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Two Zener Diodes that are End to end.
The circuit preforms clipping because the Zener diodes ‘clip’ away a portion of an input signal without distorting the remaining part of the applied waveform, clipping occurs because the maximum output voltage limit is achieved.
In this case the Zener diodes at 0V has a reverse bias (Vz), this means no current is in the circuit, leading to no voltage drop in the 100Ω resistor (R1). However, when the AC voltage is constantly increased to reach approximately 3 V (or -3 depending on the time) the clipping starts, which is the exact potential needed for the Zener diodes to reach forward bias (Vf), and that is when the Zener diode allows current to pass through R1 resistor with respect of keeping the Voltage constant at 3 V.
This happens when the AC voltage signal is at its positive and negative half of the cycle, thus having two limitations from both domains of the y- axis (i.e. the Voltage). This is due to the two Zener diodes connected in series and at similar poles (+ to +), meaning one diode will let the current flow in normally through being reversed biased at the first half cycle, leaving the other diode to do the forward bias when the voltage is positive , and vice versa, when the voltage supplied is negative.
Note: as you increase the voltage amplitude clipping remains constant.
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