If two Zener diodes are connected in series and if there is an ac input voltage ,how varies the output voltage ?

 

Show us the circuit.

 

 

You should get a limited sinewave with the limits at +(5.1V+0.7V) and -(3.3V+0.7V), roughly.

 

What you need to do (other than just parroting some answer to your homework) is develop the DC transfer characteristic for the circuit. To do this, you need to consider what the output voltage is for different states of the two diodes.

 

To add to WBahn's post, look at what the voltage is across the two diodes as the input voltage changes over the input voltage range and then what the diodes do as the voltage goes above and below the zener voltages.

Note that one diode is always reverse biased while the other is forward biased and that a zener diode in the forward direction looks like a normal diode

 

You should get a limited sinewave with the limits at +(5.1V+0.7V) and -(3.3V+0.7V), roughly.

This is a Homework Help section not a Homework Do.
It's preferred that we let the op try to solve the problem with hints from us rather than giving him the answer.

 

This is a Homework Help section not a Homework Do.
It's preferred that we let the op try to solve the problem with hints from us rather than giving him the answer.

Sorry.
I must admit I expected the OP to come back with something like "what are those voltages?" Also must admit I forgot it was the homework forum.