These terms will have a slight error due to the fact that Vmax
is not exactly equal to 2/3 VCC but is actually reduced by the
diode drop to:
Application Circuits (Continued)
Pulse generator with variable duty cycle:
The pulse generator with variable duty cycle is just a minor
modification of the basic square wave generator. Providing a
separate charge and discharge path for capacitor C1 gener-
ates a variable duty cycle. One path, through R2 and D2 will
charge the capacitor and set the pulse width (t1). The other
path, R1 and D1 will discharge the capacitor and set the time
between pulses (t2).
By varying resistor R1, the time between pulses of the gen-
erator can be changed without changing the pulse width.
Similarly, by varying R2, the pulse width will be altered with-
out affecting the time between pulses. Both controls will
change the frequency of the generator. The pulse width and
time between pulses can be found from:
Positive Peak Detector:
Positive peak detector is basically the comparator operated
as a unit gain follower with a large holding capacitor from the
output to ground. Additional transistor is added to the output
to provide a low impedance current source. When the output
of the comparator goes high, current is passed through the
transistor to charge up the capacitor. The only discharge
path will be the 1M ohm resistor shunting C1 and any load
that is connected to the output. The decay time can be al-
tered simply by changing the 1M ohm resistor. The output
should be used through a high impedance follower to a avoid
loading the output of the peak detector.
DS100080-9
FIGURE 7. Pulse Generator
DS100080-17
FIGURE 8. Positive Peak Detector
Negative Peak Detector:
For the negative detector, the output transistor of the com-
parator acts as a low impedance current sink. The only dis-
charge path will be the 1 MΩ resistor and any load imped-
ance used. Decay time is changed by varying the 1 MΩ
resistor
Solving these equations for t1 and t2
DS100080-18
=
t1 R4C1ln2
FIGURE 9. Negative Peak Detector
=
t2 R5C1ln2
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