Question 5.5: What does the circuit of Fig. 5-8 do?
What does the circuit of Fig. 5-8 do?
In most applications, zener diodes are used in voltage regulators, where they remain in the breakdown region. But there are exceptions. Sometimes, zener diodes are used in waveshaping circuits like Fig. 5-8.
Notice the back-to-back connection of two zener diodes. On the positive half-cycle, the upper diode conducts and the lower diode breaks down. Therefore, the output is clipped as shown. The clipping level equals the zener voltage (broken-down diode) plus 0.7 V (forward-biased diode).
On the negative half-cycle, the action is reversed. The lower diode conducts, and the upper diode breaks down. In this way, the output is almost a square wave. The larger the input sine wave, the better looking the output square wave.