• Oscillators are electronic circuits that generate an output
signal without the necessity of an input signal.
• Different types of oscillators produce various types of
outputs including sine waves, square waves, triangular waves,
and sawtooth waves.
• Sinusoidal oscillator operation is based on the principle of
positive feedback, where a portion of the output signal is fed
back to the input in a way that causes it to reinforce itself and
thus sustain a continuous output signal.
2
Introduction
• An oscillator is a circuit that produces a periodic
waveform on its output with only the dc supply voltage as an
input.
• The output voltage can be either sinusoidal or non-
sinusoidal, depending on the type of oscillator.
• Two major classifications for oscillators are:
Feedback Oscillators and Relaxation Oscillators.
• Essentially, an oscillator converts electrical energy from
the dc power supply to periodic waveforms.
3
Introduction (cont.)
Figure 1. The basic oscillator concept showing three common types of output
waveforms: sine wave, square wave, and sawtooth.
4
Relaxation Oscillators
•Relaxation Oscillator uses an RC timing circuit to generate
a waveform that is generally a square wave or other non-
sinusoidal waveform.
• Typically, a relaxation oscillator uses a Schmitt trigger or
other device that changes states to alternately charge and
discharge a capacitor through a resistor.
5
Relaxation Oscillators (cont.)
It use an RC timing circuit and a device that changes states to
generate a periodic waveform and nonsinusoidal waveforms.
A Triangular-Wave Oscillator
The op-amp integrator can be used as the basis for a
triangular-wave oscillator.
• A dual-polarity, switched input is used.
• When the switch is in position 1, the negative voltage is
applied, and the output is a positive-going ramp.
• When the switch is thrown into position 2, a negative-
going ramp is produced. If the switch is thrown back and
forth at fixed intervals, the output is a triangular wave
consisting of alternating positive-going and negative-going
ramps.
A Practical Triangular-Wave Oscillator
One practical implementation of a triangular-wave oscillator
utilizes an op-amp comparator with hysteresis to perform the
switching function.
• Since the comparator produces a square-wave output, this
circuit can be used as both a triangular-wave oscillator and a
square-wave oscillator.
• Devices of this type are commonly known as function
generators because they produce more than one output
function.
• The output amplitude of the square wave is set by the output
swing of the comparator, and the resistors R2and R3 set the
amplitude of the triangular output
R3 R3
VUTP Vmax , VLTP Vmax
R2 2
R
• The comparator output levels -Vmax and +Vmax are equal.
• The frequency of both waveforms depends on the R1C time
constant and the amplitude-setting resistors R2 and R3.
• By varying R1 the frequency of oscillation can be adjusted
without changing the output amplitude.
1 R2
fr
4 R1C R3
Example
Determine the frequency of the oscillator. To what value
must R1 be changed to make the frequency 20 kHz?
Sol
1 R2 1 33
fr 8.25kHz
4 R1C R3 4 10k 0.01 F 10
1 R2 1 33
R1 4.13k
4 fC R3 4 20kHz 0.01 F 10
A Sawtooth Voltage-Controlled Oscillator (VCO)
The VCO is a relaxation oscillator whose frequency can be
changed by a variable dc control voltage.
• VCOs can be either sinusoidal or nonsinusoidal.
• To build a sawtooth VCO, an op-amp integrator that uses a
switching device (PUT) in parallel with the feedback
capacitor to terminate each ramp at a prescribed level and
effectively “reset” the circuit.
• The PUT is a programmable unijunction transistor with an
anode, a cathode, and a gate terminal. The gate is always
biased positively with respect to the cathode.
When VA exceeds VG by 0.7 V, the PUT turns on and acts as a
forward-biased diode.
When VA falls below this level, the PUT turns off.
• The operation of the sawtooth VCO begins when the –ve dc
input voltage, produces a +ve going ramp on the output, and
the circuit acts as a regular integrator.
• The PUT triggers on when the output ramp exceeds the VG
by 0.7 V.
• When the PUT turns on, the capacitor rapidly discharges.
• The capacitor does not discharge completely to zero because
of the PUT’s forward voltage, VF.
• Discharge continues until the PUT current falls below the
holding value where the PUT turns off and the capacitor
begins to charge again.
• The sawtooth amplitude and period adjusted by varying the
PUT gate voltage.
