Triangular Wave Generator
Discussion
A triangular waveform is of some interest for several purposes. It has been starting waveform, for
example, for conversion to a sinusoidal waveform because of its relatively small harmonic content and
comparative simplicity in generating. There are several ways of generating a triangular waveform.. The
circuit diagram below is that of relaxation oscillator to be designed for the current project.
The central concept involved here is to charge and discharge a capacitor using (nearly) constant current
sources. The reference current source is provided by the diode-connected transistor Q1, and is mirrored
using Q2, Q3, and Q4 to provide respectively the two currents I and 2I, as illustrated. Note that Q2 and
Q3, presumed to be ‘identical’, are paralleled to provide 2I. The current 2I then is mirrored using Q5
and the diode-connected Q6, to provide a ‘push-pull’ arrangement for alternately charging and
discharging the capacitor C.
Q7 acts as a switch which when ON (saturated) essentially bypasses the Q5-Q6 mirror, so that C is
charged (linearly) by the constant current I. When the Q7 switch is OFF (cutoff) C is discharged
linearly by a current 2I – I = I.
An inverting Schmitt Trigger using the triangular output from the capacitor to generate a square wave
switching voltage for Q7.. Note the use of the diode D in the Schmitt Trigger feedback path to set
different threshold trip voltages.
Project Specification
a) Assume a square-wave pulse with a one-millesecond period drives Q7. Specify the current mirror
circuitry and C to produce a triangular waveform with a nominal five-volt peak. You may find it useful
to verify circuit operation separately for this section of the circuit, using the VPULSE part in PSpice.
Triangle Waveform Generator Project
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M H Miller
b) Design the inverting Schmitt trigger to have nominal trip voltages of 5 volts and zero. You may want
to verify circuit operation separately for this unit as well, using the VPULSE part, (Use TR = TF = 1ms,
PW=1u, and PR = 1.001ms.)
Note: Use 2N3904 and 2N3906 transistors available in the evaluation version of PSpice. Use a 741
opamp, or preferably (faster switching) an LM111 comparator. Your report should explain clearly and
completely, in appropriate technical language and detail, how and why various component values are
selected. Use PSpice to evaluate your design, relating expected performance to computed performance.
While it is not necessary to implement it specifically your report should describe how you would
provide a means of varying the waveform period.
Triangle Waveform Generator Project
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M H Miller