This circuit is suggested by National Semiconductor and is based on a double operational amplifier of the type LM358. The central frequency is given by C and the frequency variation is obtained when the input voltage goes from 0 to 2 volts. The power supply must be a simple 5 V and the transistor can be any general purpose NPN. Note that we have both an output with this triangular signal and an output with a rectangular signal. The theoretical maximum operating frequency of this circuit is 1 MHz, when the operating gain drops to 1.
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