UNIT-2

Combinational Circuits
Combinational circuit is a circuit in which we combine the different gates in the circuit, for
example encoder, decoder, multiplexer and demultiplexer. Some of the characteristics of
combinational circuits are following −

 The output of combinational circuit at any instant of time, depends only on the levels
present at input terminals.
 The combinational circuit do not use any memory. The previous state of input does
not have any effect on the present state of the circuit.
 A combinational circuit can have an n number of inputs and m number of outputs.

Block diagram

We're going to elaborate few important combinational circuits as follows.

Half Adder

Half adder is a combinational logic circuit with two inputs and two outputs. The half adder
circuit is designed to add two single bit binary number A and B. It is the basic building block
for addition of two single bit numbers. This circuit has two outputs carry and sum.

Block diagram

Truth Table
Circuit Diagram

Full Adder

Full adder is developed to overcome the drawback of Half Adder circuit. It can add two one-
bit numbers A and B, and carry c. The full adder is a three input and two output
combinational circuit.

Block diagram

Truth Table
Circuit Diagram

N-Bit Parallel Adder

The Full Adder is capable of adding only two single digit binary number along with a carry
input. But in practical we need to add binary numbers which are much longer than just one
bit. To add two n-bit binary numbers we need to use the n-bit parallel adder. It uses a number
of full adders in cascade. The carry output of the previous full adder is connected to carry
input of the next full adder.

4 Bit Parallel Adder

In the block diagram, A0 and B0 represent the LSB of the four bit words A and B. Hence Full
Adder-0 is the lowest stage. Hence its Cin has been permanently made 0. The rest of the
connections are exactly same as those of n-bit parallel adder is shown in fig. The four bit
parallel adder is a very common logic circuit.

Block diagram
N-Bit Parallel Subtractor

The subtraction can be carried out by taking the 1's or 2's complement of the number to be
subtracted. For example we can perform the subtraction (A-B) by adding either 1's or 2's
complement of B to A. That means we can use a binary adder to perform the binary
subtraction.

4 Bit Parallel Subtractor

The number to be subtracted (B) is first passed through inverters to obtain its 1's complement.
The 4-bit adder then adds A and 2's complement of B to produce the subtraction. S3 S2 S1 S0
represents the result of binary subtraction (A-B) and carry output Cout represents the polarity
of the result. If A > B then Cout = 0 and the result of binary form (A-B) then Cout = 1 and the
result is in the 2's complement form.

Block diagram

Half Subtractors

Half subtractor is a combination circuit with two inputs and two outputs (difference and
borrow). It produces the difference between the two binary bits at the input and also produces
an output (Borrow) to indicate if a 1 has been borrowed. In the subtraction (A-B), A is called
as Minuend bit and B is called as Subtrahend bit.
Truth Table

Circuit Diagram

Full Subtractors

The disadvantage of a half subtractor is overcome by full subtractor. The full subtractor is a
combinational circuit with three inputs A,B,C and two output D and C'. A is the 'minuend', B
is 'subtrahend', C is the 'borrow' produced by the previous stage, D is the difference output
and C' is the borrow output.

Truth Table
Circuit Diagram

Multiplexers

Multiplexer is a special type of combinational circuit. There are n-data inputs, one output and
m select inputs with 2m = n. It is a digital circuit which selects one of the n data inputs and
routes it to the output. The selection of one of the n inputs is done by the selected inputs.
Depending on the digital code applied at the selected inputs, one out of n data sources is
selected and transmitted to the single output Y. E is called the strobe or enable input which is
useful for the cascading. It is generally an active low terminal that means it will perform the
required operation when it is low.

Block diagram
Multiplexers come in multiple variations

 2 : 1 multiplexer
 4 : 1 multiplexer
 16 : 1 multiplexer
 32 : 1 multiplexer

Block Diagram

Truth Table

Demultiplexers

A demultiplexer performs the reverse operation of a multiplexer i.e. it receives one input and
distributes it over several outputs. It has only one input, n outputs, m select input. At a time
only one output line is selected by the select lines and the input is transmitted to the selected
output line. A de-multiplexer is equivalent to a single pole multiple way switch as shown in
fig.

Demultiplexers comes in multiple variations.

 1 : 2 demultiplexer
 1 : 4 demultiplexer
 1 : 16 demultiplexer
 1 : 32 demultiplexer
Block diagram

Truth Table

Decoder

A decoder is a combinational circuit. It has n input and to a maximum m = 2n outputs.

Block diagram

Examples of Decoders are following.

 Code converters
 BCD to seven segment decoders
 Nixie tube decoders
 Relay actuator
2 to 4 Line Decoder

The block diagram of 2 to 4 line decoder is shown in the fig. A and B are the two inputs
where D through D are the four outputs. Truth table explains the operations of a decoder. It
shows that each output is 1 for only a specific combination of inputs.

Block diagram

Truth Table

Logic Circuit
Encoder

Encoder is a combinational circuit which is designed to perform the inverse operation of the
decoder. An encoder has n number of input lines and m number of output lines. An encoder
produces an m bit binary code corresponding to the digital input number. The encoder
accepts an n input digital word and converts it into an m bit another digital word.

Block diagram

Examples of Encoders are following.

 Priority encoders
 Decimal to BCD encoder
 Octal to binary encoder
 Hexadecimal to binary encoder

Priority Encoder

This is a special type of encoder. Priority is given to the input lines. If two or more input line
are 1 at the same time, then the input line with highest priority will be considered. There are
four input D0, D1, D2, D3 and two output Y0, Y1. Out of the four input D3 has the highest
priority and D0 has the lowest priority. That means if D3 = 1 then Y1 Y1 = 11 irrespective of
the other inputs. Similarly if D3 = 0 and D2 = 1 then Y1 Y0 = 10 irrespective of the other
inputs.

Block diagram
Truth Table

Logic Circuit