Transcript ppt - EC - Unit 6 - sine wave -1

Part 1
Unit 6
Sinusoidal Oscillators, Waveshaping Circuits
Objectives:
• Sinusoidal Oscillators
–
–
–
–
–
–
–
Classification of oscillators
Conditions for oscillations
Barkhausen’s criteria
Types of oscillators
Crystal oscillators
Voltage-controlled oscillators
Frequency stability
• Wave-shaping circuits
–
–
–
–
RC, RL low-pass & high-pass circuits
RC, RL integrator & differentiator circuits
Multivibrators
IC Multivibrators
Sinusoidal Oscillators:
Classification:
Oscillator (AC generating circuits)
Sinusoidal:
Generate sine wave
Non-Sinusoidal
(Multivibrators):
Generate square wave or
pulsed waveforms
TON << TOFF
12.2 Conditions for oscillations:
Barkhausen's Criterion
Types of feedback systems:
• Negative feed back system
• Positive feed back system
--------Notes:
β is gain of frequency selective feedback
network, which uses resonant circuit.
Negative feedback system:
Positive feedback system:
Barkhausen's Criterion:
Initialization of oscillations:
• Generation of oscillations is initialized due to
some inevitable noise at input
• The amplified output due to noise has all
frequency components
• Feedback network is frequency selective
• Barkhausen’s criterion is satisfied
• Oscillations start
12.3 Types of oscillators:
• RC oscillators
• LC oscillators
• Crystal oscillator
RC oscillators:
• A single RC or RL section provides a maximum
of 90° phase shift
• Multiple RC sections are used to provide the
required phase shift
– 3 RC sections  3 x 60° =180°
• CE amplifier  180°
• Total 180 + 180 = 360°
LC oscillators:
• A single LC section provides 180°
• CE amplifier provides 180 °
• Total 360 °
15.15 Crystal oscillator:
• A quartz crystal with the desired value of the
resonant frequency forms the frequencyselective feedback network
• More accuracy and stability
AC equivalent circuit of Quartz crystal:
crystal
AC equivalent circuit
Resonant frequencies:
• Series resonant frequency:
1
fs = -------------------2 Π  LCS
• Parallel resonant frequency:
1
fp = -------------------2 Π  LCp
Where Cp = (CM x CS) / (CM + CS) in the above figure
Crystal controlled Colpitt oscillator:
Resonant /
tank circuit
Crystal in
feedback path
to control
frequency
Crystal based Colpitt oscillator:
Resonant /
tank circuit
Crystal in
resonant / tank
track to decide
frequency
12.16 Voltage controlled oscillator (VCO):
• A VCO is an oscillator circuit in which the
frequency can be varied by an applied voltage
• This is achieved by “varicap”, whose
capacitance varies with applied voltage.
• Varicap is used in tank circuit, which
determines frequency
Voltage-controlled Hartley oscillator:
•Varicap used as voltage
controlled capacitance in
resonant / tank circuit
•FET used as common-drain
amplifier (Av little < 1)
•Feedback factor β is large
•To avoid noise effect 2
varicaps are used
•Fine tuning the frequency is
easy with VCO
Feedback
path
12.17 Frequency stability:
• Oscillators ability to maintain constant
frequency, for as long a period as possible
• Frequency depends on: (large set of elements)
– Circuit components
– Stray elements (inter-electrode reactance)
– Supply voltages
– Active device’s characteristics
• But largely frequency depends on RC or LC
values (small set of elements)
Frequency stability criterion:
θ=phase-shift
Ω=2Πf=frequency
• A small set of elements (RC / RL) introduces a
large change of phase-shift dθ for a given
change in frequency dω, then higher the value
of
dθ
------,
dω
more will be the dependence on the these (RC
/ RL) circuit features
• When
dθ
-----dω
approaches infinity, ω is independent of other
circuit features (large set of elements)
• More the above ratio more the stability
13.1 Basic RC low pass circuit:
Xc = R
1
XC = ----------------2Πf C
Step & pulse response of LP circuit:
13.2 RC low pass circuit as integrator:
Basics / fundas:
1. C=Q / V  C= dq / dv ---- in integral form
2. I=Q / T  i = dq / dt
from 1  dv = dq / C
from 2 
= (1/C) i dt
1
v = ---- ∫ i dt
C
