Download Chapter 5 - Oscillators (Part 1)

CHAPTER 5 OSCILLATORS
Oscillators
Objectives
Describe the basic concept of an oscillator
Discuss the basic principles of operation of an
oscillator
Analyze the operation of RC, LC and crystal
oscillators
Describe the operation of the basic relaxation
oscillator circuits
Oscillators
Introduction
• An oscillator is a circuit that produces a periodic
waveform on its output with only the dc supply
voltage as an input.
• A repetitive input signal is not required except to
synchronize oscillations in some applications
• The output voltage can be sinusoidal or
nonsinusoidal,
oscillator.
depending
on
the
type
of
Oscillators
Oscillators
Two major classifications for oscillators are
feedback oscillators and relaxation oscillators.
• The feedback oscillator returns a fraction of
the output signal to the input with no net phase
shift, resulting in a reinforcement of the output
signal.
• The relaxation oscillator makes use of an RC
timing circuit to generate a waveform that is
generally square wave or other nonsinusoidal
waveform.
Oscillators
Feedback Oscillators
Feedback Oscillators
Positive feedback:
• is characterized by the condition wherein the portion
of the output voltage of an amplifier is fed to the input
with no net phase shift, resulting in a reinforcement of
the output signal.
In phase
• This is called “oscillation”.
If the feedback circuit
returns the signal out of
phase, an inverting
amplifier produces
positive feedback.
Vf
Av
Noninverting
amplifier
Feedback
circuit
Vo
Feedback Oscillators
Conditions for oscillation:
1. The phase shift around the feedback loop must be 0°.
2. The voltage gain ACL, around the closed feedback loop
(loop gain) must equal to 1 (unity).
Feedback Oscillators
Start-Up Conditions:
• When oscillation starts at t0, the condition ACL > 1 causes
the sinusoidal output voltage amplitude to build up to a
desired level.
• Then ACL decreases to 1 and maintains the desired
amplitude.
Oscillators
Oscillators with RC
Feedback Circuits
RC Feedback Circuits
Three types of feedback oscillators that use RC
circuits to produce sinusoidal outputs are:
• Wien-bridge oscillator – most widely used type
• Phase-shift oscillator
• Twin-T oscillator
The Wien-Bridge Oscillator
RC Oscillators
– Wien–bridge oscillator
• Wien-Bridge Oscillator is a lead-lag circuit.
• R1 and C1 form the lag portion and R2 and C2 form the lead portion
• At lower frequencies, the lead circuit dominates due to the high
reactance of C2.
•As the frequencies increases, XC2 decreases, thus allowing the output
voltage to increase.
V in
V out
RC Oscillators – Wien-bridge
The lead-lag circuit of a Wien-bridge oscillator
reduces the input signal by 1/3 and yields a
response curve as shown.
• The resonant frequency can be determined
by the formula below.
1
fr 
2RC
• Below fr, the lead circuit dominates and the
output leads the input. Above fr, the lag circuit
dominates and the output lags the input.
RC Oscillators – Wien-bridge
It is a low frequency oscillator which ranges from a few
kHz to 1 MHz.
Structure of this oscillator is shown below;
Voltagedivider
R1
–
Vout
R2
+
R3
C1
C2
R4
Lead-lag
network
RC Oscillators – Wien-bridge
The lead-lag circuit
is in the positive
feedback loop of
Wien-bridge
oscillator.
The
voltage
divider
limits the gain.
The lead lag circuit
is basically a bandpass with a narrow
bandwidth.
RC Oscillators – Wien-bridge
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.
RC Oscillators – Wien-bridge
To start the oscillations an initial loop gain
greater than 1 must be achieved.
RC Oscillators – Wien-bridge oscillator using backto-back zener diode
The back-to-back zener diode arrangement is
one way of achieving this.
D1
R1
D2
R3
+
V out
.
-
R2
f r Lead-lag
1/3
RC Oscillators – Wien-bridge
When dc is first applied the zeners appear
as opens. This allows the slight amount of
positive feedback from turn on noise to pass.
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.
RC Oscillators – Wien-bridge oscillator using a JFET
negative feedback loop
•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.
RC Oscillators – Wien-bridge
RC Oscillators – Wien-bridge
EXAMPLE
What is fr for the Wien bridge shown in figure below?
fr 
1
2πRC
1

2π  680 W  4.7 nF 
Rf
C1
10 kW
4.7 nF
R1
–
Vout
680 W
+
D1
Q1
= 48.9 kHz
R2
680 W
C2
4.7 nF
R3
1.0 kW
R4
10 kW
C3
1.0 mF
RC Oscillators – Phase-shift
The attenuation, B of the three-section RC feedback circuit is:
1
B
29
Where,
R3
B
Rf
RC Oscillators – Phase-shift
• Each of the three RC circuits in the feedback
loop can provide a maximum phase shift
approaching 90o.
• Oscillation occurs at the frequency where the
total phase shift through the three RC circuits is
180o.
• The frequency of oscillation is shown in the
following equation:
1
fr 
2 6 RC
where R1 = R2 = R3 = R and C1 = C2 = C3 = C
RC Oscillators – Phase-shift - EXAMPLE
(a) Determine the value of Rf necessary for the
circuit below to operate as an oscillator.
(b) Determine the frequency of oscillation
Answer: Rf = 290 kOhm, fr = 6.5 kHz
RC Oscillators – Twin-T Oscillator
RC Oscillators – Twin-T Oscillator
• Two T-type RC filters used in the feedback
loop
• One of the twin-T filters has a low-pass
response, and the other has a high-pass
response
• The combined parallel filters produce a
band-stop response with a center frequency
equal to frequency of oscillation, fr.