we use diodes in series
The basic purpose of diode is limiting direction of current.
As we all know the current flow will be from + ve terminal to the -ve terminal. but during the ac current case, the polarity will vary continuously and if you have a circuit that needs a current to flow in a certain direction then you can use a diode to do the job.
The most common function of a diode is to allow the electric current to pass in one direction (called the forward direction of the diode), while blocking it on the reverse (vice versa). thus, the diode can be seen as an electronic version of the valve. this unidirectional behavior is called rectification, and is used to convert alternating current (AC) into direct current (dc). shape rectifier, the diode can be used for tasks such as extracting modulation from radio signals in the radio receiver.
However, the diode can have more complicated behavior than this simple on-off action, due to its nonlinear current voltage characteristics. the semiconductor diode begins to conduct electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction (the state at which the diode is said to bias forward). the voltage drop across the forward biased diode varies only slightly with the current, and is a function of temperature, this effect can be used as a temperature sensor or as a voltage reference.The basic purpose of diode is limiting direction of current.
The diode operates either in forward bias or reverse biased by the voltage applied across the terminals (anode & cathode). It has a 0.7 volt barrier voltage. This is if we apply above 0.7 on the anode then it will only be in forward bias or if -0.7 is applied to the cathode it will be in reverse biased condition.
So in our circuit you can operate it in any bias condition based on our needs. For example if you just want a positive cycle then it will be set forward bias, for negative cycle it will be set to reverse bias.
It can also be used as a peak detector that detects the max voltage value.