CBSE Class 12 · 2012 · Delhi · SemiconductorsLong Answer

(a) Full-wave rectifier: the ac is applied to a centre-tapped transformer; each secondary end feeds one diode, and the load $R_L$ is between the centre tap and the common output.
During the first half-cycle $D_1$ is forward biased and conducts; during the next half-cycle $D_2$ conducts; both send current through $R_L$ in the same direction, so output appears for both halves of the input (pulsating dc).
(b) For the given inputs, apply each rule interval by interval:
(i) OR gate, $Y=A+B$: output is high whenever at least one input is high, and low only when both A and B are low.
(ii) NAND gate, $Y=\overline{A\cdot B}$: output is low only when both A and B are high, and high in every other interval.
$\boxed{\text{OR: }Y=A+B;\text{NAND: }Y=\overline{A\cdot B}.}$