feedback Oscillators

Two types :

RC - limited to frequencies of 1 MHz/less


Wien-bridge, the phase-shift, and the twin-T.
LC - for frequencies above 1 MHz.

Colpitts, Clapp, Hartley, Armstrong, and crystal
controlled oscillators
FEEDBACK OSCILLATORS:
Oscillators With
RC Feedback Circuits
Oscillators With RC Feedback Circuits
Since there is a loss of about 1/3 of the signal in the positive
feedback loop, the voltage-divider ratio must be adjusted
such that a positive feedback loop gain of 1 is produced. This
requires a closed loop gain of 3. The ratio of R1 and R2 can
be set to achieve this.
Oscillators With RC Feedback Circuits
The lead-lag circuit of a Wien-bridge oscillator reduces
the input signal by 1/3 and yields a response curve as
shown. The frequency of resonance can be determined
by the formula below.
fr = 1/2RC
Oscillators With RC Feedback
Circuits
The lead-lag circuit
is in the positive
feedback loop of
Wien-bridge
oscillator. The
voltage divider
limits gain. The
lead lag circuit is
basically a
bandpass with a
narrow bandwidth
(high Q).
Exercise 4.1 : Wien-bridge oscillator
Floyd page 781(Example 16.1)
Do it now!
Quiz / exercise?
Oscillators With RC Feedback Circuits
•To start the oscillations, initially the
close loop gain,Acl >3 must be
achieved.
•The back to back zener diode
arrangement are one way of
achieving this.
•Voltage divider circuit has been
modified
•When dc is first applied the zeners
appear as opens-R3 series with R1
• Increasing the Acl.
Oscillators With RC Feedback Circuits
• Zener arrangement for gain control is simple but produces
distortion because of the non-linearity of zener diodes.
•Automatic gain control (AGC) - maintain a gain of exact
unity.
•A JFET in the negative feedback loop can be used to precisely control
the gain. After the initial startup and the output signal increases the
JFET is biased such that the negative feed back keeps the gain at
precisely 1.
Gain is controlled by the components
in the colored box
Oscillators With RC Feedback Circuits
•The phase shift oscillator –
•3 RC circuits to provide 180º phase shift + 180º of the op-amp
itself provides 360º or 0º - to sustain oscillations.
•The close loop gain, ACL must be at least 29 to maintain the
oscillations.
•Attenuation of 3 RC feedback circuit, B = 1/29=R3/Rf
•The frequency of resonance for the this type is similar to any RC
circuit oscillator.
fr = 1/26RC
Oscillators With RC Feedback Circuits
•The twin-T – 2 types RC filters (LPF & HPF) used in feedback looputilizes a bandstop/notch arrangement of RC circuits to block all but the
frequency of operation in the negative feedback loop.
•The only frequency allowed to effectively oscillate is the frequency of
resonance, fr
•Oscillation only occur at fr where positive f/b through the voltage divider
exist. Negative f/b is negligible.
FEEDBACK OSCILLATORS:
Oscillators With
LC Feedback Circuits
Oscillators With LC Feedback Circuits
The Colpitts oscillator uses
a tank circuit (LC) in the
feedback loop-phase shift
and act as a resonant
filter.
The resonant frequency;
fr = 1/2LCT, CT= C1//C2
The resonant frequency with
loading of feedback cct;
fr = (1/2LCT)((Q2/Q2+1)),
CT= C1//C2
For Q less than 10

Oscillators With LC Feedback Circuits
The Clapp is similar to the
Colpitts except the
additional C in the tank
circuit. The same formula
applies as the Colpitts.
FET is replaced with
BJT-reduce the loading
effect of the transistor’s input
impedance
Oscillators With LC Feedback Circuits
The Hartley oscillator similar
to the Clapp and Colpitts.
The tank circuit has two
inductors and one
capacitor.
The calculation of the
resonant frequency is the
same.
Oscillators With LC Feedback Circuits
The Armstrong uses transformer coupling in the feedback
loop. For this reason the Armstrong is not as popular.
Oscillators With LC Feedback Circuits
The crystal-controlled oscillator is the most stable and
accurate of all oscillators. A crystal has a natural frequency
of resonance. Quartz material can be cut or shaped to have
a certain frequency. We can better understand the use of a
crystal in the operation of an oscillator by viewing it’s
electrical equivalent.
Oscillators With LC Feedback Circuits
Since crystal have
natural resonant
frequencies of 20 MHz
or less, generation of
higher frequencies are
attained by operating
the crystal in what is
called the overtone
mode. Overtones are
usually odd multiples of
a crystal’s fundamental.
Relaxation Oscillators
Relaxation oscillators make use of an RC timing
and a device that changes states to generate a
periodic waveform.
Used to produce nonsinusoidal waveforms
Relaxation Oscillators
 Triangular wave oscillator
 VCO
 Square wave
Relaxation Oscillators
Basic idea of Triangular wave oscillator
Dual polarity switched input (not for practical)
Position 1, negative voltage, positive going ramp
Position 2, Positive voltage, negative going ramp
Relaxation Oscillators
•
Practical circuit:
•
This triangular-wave oscillator makes use of
a comparator and integrator to actually
produce both a triangle-wave and squarewave.
Relaxation Oscillators
UTP-upper trigger point-comparator at max positive level
LTP-lower trigger point- comparator at max positive level
Output levels are set by the ratio of R2 and R3 times the
maximum output of the comparator. The frequency of
output can be determined by the formula below.
fr = 1/4R1C(R2/R3)
Relaxation Oscillators
•The voltage-controlled sawtooth oscillator’s
frequency can be changed by a varied by a given dc
control voltage. One possible type uses a
programmable unijunction transistor (PUT).
•Can be either sinus or non-sinusoidal O/p
•Consists of :•Op-amp integrator
•Switching device, PUT parallel with f/b Capacitor
(used to terminate the each ramp)
Relaxation Oscillators
•Anode voltage exceeds VG=0.7V, PUT turn on, FWD
biased diode
•If anode voltage less than VG=0.7V, PUT turn off.
•The forward voltage of the PUT (VF) determines the
frequency of the output. The formula below shows
the relationship.
f = \VIN\/RiC(1/Vp-VF)
Exercise 4.3 : Sawtooth VCO
Floyd page 796(Example 16.5)
Do it now!
Relaxation Oscillators
A square wave oscillator relaxation oscillator use the charging
and discharging of the capacitor to cause the op-amp to switch
states rapidly and produce a square wave. The RC time
constant determines the frequency.
The 555 Timer As An Oscillator
The 555 timer is an
integrated circuit that can
be used in many
applications. We will
discuss it’s operation as a
square wave oscillator.
The frequency of output
is determined by the
external components R1,
R2, and C. The formula
below shows the
relationship.
fr = 1.44/(R1 + 2R2)C
Detailed operation is
described within the text.
The 555 Timer As An Oscillator
Duty cycles can be adjusted by values of R1 and R2. The
duty cycle is limited to 50% with this arrangement. To have
duty cycles less than 50%, a diode is placed across R2. The
two formulas show the relationship. (see following slide)
Duty Cycle >50 % = R1 + R2/R1 + 2R2 x 100%
Duty Cycle <50 % w/diode = R1/R1 + R2 x 100%
The 555 Timer As An Oscillator
The 555 Timer As An Oscillator
The 555 timer by be operated as a VCO with a control
voltage applied to the CONT input (pin 5).
Summary
 Sinusoidal oscillators operate with positive feedback.
 Two conditions for oscillation are 0º feed back phase
shift and feedback loop gain of 1.
 The initial startup requires the gain to be
momentarily greater than 1
 RC oscillators include the Wien-bridge, phase shift,
and twin T.
 LC oscillators include the Colpitts, Clapp, Hartley,
Armstrong, and crystal.
Summary
 The crystal actually uses a crystal as the LC tank circuit
and is very stable and accurate.
 A voltage controlled oscillator’s (VCO) frequency is
controlled by a dc control voltage.
 A 555 timer is a versatile integrated circuit which can
be used a square wave or pulse generator.