The XNOR gate symbol looks
just like that for the XOR except that outputs are the
opposite of the XOR outputs, so it has a bubble
(meaning; inverted). The truth table shows
that if both inputs are the same, the output is High,
the opposite of XOR operation. The
equivalent electrical circuit validates the truth table.
With switches S1
and S2 in either
positions A and B, the lamp lights and
the output is High. If S1 is in position A while S2
is in the position B, however, no current can
flow, the lamp is off, and the output is Low.
A similar output condition results when S1
is in position A and S2
is in position B.
The timing diagram tells us that
the output will be High anytime both inputs are either
Low or they are High; otherwise the output is Low.
The transistor equivalent circuit
is that for the XOR gate with Q3,
Q4 and R5
removed.
If both inputs are Low, Q1
and Q2 remain off
and output X is pulled High through R3. If
both inputs are High, the transistors are still cut-off
and the output X remains High. With
either input High, however, a respective transistor turns
on and the output at X is brought
Low.
The Boolean expression states;
"Perform an XOR operation and invert the result".