Combining gates

We can combine two or more gates to perform more complex functions.

Combining gates

In this diagram, we have three inputs (A, B and C) and one output (Q). We are using an AND gate and an OR gate.

gate

We have also labelled the output of the AND gate as D. D is not an input, it is an intermediate value which we will use when calculating the truth table.

Truth table

Since this circuit has 3 inputs, the truth table will have 8 rows. Here is the basic truth table, with space for D and Q to be added:

table

D is equal to A AND B, so we can easily fill out the D column in the table. We just put 1 everywhere that A and B are both 1, and 0 everywhere else. Notice that D doesn't depend on C in any way:

table

Q is equal to D OR C, so we can now fill out the Q column in the table - the actual output of the combination gate. We just put 1 everywhere that either D or C are 1, and 0 everywhere else:

table

Interactive example

Here is an interactive combined gate. Click on the inputs A, B and C to change their state, and see the output value Q.

Combining gates

Combining gates

Truth table

Interactive example

Video

Logic gates

See also

Links