TL/H/8729
Precise Tri-Wave Generation LB-23
National Semiconductor
Linear Brief 23
March 1986
Precise Tri-Wave
Generation
INTRODUCTION
The simple Tri-wave generator has become an often used
analog circuit. Tri-wave oscillators are more easily designed,
require less circuitry, and are more easily stabilized than
sine wave oscillators. Further, the highly linear output of to-
days Tri-wave generators make them useful in many
‘‘sweep’’ circuits and test equipment.
This article describes a triangle wave generator with an eas-
ily controlled peak-to-peak amplitude. The positive and neg-
ative peak amplitude is controllable to an accuracy of about
g0.01V by a DC input. Also, the output frequency and sym-
metry are easily adjustable.
CIRCUIT DESCRIPTION
The Tri-wave oscillator consists of an integrator and two
comparatorsÐone comparator sets the positive peak and
the other the negative peak of the Tri-wave. To understand
the operation, assume that the output of the comparator is
low (b5V). Then b5.0V is applied through R1 to the input
of the integrator. The LM118 will integrate positive until its
output is equal to the positive reference on pin 9 of the
LM119. Since the comparator outputs are low, D1 is reverse
biased and the full output of the integrator is applied to the
non-inverting input of comparator A. As the integrator output
crosses the positive reference, comparator A switches
‘‘plus’’ and latches ‘‘plus’’ from positive feedback through
D1 and R4. Now the polarity of the current to the integrator
has changed and the integrator starts ramping negative.
When the output reaches the negative reference voltage,
comparator B swings negative. This forces the output of
comparator A negative, also, and stops the positive feed-
back through D1 from holding the comparator’s outputs
positive. Once the positive feedback loop is broken, the out-
puts of the comparators stay low. With the comparator’s
outputs low, the integrator ramps positive again.
The frequency of operation is dependent upon R1, C1 and
the reference voltages. Frequency is given by:
Fe5.0V
2R1 C1 (VREFabVREFb)
The maximum frequency of operation is limited by the circuit
delay to about 200 kHz. Also, the maximum difference in
reference voltages is 5.0V.
APPLICATIONS
Regular or op amp testing is made easier with precise trian-
gle waves. For example, IC voltage regulators are usually
specified to operate over a certain input voltage range such
as 7.0V to 25V. The Tri-wave generator can be set to deliver
a 0.7V to 2.5V output. This output is then amplified by a
factor of 10 by an op amp and used to sweep the regulator
input over its operating range. With op amps, the generator
can be used to sweep common mode voltages, power sup-
ply voltages, or even to test output swing. The output of the
device can be displayed on an oscilloscope and perform-
ance monitored over the entire operating range.
Another application is a voltage controlled oscillator. Since
the frequency depends on the input reference voltage, vary-
ing the reference varies the frequency. The useful VCO
range is about 2 decades. The output is then taken from the
comparators as the Tri-wave changes in amplitude.
Many sine wave oscillators use a non-linear network to con-
vert triangle wave to sines. It is usually necessary to set
triangle amplitude precisely for minimum distortion. If R1 is
replaced by a pot, frequency can be varied over at least 10
to 1 range without affecting amplitude.
Symmetry is also easily adjustable. Current can be injected
into the inverting input of the LM118 to change ramp time.
The easiest way to achieve this is to connect a 50 kXresis-
tor from the inverting input of the LM118 to the arm of a 1
kXpot. The ends of the pot are connected across the sup-
plies. Current from the resistor either adds or subtracts from
the current through R1, changing the ramp time.
TL/H/8729–1
FIGURE 1. Precision Tri-Wave Generator
C1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.
LB-23 Precise Tri-Wave Generation
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