generators have a weakness in that the waveform of their output signal
normally cannot be modified. The circuit presented here makes it
possible to smoothly alter the waveform of a linearly rising and steeply
trailing saw-tooth signal through a symmetrical triangle-wave to a
slowly trailing, steeply rising linear sawtooth. The wanted waveform may
be selected independently of the frequency, which can also be varied
uniformly from 0.2 Hz to 8 kHz. At the same time, a rectangular signal
with variable duty cycle (also independent of frequency) is available at
the rectangular-signal output of the circuit.
The circuit consists of integrator IC1b, whose output is applied to
comparator IC1c. The output of the comparator is a rectangular signal
The output of IC1b is raised by amplifier IC1d to a level that allows
the full output voltage range of the operational amplifier to be used.
Op amp IC1a provides a stable virtual earth, whose level is set to half
the supply voltage with P1. The smooth setting of the frequency is made
possible by feedback of part of the output of the comparator to the
input of the integrator via P2. This preset is usually not provided in
standard triangle-wave generators. Network D1-R1-D2-R2-P3 makes it
possible to give integrator capacitor C3 different charging and
This arrangement enables the output signal at A1 and the duty cycle
of the rectangular wave signal at A2 to be varied. Varying the
amplification factor with P5 has no effect on the frequency set with P2.
The slope of the signal edges, the transient responses, and the output
voltage range (rail-to-rail or with some voltage drop) depend on the
type of op amp used. The TL084 used in the prototype offers a good
compromise between price and meeting the wanted parameters. The circuit
is best built on a small piece of prototyping board. The circuit draws a
current of not more than 12 mA.
Provides triangle-wave, sawtooth or rectangular signal
Waveform variable independently of frequency (triangle wave and sawtooth)
Duty cycle of rectangular signal can be set independently of frequency
Test and measurement
Summary of preset action:
P1 – sets virtual earth to a level equal to Ucc/2;
P2 – sets the frequency;
P3– sets the waveform;
P4 – sets the hysteresis of the comparator (frequency and amplitude of the triangle-wave signal)
P5 – sets the amplification of the triangle-wave and sawtooth signals.