Application Circuits
Square Wave Oscillator
(Continued)
Comparators are ideal for oscillator applications. This square
wave generator uses the minimum number of components.
The output frequency is set by the RC time constant of the
capacitor C
1
and the resistor in the negative feedback R
4
.
The maximum frequency is limited only by the large signal
propagation delay of the comparator in addition to any ca-
pacitive loading at the output, which would degrade the out-
put slew rate.
Capacitor C
1
must now discharge through R
4
towards
ground. The output will return to its high state when the volt-
age across the capacitor has discharged to a value equal to
V
a2
.
For the circuit shown, the period for one cycle of oscillation
will be twice the time it takes for a single RC circuit to charge
up to one half of its final value. The time to charge the ca-
pacitor can be calculated from
Where V
max
is the max applied potential across the capaci-
tor = (2V
cc
/3)
and V
C
= Vmax/2 = V
CC
/3
One period will be given by:
1/freq = 2t
or calculating the exponential gives:
1/freq = 2(0.694) R
4
C
1
Resistors R
3
and R
4
must be at least two times larger than
R
5
to insure that V
o
will go all the way up to V
cc
in the high
state. The frequency stability of this circuit should strictly be
a function of the external components.
Free Running Multivibrator
A simple yet very stable oscillator that generates a clock for
slower digital systems can be obtained by using a resonator
as the feedback element. It is similar to the free running mul-
tivibrator, except that the positive feedback is obtained
through a quartz crystal. The circuit oscillates when the
transmission through the crystal is at a maximum, so the
crystal in its series-resonant mode.
The value of R
1
and R
2
are equal so that the comparator will
switch symmetrically about +V
cc
/2. The RC constant of R
3
and C
1
is set to be several times greater than the period of
the oscillating frequency, insuring a 50% duty cycle by main-
taining a DC voltage at the inverting input equal to the abso-
lute average of the output waveform.
When specifying the crystal, be sure to order series resonant
with the desired temperature coefficient
DS100080-8
DS100080-24
FIGURE 5. Squarewave Oscillator
To analyze the circuit, assume that the output is initially high.
For this to be true, the voltage at the inverting input V
c
has to
be less than the voltage at the non-inverting input V
a
. For V
c
to be low, the capacitor C
1
has to be discharged and will
charge up through the negative feedback resistor R
4
. When
it has charged up to value equal to the voltage at the positive
input V
a1
, the comparator output will switch.
V
a1
will be given by:
If:
R
1
= R
2
= R
3
Then:
V
a1
= 2V
cc
/3
When the output switches to ground, the value of V
a
is re-
duced by the hysteresis network to a value given by:
V
a2
= V
cc
/3
DS100080-7
FIGURE 6. Crystal controlled Oscillator
9
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