Zener Diode Adjustable Equivalent Circuit

But the ripples are not well attenuated as they would be with a zener. I suggest you split the 10K resistor into 2x4.7k resistors and add a capacitor with one end to the central tap the other end to ground. The cap value should be around 1uF-100uF. This way you sorta filter the ripples before they reach the regulator.

As I changed the simulation time I did not see any ripple on the 1.5 volt output.

Because the DC supply in EC is ideal. That means completely rippleless and with infinite current capabilities. In real life you usually use a transformer to step down the voltage, then a rectifier and some filters but in the end no matter the filtration there are ripples, that get bigger as the load increases (as it is in amplifiers, where the consumption is not constant). Anyway these ripples can be as much as a few volts even with heavy filtering, so sensitive electronics like preamps, oscillators and such can be quite susceptible to noise and errors. By using filtering and zener regulation you you smoothen these ripples for the sensitivity equipment to work properly. This is how you can simulate this easily. Replace the DC supply with an AC. Set the AC supply parameters as it follows. Frequency 50-60Hz. Amplitude 1-3V. DC voltage - 15-20V.

Set the frequency to 100Hz - 120Hz .

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