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Sawtooth wave
generators using opamp are very common. But the disadvantage is
that it requires a bipolar power supply.
A sawtooth
wave generator can be built using a simple 555 timer IC and a
transistor as shown in the circuit diagram.
The working
of the circuit can be explained as follows:
The part of the circuit consisting of the capacitor C,
transistor,zener diode and the resistors form a constant current
source to charge the capacitor. Initially assume the capacitor is
fully discharged. The voltage across it is zero and hence the
internal comparators inside the 555 connected to pin 2 causes the
555's output to go high and the internal transistor of 555
shorting the capacitor C to ground opens and the capacitor starts
charging to the supply voltage. As it charges, when its voltage
increases above 2/3rd the supply voltage, the 555's output goes
low, and shorts the C to ground, thus discharging it. Again the
555's output goes high when the voltage across C decreases below
1/3rd supply. Hence the capacitor charges and discharges between
2/3rd and 1/3rd supply.
Note that the
output is taken across the capacitor. The 1N4001 diode makes the
voltage across the capacitor go to ground level (almost).
The frequency
of the circuit is given by:
f =
(Vcc-2.7)/(R*C*Vpp)
where:
Vcc= Supply
voltage.
Vpp= Peak to peak voltage of the output required.
Choose proper R,C,Vpp and Vcc values to get the required 'f'
value.
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