(I) When both A and B are connected to ground, both diodes
(D1 and D2) are forward biased and
hence they conduct current. Consequently, the two diodes
are grounded and output voltage is zero. In terms of binary,
when A = 0 and B = 0, then Y = 0 as shows in the truth table.
(II) When A is connected to ground and B connected to the
positive terminal of the battery, diode D1 is
forward biased and diode D2 will not conduct.
Therefore diode D1 conducts and is grounded.
Again output voltage will be zero. In terms of binary, When
A = 0 and B = 1,then Y = 0 as
shows in the truth table.
(III) When both B is connected to the ground and A connected
to the positive terminal of the battery, the roles of diodes
are interchanged. Now diode D2 will conduct while
diode D1 dose not conduct. As a result, diode D2
is grounded and again output voltage is zero. In binary terms
when A = 1 and B = 0 then Y = 0 as shows in the truth table.
(IV) When both A and B are connected to the positive terminal
of the battery, both diodes don't conduct. Now, the output voltage
is +5V because there is no current through RL.
It is clear from the truth table that for
AND gate, the output is high if all the input are high. However,
the output is low if any or all input are low. Fig (c) shows
the logic symbol of the AND gate. This is the symbol you will
memories and use from now on for AND gates.