Transcript emt212_ch.4 oscillators

Chapter 4
Oscillator is an electronic circuit that generates a periodic
waveform on its output without an external signal
source. It is used to convert dc to ac.
The waveform can be sine wave, square wave, triangular
wave, and sawtooth wave.
Sine wave
Square wave
Sawtooth wave
Fig. 4-1: Basic oscillator concept showing three types of output waveforms.
Chapter 4
If the output signal varies sinusoidally, the circuit is
referred to as a sinusoidal oscillator.
If the output voltage rises quickly to one voltage level and
later drops quickly to another voltage level, the circuit is
referred to as a pulse or square-wave oscillator.
Chapter 4
Based on the waveform, oscillators are divided into
following two groups:
1. Sinusoidal (or harmonic) oscillators—which produce
an output having sine waveform.
2. Non-sinusoidal (or relaxation) oscillators—which
produce an output having square, rectangular or
sawtooth waveform or is of pulsa shape.
Oscillators are widely used in most communications
system as well as in digital systems, including
computers, to generate required frequencies and timing
signals.
Chapter 4
Feedback oscillators operation is based on the principle
of positive feedback, where a portion of the output
signal is fed back to the input.
There are five types of feedback oscillators to produce
sinusoidal outputs:
1. Phase-shift oscillator;
Using RC circuits
2. Wien bridge oscillator;
3. Colpitts oscillator;
Using LC circuits
4. Hartley oscillator;
5. Crystal oscillator;
Fig. 2 illustrates the creation of a loop in which causes
the signal reinforces its self and sustains a continuous
output signal. This phenomenon is called oscillation.
Chapter 4
The requirements for oscillation are:
 the loop gain βA is greater than unity;
 the phase-shift around the feedback network is 180o
(providing positive feedback).
+
Vi
-
+
A
+
Vo = AVi
-
β
+
Vf = β(AVi)
-
-
Fig. 2: Feedback circuit used as an oscillator.
Chapter 4
Fig. 3 shows a circuit containing three RC circuits in its
feedback network called the phase-shift oscillator. The
three RC circuits combine to produce a phase shift of 180o.
Fig. 3: Idealized phase-shift oscillator.
Chapter 4
This circuit is drawn to show clearly the amplifier and feedback
network.
Fig. 4: Phase-shift oscillator circuits: (a) FET version; (b) BJT version.
Chapter 4
The IC phase-shift oscillator consist of an inverting
amplifier for the required gain and three RC circuits for
the feedback network.
The inversion of the op-amp itself provides the additional
180o to meet the requirement for oscillation of a 360o phase
shift around the feed back loop.
Fig. 5: Phase-shift oscillator using an op-amp.
Chapter 4
The oscillation frequency in rad/sec can be calculated
using the following equation:
1
0.065
f 

RC
2 6 RC
1

29
(4-1)
Chapter 4
The Wien bridge oscillator is one of the more commonly
used low-frequency RC oscillators. It contains an opamp and two feedback networks.
Fig. 6: Wien bridge oscillator using an op-amp.
Chapter 4
Resistors R1 and R2 and capacitors C1 and C2 form the
frequency-adjustment elements.
Resistors R3 and R4 form part of the feedback path.
The op-amp output is connected as the bridge input at points
a and c. The bridge circuit output at points b and d is the input
to the op-amp.
Chapter 4
The analysis of the bridge circuit results in:
R3 R1 C2


R4 R2 C1
and the resonant frequency of the Wien bridge
oscillator is
1
fo 
2 R1C1 R2C2
(4-2)
Chapter 4
If, in particular, the values are R1 = R2 = R and C1 = C2
= C, the resulting oscillator frequency is
1
fo 
2 RC
and
R3
2
R4
(4-3)
Chapter 4
The Colpitts oscillator is a
type of oscillator that uses an
LC circuit in the feed-back
loop.
The feedback network is
made up of a pair of tapped
capacitors (C1 and C2) and
an inductor L to produce a
feedback necessary for
oscillations.
The output voltage is
developed across C1.
The feedback voltage is
developed across C2.
Fig. 7: Colpitts oscillator.
Chapter 4
For a Colpitts configuration, the oscillator frequency is
set by an LC feedback network and given as:
1
fo 
2 LCeq
where,
C1C2
Ceq 
C1  C2
(4-4)
Chapter 4
The Hartley oscillator is
almost identical to the
Colpitts oscillator.
The primary difference is
that the feedback network
of the Hartley oscillator
uses tapped inductors
(L1 and L2) and a single
capacitor C.
Fig. 8: Hartley oscillator.
Chapter 4
The oscillator frequency is given by
1
fo 
2 Leq C
with
Leq  L1  L2
(4-5)
Chapter 4
Most communications and digital applications require the
use of oscillators with extremely stable output. Crystal
oscillators are invented to overcome the output fluctuation
experienced by conventional oscillators.
Crystals used in electronic applications consist of a
quartz wafer held between two metal plates and
housed in a a package as shown in Fig. 9 (a) and (b).
Fig. 9: A quartz crystal.
Chapter 4
The quartz crystal is made of silicon oxide (SiO2) and
exhibits a property called the piezoelectric.
When a changing an alternating voltage
is applied across the crystal, it vibrates at
the frequency of the applied voltage. In
the other word, the frequency of the
applied ac voltage is equal to the natural
resonant frequency of the crystal.
The thinner the crystal, higher its
frequency of vibration. This phenomenon
is called piezoelectric effect.
Chapter 4
The crystal can have two resonant
frequencies;
R
One is the series resonance
frequency f1 which occurs when XL
= XC. At this frequency, crystal
offers a very low impedance to the
external circuit where Z = R.
L
C
The other is the parallel resonance
(or antiresonance) frequency f2
which occurs when reactance of
the series leg equals the
reactance of CM. At this frequency,
crystal offers a very high
impedance to the external circuit.
R
Fig. 10: Crystal impedance versus frequency.
CM
The crystal is connected as a series element in the feedback
path from collector to the base so that it is excited in the
series-resonance mode.
Chapter 4
The circuit was suggested by Pierce.
BJT
FET
Fig. 11: Crystal oscillator using a crystal (XTAL) in a series-feedback path.
Since, in series resonance, crystal impedance is the smallest
that causes the crystal provides the largest positive feedback.
Chapter 4
Resistors R1, R2, and RE provide a voltage-divider stabilized
dc bias circuit. Capacitor CE provides ac bypass of the
emitter resistor, RE to avoid degeneration.
The RFC coil provides dc collector load and also prevents
any ac signal from entering the dc supply.
The coupling capacitor CC has negligible reactance at circuit
operating frequency but blocks any dc flow between collector
and base.
The oscillation frequency equals the series-resonance
frequency of the crystal and is given by:
1
fo 
2 LCC
(4-6)
Chapter 4
The unijunction transistor
can be used in what is
called a relaxation
oscillator as shown by
basic circuit of Fig. 12.
The unijunction oscillator
provides a pulse signal
suitable for digital-circuit
applications.
UJT
Resistor RT and capacitor
CT are the timing
components that set the
circuit oscillating rate.
Fig. 12: Basic unijunction oscillator circuit.
Chapter 4
Fig. 13 shows three waveforms that can be expected from the
UJT oscillator.
Sawtooth wave appears
at the emitter of the
transistor. This wave
shows the gradual
increase of capacitor
voltage.
The oscillating frequency is calculated as follows:
Chapter 4
1
fo 
RT CT ln 1 / 1   
(4-7)
where, η = the unijunction transistor intrinsic stand- off
ratio.
Typically, a unijunction transistor has a stand-off ratio
from 0.4 to 0.6.