Positive feedback system:
Barkhausen's Criterion:
Initialization of oscillations:
• Generation of oscillations is initialized due to
some inevitable noise at input
• The amplified output due to noise has all
frequency components
• Feedback network is frequency selective
• Barkhausen’s criterion is satisfied
• Oscillations start
12.3 Types of oscillators:
• RC oscillators
• LC oscillators
• Crystal oscillator
RC oscillators:
• A single RC or RL section provides a maximum
of 90° phase shift
• Multiple RC sections are used to provide the
required phase shift
– 3 RC sections  3 x 60° =180°
• CE amplifier  180°
• Total 180 + 180 = 360°
LC oscillators:
• A single LC section provides 180°
• CE amplifier provides 180 °
• Total 360 °
15.15 Crystal oscillator:
• A quartz crystal with the desired value of the
resonant frequency forms the frequencyselective feedback network
• More accuracy and stability
AC equivalent circuit of Quartz crystal:
crystal
AC equivalent circuit
Resonant frequencies:
• Series resonant frequency:
1
fs = -------------------2 Π  LCS
• Parallel resonant frequency:
1
fp = -------------------2 Π  LCp
Where Cp = (CM x CS) / (CM + CS) in the above figure
Crystal controlled Colpitt oscillator:
Resonant /
tank circuit
Crystal in
feedback path
to control
frequency
Crystal based Colpitt oscillator:
Resonant /
tank circuit
Crystal in
resonant / tank
track to decide
frequency
12.16 Voltage controlled oscillator (VCO):
• A VCO is an oscillator circuit in which the
frequency can be varied by an applied voltage
• This is achieved by “varicap”, whose
capacitance varies with applied voltage.
• Varicap is used in tank circuit, which
determines frequency
Voltage-controlled Hartley oscillator:
•Varicap used as voltage
controlled capacitance in
resonant / tank circuit
•FET used as common-drain
amplifier (Av little < 1)
•Feedback factor β is large
•To avoid noise effect 2
varicaps are used
•Fine tuning the frequency is
easy with VCO
Feedback
path
12.17 Frequency stability:
• Oscillators ability to maintain constant
frequency, for as long a period as possible
• Frequency depends on: (large set of elements)
– Circuit components
– Stray elements (inter-electrode reactance)
– Supply voltages
– Active device’s characteristics
• But largely frequency depends on RC or LC
values (small set of elements)
Frequency stability criterion:
θ=phase-shift
Ω=2Πf=frequency
• A small set of elements (RC / RL) introduces a
large change of phase-shift dθ for a given
change in frequency dω, then higher the value
of
dθ
------,
dω
more will be the dependence on the these (RC
/ RL) circuit features
• When
dθ
-----dω
approaches infinity, ω is independent of other
circuit features (large set of elements)
• More the above ratio more the stability
13.1 Basic RC low pass circuit:
Xc = R
1
XC = ----------------2Πf C
Step & pulse response of LP circuit:
13.2 RC low pass circuit as integrator:
Basics / fundas:
1. C=Q / V  C= dq / dv ---- in integral form
2. I=Q / T  i = dq / dt
from 1  dv = dq / C
from 2 
= (1/C) i dt
1
v = ---- ∫ i dt
C
Numerical:
13.3 Basic RC high pass circuit:
13.4 RC high pass circuit as differentiator:
13.5 Basic RL circuit as integrator:
13.5 Basic RL circuit as differentiator:
13.9 Multivibrators:
Multivibrator is a non-sinusoidal oscillator circuit
with regenerative feedback with pulsed
waveform.
Types:
1. Bistable
2. Monostable
3. Astable
Chapter 11: Oscillators
The 555
Timer:
555 is a IC (integrated circuit) that is used to generate –
1.
Accurate time delay (from
µS to hours)
–
Monostable Operation (one
stable state)
2.
Rectangular signal
–
Astable Operation (no stable
states)
Both are non-stable states
Non-stable state
Stable state
Here triggered
This is Pulse Waveform
Need not be triggered
This is Rectangular Waveform
Note: 1. Stable state = does not change, unless triggered.
2. Non-stable state = change state after fixed (designed) time.
Chapter 11: Oscillators
The 555
Timer:
Refer
next slide
for SR FF
Functional Block Diagram: