• Parul Poltehynic Institute
Subject Code : 3320701
Name Of Subject : Basic Electronics
Name of Unit : Ch-3 Oscillator’s
Topic : Oscillator’s
Name of Faculty : H.M.Avaiya
• Introduction
• 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.
• 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
Sub: Basic Electronics Topic: Oscillator’s
•Introduction
• 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
Sub: Basic Electronics Topic: Oscillator’s
Sub: Basic Electronics Topic: Oscillator’s
Oscillators
• Oscillation: an effect that repeatedly and regularly fluctuates about the mean value
• Oscillator: circuit that produces oscillation
• Characteristics: wave-shape, frequency, amplitude, distortion, stability
Sub: Basic Electronics Topic: Oscillator’s
Types of oscillators
1. RC oscillators– Wien Bridge– Phase-Shift
2. LC oscillators– Hartley– Colpitts– Crystal
3. Unijunction / relaxation oscillators
Sub: Basic Electronics Topic: Oscillator’s
•Basic principles for oscillation
A
b
V e
V f
V sV o
+(1) fse VVV (2) of βVV (3) osfseo βVVAVVAAVV
•An oscillator is an amplifier with positive feedback
Sub: Basic Electronics Topic: Oscillator’s
•Basic principles for oscillation
osfs
eo
βVVAVVA
AVV
oso VAAVV b
so AVVA b1
•The closed loop gain is:
Aβ
A
V
VA
s
of
1
Sub: Basic Electronics Topic: Oscillator’s
•Basic principles for oscillation
sβsA1
sAs
V
VsA
s
of
•In general A and are functions of frequency and thus may be written as;
sβsA is known as loop gain
Sub: Basic Electronics Topic: Oscillator’s
•Basic principles for oscillation
100 jωβjωA
•Thus, the condition for sinusoidal oscillation of frequency f0 is;
•This is known as Barkhausen criterion.• The frequency of oscillation is solely determined by the phase characteristic of the feedback loop – the loop oscillates at the frequency for which the phase is zero.
Sub: Basic Electronics Topic: Oscillator’s
•Basic principles for oscillation
In phase
Noninverting amplifier
V f V oA v
Feedback circuit
Sub: Basic Electronics Topic: Oscillator’s
R L1
L2
C
RC1
C2
L
•Hartley Oscillator •Colpitts Oscillator
T
oLC
1
1
2
RC
Cgm
21
21
CC
CCCT
CLL
o)(
1
21
2
1
RL
Lgm
Sub: Basic Electronics Topic: Oscillator’s
•Colpitts Oscillator
• 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.Sub: Basic Electronics Topic: Oscillator’s
•Colpitts Oscillator
Sub: Basic Electronics Topic: Oscillator’s
•Colpitts Oscillator
011
21
sCsL
VVg
R
V
sC
V ogsm
oo
• KCL at the output node
ogs VsL
sC
sCV
2
2
1
1
•voltage divider produces:
Sub: Basic Electronics Topic: Oscillator’s
•Colpitts Oscillator
01
1 122
2
sCR
LCssCgV mo
•substitute eq(2) into eq(1):
01
212
2
213
RgCCs
R
LCsCLCs m
•Assume that oscillation has started, then Vo≠0
Sub: Basic Electronics Topic: Oscillator’s
•Colpitts Oscillator
01
212
212
2
CLCCCj
R
LC
Rgm
• Let s=jω
21
21
1
CCCC
L
o
• Both real & imaginary component must be zero Imaginary component
Sub: Basic Electronics Topic: Oscillator’s
•Colpitts Oscillator
Rg
R
LCm
122
•both real & imaginary component must be zeroImaginary component
RgC
Cm
1
2
•Combining Eq(3) and Eq(4):
1
2
C
CRgm
•to initiate oscillations spontaneously:
Sub: Basic Electronics Topic: Oscillator’s
•Hartley Oscillator
• 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.
Sub: Basic Electronics Topic: Oscillator’s
•Hartley Oscillator
Sub: Basic Electronics Topic: Oscillator’s
•Hartley Oscillator
CLLo
21
1
•the analysis of Hartley oscillator is identical to that Colpitts oscillator.the frequency of oscillation:
Sub: Basic Electronics Topic: Oscillator’s