Two-level circuits

Two-level circuits are direct implementations of sum of products and product of sums forms, either in fundamental form(straight from the truth table) or after minimisation. We now consider the advantages and disadvantages of this type o circuit:

Advantages:

• Any combinational logic function can be realised as a two-level circuit.

• This is theoretically the fastest implementation since signals have only to propagate through two gates.

• They can be implemented in variety of ways,e.g.and-or,or-and,etc.

Disadvantages:

• Avery large number of gates may be required.

• Gates with a prohibitively large number of inputs may be needed.

• Signals may be required to feed to more gates than is possible(because of the electrical characteristics of the circuit).

• The task of minimisation increases exponentially with the number of input variables (although computer programs can obviously help reduce this problem).

The effect of minimising a fundamental two-level circuit is to reduce the first three disadvantages although it cannot be guaranteed to remove them. Note that the second disadvantage can always be overcome by using more gates (e.g. by using three two-input  AND gates to implement a four-input AND gate) but that this means a single-level gate has itself become a two-level circuit.