Application Notes (Continued)
10006081
FIGURE 16. Pulse Generator
10006086
When the output voltage VO is first at its high, VOH, the
capacitor C is charged toward VOH through R2. The voltage
across C rises exponentially with a time constant τ = R2C,
and this voltage is applied to the inverting input of the op
amp. Meanwhile, the voltage at the non-inverting input is set
at the positive threshold voltage (VTH+) of the generator. The
FIGURE 17. Waveforms of the Circuit in Figure 16
As shown in the waveforms in Figure 17, the pulse width (T1)
is set by R2, C and VOH, and the time between pulses (T2) is
set by R 1, C and VOL. This pulse generator can be made to
have different frequencies and pulse width by selecting dif-
ferent capacitor value and resistor values.
capacitor voltage continually increases until it reaches VTH+
,
at which point the output of the generator will switch to its
low, VOL (= 0V in this case). The voltage at the non-inverting
input is switched to the negative threshold voltage (VTH-) of
the generator. The capacitor then starts to discharge toward
VOL exponentially through R1, with a time constant τ = R1C.
When the capacitor voltage reaches VTH-, the output of the
pulse generator switches to V OH. The capacitor starts to
charge, and the cycle repeats itself.
Figure 18 shows another pulse generator, with separate
charge and discharge paths. The capacitor is charged
through R1 and is discharged through R2.
10006077
FIGURE 18. Pulse Generator
Figure 19 is a squarewave generator with the same path for
charging and discharging the capacitor.
19
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