• The frequency of oscillation is determined by the RiC time
constant of the integrator and the peak voltage set by the PUT.
VIN
the charging rate of a capacitor is
Ri C
• The time it takes a capacitor to charge from VF to VP is the
period, T, (neglecting the rapid discharge time).
VP VF VIN 1
T f
VIN RiC Ri C VP VF
Example
(a) Find the amplitude and frequency of the sawtooth
output. Assume that the forward PUT voltage, VF is 1V.
(b) Sketch the output waveform.
Sol
find the gate voltage
R4 10k
VG V 15
R4 R3 20k
7.5V
The max peak (neglecting the 0.7 V).
VP 7.5V
The minimum peak value (low point) is VF=1V.
So the peak-to-peak amplitude is
VPP =VP –VF = 6.5V
R2 10k
VIN V 15 1.92V
R1 R2 78k
VIN 1 1.92 1
f 628Hz
Ri C VP VF 100k 0.0047 F 6.5
1 1
T 1.59 ms
f 628
A Square-Wave Oscillator
Is a type of relaxation oscillator because its operation is based
on the charging and discharging of a capacitor.
When the circuit is first turned on,
C is uncharged, and thus the inverting
input is at 0 V.
This makes the output a positive
maximum, and C begins to charge toward Vout through R1.
• When VC reaches VF, the op-amp switches to the maximum
negative state. At this point, the capacitor begins to discharge
from +VF toward -VF .
• This action continues to repeat, and a square-wave output
voltage is obtained.
The 555 Timer as an Oscillator
Astable Operation
(a free-running relaxation oscillator) (Astable
multivibrator)
• Notice that the threshold
(THRESH) is now connected
to the trigger (TRIG).
• The external components R1,
R2 and Cext form the timing
circuit that sets the frequency
of oscillation.
• Initially, when the power is turned on, the capacitor Cext is
uncharged and thus the trigger voltage (pin 2) is at 0 V.
• This causes the output of the lower comparator to be high
and the output of the upper comparator to be low, forcing the
output of the flip-flop, and thus the base of Qd low and keeping
the transistor off.
• Cext begins charging through R1 and R2. When the VC reaches
1⁄3VCC, the lower comparator switches to its low output state,
and when the capacitor voltage reaches 2⁄3VCC, the upper
comparator switches to high. This resets the flip-flop, and
turns on the transistor.
The frequency of oscillation
1.44
fr
R1 2R2 Cext
• By selecting R1 and R2 the duty cycle of the output can be
adjusted. Since Cext charges through R1 + R2 and discharges only
through R2 .
• The time that the output is high (tH) is how long it takes Cext
to charge from 1⁄3VCC to 2⁄3VCC. It is expressed as
tH 0.694 R1 R2 Cext
- The time that the output is low
t L 0.694 R2Cext
The period, T
T tH tL 0.694 R1 2R2 Cext
tH tH
Duty cycle 100% 100%
T tH tL
R1 R2
Duty cycle 100%
R1 2 R2
• To achieve duty cycles of less than 50 percent, the circuit
can be modified so that Cext charges through only R1 and
discharges through R2 . This is achieved with a diode, D1,
placed as shown.
The duty cycle can be made less than 50 percent by making
R1 less than R2. Under this condition,
1.44
fr
R1 R2 Cext
R1
Duty cycle 100%
R1 R2
Example
A 555 timer configured to run in the astable mode.
Determine the frequency of the output and the duty cycle.
Sol 1.44
fr
R1 2 R2 Cext
1.44
2.2 2 4.7 0.022 F
5.64kHz
R1 R2
Duty cycle 100%
R1 2 R2
2.2 4.7
100% 59.5%
2.2 9.4
Operation as a Voltage-Controlled Oscillator (VCO)
A 555 timer can be set up to operate as a VCO by using the
same external connections
as for astable operation, with
the exception that a variable
control voltage is applied to
The CONT input.
• The control voltage (VCONT) changes the threshold values of
1⁄3VCC and 2⁄3VCC for the internal comparators.
• With the control voltage, the upper value is VCONT and the
lower value is 1⁄2VCONT.
• An increase in VCONT increases charging and discharging
time of the external capacitor and causes the frequency to
decrease.
• An application of the VCO is in phase-locked loops.
