The XOR gate symbol is
similar to that used for an OR gate, with the additional
of a 'rear curve' on the input end. The truth table tells
us that when both inputs are
unequal, opposite or different, the output is High. If
the inputs are both Low or both
High, the output is Low. The equivalent electrical circuit
confirms the logic of the
truth table. With switches S1
and S2 in position
A
and B, the output will be Low
because the Lamp will not light since there is no complete
path for current flow. The
same output will result if the switches are in position
A
and B (the equivalent of two
Low inputs); still no path for current flow exists and
the lamp is off. But if S1
is in
position A, while S2
is in position B, or vice versa, current will flow, the lamp will
light, and a High output is the result.
The timing diagram says that
only when the inputs are different at the same time
the output is High; if not, the output will remain Low.
Three (3) transistors are needed
to construct an equivalent XOR gate. If inputs
A and B are both Low, transistors Q1
and Q2 remain off
and transistor Q3
turns on
with a positive voltage to its base through resistor
R3
and R4. Thus the X output is
brought Low. The same output voltage exists when inputs
A
and B are both High
because Q1
and Q2 still remain
off. But lets say input B is High while input A is Low.
Now Q1 turns on, bringing the base of Q3
Low, turning the transistor off, and pulling
output X High through resistor R5.
In similar manner, if the input A is High, but The B
input is Low, Q2
now turns on, Q3
is off, and output X is also High.
The Boolean expression states,
"A or B, buth not both". The + sign with a circle
means exclusive.