CBSE Class 12 · 2018 · SemiconductorsShort Answer

(a) Two diodes are used in a centre-tapped full-wave rectifier: the ac feeds a centre-tapped transformer, each secondary end connects to one diode, and the load is between the centre tap and the common output.
In one half-cycle $D_1$ is forward biased and conducts; in the next half-cycle $D_2$ conducts; both drive current through the load in the same direction, converting ac to unidirectional (dc) output.
(b) A NAND gate gives $Y=\overline{A\cdot B}$; its truth table is $00\to 1,01\to 1,10\to 1,11\to 0$, and its symbol is the AND-gate symbol with a small circle at the output.
$\boxed{\text{Full-wave rectifier; NAND: }Y=\overline{A\cdot B}.}$