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Transcript
OSCILLATORS
OBJECTIVES :
• Understand sinusoidal oscillator circuits
and state their characteristics.
• Know types of sinusoidal oscillator
2
INTRODUCTION
The basic signal-generating source for
various applications in electronic circuits
is 'oscillator'.
It will change the dc to an ac signal and
can generate any frequency required by
the circuit.
3
2.1 Understand sinusoidal oscillator
circuits and state their characteristics
Block Diagram Of an oscillator
•Oscillators are devices that convert DC voltage
into ac voltage without any external source at a
particular frequency
4
Harmonic & Non-Harmonic An
Oscillators
• There are two types of an oscillators:
i. Harmonic oscillator - the sine wave.
ii. Non-harmonic oscillator - in the fourth wave, triangle
wave, etc..
 Nowadays, Non- harmonic oscillator become important
due to digital circuit mostly used in electrical equipment
and the circuits required timer device for the timing.
5
REQUIREMENT OF AN OSCILLATOR
CIRCUITS
The basic oscillator circuit consists of :
i. Amplifier
ii. Feedback
iii. Frequency generation circuit.
Block diagram of an oscillator
1) Amplifier

An “oscillator” is device that produces oscillations (back-and –
forth) charges-usually an electronic circuit that produces AC-from
a steady (DC) source of power.

The amplifier circuit requires a DC power supply source to bias
the transistor.
6
REQUIREMENT OF AN OSCILLATOR
CIRCUITS-contd
2) Feedback
 Feedback is a condition in which part of the output
signal supplied to the input.
 When the oscillator has no input signal, the feedback
signal is the input signal for the amplifier in the
oscillator circuit.
 There are two principles of the feedback, negative
feedback and positive feedback.
 Oscillators using the principle of positive feedback.
Figure 2.1.2a shows the basic block diagram of a
feedback system:
7
REQUIREMENT OF AN OSCILLATOR
CIRCUITS-contd
3) Frequency generation circuit
Feedback signal and the amplifier can not be sure of
the swing, it requires the control / frequency
generation and it usually placed in the feedback.
The frequency generation circuit of the oscillator in the
consumer products can divided into:
i. Oscillator that generates audio frequency
RC Oscillator Network (Resistance-Capacitor). It is to
produce medium and low frequency signals.
Examples of types of RC oscillator is the oscillator
phase shift and Wein bridge oscillator.
ii. Oscillator that generates radio frequency:
The oscillator LC network (inductor-capacitor). . It is to
produce high frequency signals (> 1MHz), and often it
produces a stable frequency.
Examples of the type LC oscillator is Armstrong
8
Oscillator, Colpitts, Hartley and Crystal.
Basic LC Oscillator Tank Circuit
• The circuit consists of an inductive coil, L and
a capacitor, C. The capacitor stores energy
in the form of an electrostatic field and which
produces a potential (static voltage) across
its plates, while the inductive coil stores its
energy in the form of an electromagnetic
field. The capacitor is charged up to the DC
supply voltage, V by putting the switch in
position A. When the capacitor is fully
charged the switch changes to position B.
The charged capacitor is now connected in
parallel across the inductive coil so the
capacitor begins to discharge itself through
the coil. The voltage across C starts falling as
the current through the coil begins to rise.
This rising current sets up an
electromagnetic field around the coil which
resists this flow of current. When the
capacitor, C is completely discharged the
energy that was originally stored in the
capacitor, C as an electrostatic field is now
stored in the inductive coil, L as an
electromagnetic field around the coils
windings.
Basic LC Oscillator Tank Circuit
• As there is now no external voltage in the circuit to maintain the current within
the coil, it starts to fall as the electromagnetic field begins to collapse. A back
emf is induced in the coil (e = -Ldi/dt) keeping the current flowing in the
original direction. This current now charges up the capacitor, C with the
opposite polarity to its original charge. C continues to charge up until the
current reduces to zero and the electromagnetic field of the coil has collapsed
completely. The energy originally introduced into the circuit through the
switch, has been returned to the capacitor which again has an electrostatic
voltage potential across it, although it is now of the opposite polarity. The
capacitor now starts to discharge again back through the coil and the whole
process is repeated. The polarity of the voltage changes as the energy is
passed back and forth between the capacitor and inductor producing an AC
type sinusoidal voltage and current waveform. This then forms the basis of an
LC oscillators tank circuit and theoretically this cycling back and forth will
continue indefinitely. However, every time energy is transferred from C to L or
from L to C losses occur which decay the oscillations
Resonant Frequency of a LC Oscillator
The frequency of the oscillatory voltage depends upon the value of
the inductance and capacitance in the LC tank circuit. For
resonance to occur in the tank circuit, there must be a frequency
point were the value of XC, the capacitive reactance is the same as
the value of XL, the inductive reactance (XL = XC).
Where:
 L is the Inductance in Henries
 C is the Capacitance in
Farads
 ƒr is the Output Frequency in
Hertz
2.2 Types of Sinusoidal oscillator
2.2.1a Amstrong Oscillator
 Resistors R1, R2 and R3 are provided bias voltage to the
transistor. Capacitors C1 and C2 are avoid the alternating
signal.
 The transformer Tr is to produce a phase shift of 180o to get
a feedback voltage in phase with the input transistors.
12
2.2.1a Amstrong Oscillator-contd
The ratio between the coil windings L1 to
L2 coils is the product of the gain, A, with
the feedback factor, , is a (| A | = 1).
 Example 1
if the amplifier gain is 10, the ratio of
windings must be
1: 10.
A  = 10 x 0.1 = 1
13
2.2.1(a) Amstrong Oscillator:
Frequency of Oscillation
 Oscillation frequency is determined by the circuit L2.C2
given by:
1
f 
2 L 2C2
 The ratio between the transformer windings is 1:10
and capacitors used in the resonant circuit is 50  F
Get the resonance frequency for this circuit.
f 
f 
1
2 L2C2
2
1
10 50 F 
f  7.14 Hz
14
2.2.1b Colpitts oscillator
2.1.1(b) Circuit Connection of Colpitts
oscillator:
 Colpitts oscillator using two capacitors and an inductor in
the frequency generation circuit
Figure 2.1.1b: Colpitts
oscillator
15
2.2.1b Colpitts oscillator:operation
• Transistors and resistors R1, R2, R3 and R4
are a combination of amplifier circuits.
Capacitors C3 and C4 are used to overtake the
signal s shuttle to earth.
•
The amplifier will provide different phase of the
output signal 180o. The LC circuit in the
feedback loop are produced a phase shift of
180o. Therefore, the feedback voltage will be in
phase with the input voltage on the transistor.
16
2.2.1b Colpitts oscillator-contd
Frequency of Oscillation:
 Oscillation frequency is determined by the circuit
LC given by:
1
fr 
2 LCT
 The capacitor in above circuit is connected in
series
C1.C2
CT 
C1  C2
17
2.2.1c Hartley oscillator
2.1.1(c) Circuit Connection of Hartley oscillator:
 Hartley oscillator using two inductors and a capacitor in
the frequency generation circuit
2.2.1c Hartley oscillator-contd
Frequency of Oscillation:
 Oscillation frequency is determined by the circuit
LC given by:
1
fr 
2 LT C
 The inductors in above circuit is connected in
series
LT  L1  L2
19
2.1.1(d) Crystal Oscillator:
 Oscillator is the most stable and precisely when using a
piezoelectric crystal in a feedback circuit. When the
alternating voltage is applied to the crystal produced the
mechanical vibrations.
 These vibrations has the natural resonant frequencies
are depend on the thickness of the crystal (The high
frequency, the thinner of the crystal).
Symbol Figure
Figure 2.1.1d Electric Circuits
connection
20
2.1.1(d) Crystal Oscillator:contd
•




