Chapter 16 Oscillators

1
Chapter 16Oscillators
2
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

3
Introduction
Oscillators are circuits that produce a
continuous signal of some type without the need
of an input. These signals serve a variety of
purposes. Communications systems, digital systems
(including computers), and test equipment make
use of oscillators.
4
The Oscillator
An oscillator is a circuit that produces a
repetitive signal from a dc voltage. The
feedback oscillator relies on a positive feedback
of the output to maintain the oscillations. The
relaxation oscillator makes use of an RC timing
circuit to generate a nonsinusoidal signal such
as square wave.
5
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.
6
Oscillators With RC Feedback Circuits
RC feedback oscillators are generally limited to
frequencies of 1 Mhz or less. The three types of
RC oscillators we will discuss are the
Wien-bridge, the phase-shift, and the twin-T.
7
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
8
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 band-pass with a narrow bandwidth
(high Q).
9
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.
10
Oscillators With RC Feedback Circuits
To start the oscillations an initial gain greater
than 1 must be achieved. The back-to-back zener
diode arrangement is one way of achieving this.
When dc is first applied the zeners appear as
opens. This allows the slight amount of positive
feedback from turn on noise to pass.
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Oscillators With RC Feedback Circuits
The lead-lag circuit narrows the feedback to
allow just the desired frequency of these turn
transients to pass. The higher gain allows
reinforcement until the breakover voltage for the
zeners is reached.
12
Oscillators With RC Feedback Circuits
Automatic gain control is necessary to maintain a
gain of exact unity. The zener arrangement for
gain control is simple but produces distortion
because of the nonlinearity of zener diodes. 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 feedback keeps
the gain at precisely 1.
13
Oscillators With RC Feedback Circuits
The phase shift oscillator utilizes three RC
circuits to provide 180º phase shift that when
coupled with the 180º of the op-amp itself
provides the necessary feedback to sustain
oscillations. The gain must be at least 29 to
maintain the oscillations. The frequency of
resonance for the this type is similar to any RC
circuit oscillator. fr 1/2??6RC
14
Oscillators With RC Feedback Circuits
The twin-T utilizes a band-stop 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.
15
Oscillators With LC Feedback Circuits
For frequencies above 1 Mhz, LC feedback
oscillators are used. We will discuss the
Colpitts, Clapp, Hartley, Armstrong, and
crystal-controlled oscillators. Transistors are
used as the active device in these types.
16
Oscillators With LC Feedback Circuits
The Colpitts oscillator utilizes a tank circuit
(LC) in the feedback loop. The resonant frequency
can be determined by the formula below. Since the
input impedance affects the Q, an FET is a better
choice for the active device. fr 1/2??LCT
17
Oscillators With LC Feedback Circuits
The Clapp is similar to the Colpitts with
exception to the additional capacitor in the tank
circuit. The same formula applies as for the
Colpitts.
18
Oscillators With LC Feedback Circuits
The Hartley oscillator is similar to the Clapp
and Colpitts. The tank circuit has two inductors
and one capacitor. The calculation of the
resonant frequency is the same.
19
Oscillators With LC Feedback Circuits
The Armstrong uses transformer coupling in the
feedback loop. For this reason the Armstrong is
not as popular.
20
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 its electrical equivalent.
21
Oscillators With LC Feedback Circuits
Since crystal has natural resonant frequencies of
20 Mhz or less, generation of higher frequencies
is attained by operating the crystal in what is
called the overtone mode. Overtones are usually
odd multiples of a crystals fundamental.
22
Relaxation Oscillators
Relaxation oscillators make use of an RC timing
and a device that changes states to generate a
periodic waveform. This triangular-wave
oscillator makes use of a comparator and
integrator to actually produce both a triangle
wave and square wave.
23
Relaxation Oscillators
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)
24
Relaxation Oscillators
The voltage-controlled sawtooth oscillators
frequency can be changed by a variable dc control
voltage. One possible type uses a programmable
unijunction transistor.
25
Relaxation Oscillators
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)
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Relaxation Oscillators
A square wave relaxation oscillator uses 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.
27
The 555 Timer As An Oscillator
The 555 timer is an integrated circuit that can
be used in many applications. We will discuss its
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.
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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 gt50 R1 R2/R1 2R2 x 100 Duty Cycle
lt50 w/diode R1/R1 R2 x 100
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The 555 Timer As An Oscillator
30
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).
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Summary

• Sinusoidal oscillators operate with positive
  feedback.

• Two conditions for oscillation are 0º feedback
  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.

32
Summary

• The crystal actually uses a crystal as the LC
  tank circuit and is very stable and accurate.

• A voltage controlled oscillators (VCO)
  frequency is controlled by a dc control voltage.

• A 555 timer is a versatile integrated circuit
  that can be used as a square wave oscillator or
  pulse generator.