I just saw a circuit in Forrest Mims book. It is a triangle wave converted into a sine, he just use two diode , so simple. But I can't understand.

 

I just saw a circuit in Forrest Mims book. It is a triangle wave converted into a sine, he just use two diode , so simple. But I can't understand.

It's only an approximate sine wave that is produced. The harmonic distortion in the waveform will be quite significant unless further filtering is used.

The reason why it works is because diodes have an exponential relation between voltage and current. The 10K resistor helps control the current into diodes. Note that the amplitude of input and output voltages are specified very carefully. These values are important to get the best approximation to a sinusoidal output. Also note that this circuit is going to be temperature dependent.