Referring to the figure 2.1.1 d:
Lh: The equivalent electric of the crystals mass
Ch: The elasticity of crystals
Rh: The friction resistance in the crystal structure
Cm: The capacitance capacitor containers loaded crystals
* The f1 generated by the series circuit Rh-LH-Ch. f2
components of the same series reaktan, Cm.
z
Parallel resonant
Series resonant
f
f1
f2
21
2.1.1(d) Crystal Oscillator:CIRCUIT
Frequency of Oscillation:
Series resonance :
f1 
1
2 LhCh
Parallel resonance:
1
f2 
2 LhC
Figure 2.1.1 (d): Crystal Oscillator
Cm .Ch
C
Cm  Ch
22
2.1.1e OSCILLATOR PHASE SHIFT (RC):
• Phase shift oscillator consists of amplifier and feedback
network with three RC circuits (as shown in Figure
2.1.1e):
• Circuit connection of the Phase Shift Oscillator:
Circuit Operation:
The output signal from the amplifier of
180o out of phase with the input signal.
For a positive feedback signal, the
output signal phase shifted by 180o, so
that to be in phase with the input. RC
network produces a phase shift of 180o
23
The amount of actual phase shift in the circuit depends upon the values of the
resistor and the capacitor, and the chosen frequency of oscillations with the
phase angle ( Φ ) being given as:
Basic RC Oscillator Circuit
2.1.1e OSCILLATOR PHASE SHIFT (RC):
Frequency of Oscillation:
• If all the resistors, R and the capacitors, C
in the phase shift network are equal in
value, then the frequency of oscillations
produced by the RC oscillator is given as:
• Determine the frequency of oscillations of
a RC Oscillator circuit having 3-stages
each with a resistor and capacitor of equal
values. R = 10kΩ and C = 500pF
Answer:
1
fr 
2RC 2 N
1

2 10k 500 p  2  3
 13kHz