Magnitude Comparator

Magnitude Comparator

A magnitude comparator is a combinational circuit that compares two given numbers and determines whether one is equal to, less than or greater than the other. The output is in the form of three binary variables representing the conditions A = B_A>B and A<B, if A and B are the two numbers being compared. Depending upon the relative magnitude of the two numbers, the relevant output changes state. If the two numbers, let us say, are four-bit binary numbers and are designated as (A3 A2 A1 A0) and (B3 B2 B1 B0), the two numbers will be equal if all pairs of significant digits are equal,

that is, A3= B3, A2 = B2, A1= B1 and A0 = B0. In order to determine whether A is greater than or less than B we inspect the relative magnitude of pairs of significant digits, starting from the most significant position. The comparison is done by successively comparing the next adjacent lower pair of digits if the digits of the pair under examination are equal. The comparison continues until a pair of unequal digits is reached. In the pair of unequal digits, if Ai = 1 and Bi = 0, then A > B, and if Ai = 0, Bi= 1 then A < B. If X, Y and Z are three variables respectively representing the A = B, A > B and A < B conditions, then the Boolean expression representing these conditions are given by the equations

Let us examine equation (7.25). x3 will be ‘1‘ only when both A3 and B3 are equal. Similarly, conditions for x2, x1 and x0 to be ‘1‘ respectively are equal A2 and B2, equal A1 and B1 and equal A0 and B0. ANDing of x3, x2, x1 and x0 ensures that X will be ‘1‘ when x3, x2, x1 and x0 are in the logic ‘1‘ state. Thus, X = 1 means that A = B. On similar lines, it can be visualized that equations (7.26) and (7.27) respectively represent A > B and A < B conditions. Figure 7.36 shows the logic diagram of a four-bit magnitude comparator.

Magnitude comparators are available in IC form. For example, 7485 is a four-bit magnitude comparator of the TTL logic family. IC 4585 is a similar device in the CMOS family. 7485 and 4585 have the same pin connection diagram and functional table. The logic circuit inside these devices determines whether one four-bit number, binary or BCD, is less than, equal to or greater than a second four-bit number. It can perform comparison of straight binary and straight BCD (8-4-2-1) codes.

These devices can be cascaded together to perform operations on larger bit numbers without the help of any external gates. This is facilitated by three additional inputs called cascading or expansion inputs available on the IC. These cascading inputs are also designated as A = B, A > B and A < B inputs. Cascading of individual magnitude comparators of the type 7485 or 4585 is discussed in the following paragraphs. IC 74AS885 is another common magnitude comparator. The device is an eight bit magnitude comparator belonging to the advanced Schottky TTL family.

It can perform high-speed arithmetic or logic comparisons on two eight-bit binary or 2‘s complement numbers and produces two fully decoded decisions at the output about one number being either greater than or less than the other. More than one of these devices can also be connected in a cascade arrangement to perform comparison of numbers of longer lengths.

4 bit magnitude comparator

A magnitude digital Comparator is a combinational circuit that compares two digital or binary numbers in order to find out whether one binary number is equal, less than, or greater than the other binary number. We logically design a circuit for which we will have two inputs one for A and the other for B and have three output terminals, one for A > B condition, one for A = B condition, and one for A < B condition. 

1 Bit Magnitude Comparator :

A comparator used to compare two bits is called a single-bit comparator. It consists of two inputs each for two single-bit numbers and three outputs to generate less than, equal to, and greater than between two binary numbers. 

The truth table for a 1 bit comparator is given below:  

From the above truth table logical expressions for each output can be expressed as follows: 

A>B: AB'
A<B: A'B
A=B: A'B' + AB

From the above expressions we can derive the following formula: 

By using these Boolean expressions, we can implement a logic circuit for this comparator as given below: 

Logic Diagram is wrong

2 Bit Magnitude Comparator :

A comparator used to compare two binary numbers each of two bits is called a 2 bit Magnitude comparator. It consists of four inputs and three outputs to generate less than, equal to, and greater than between two binary numbers. 

The truth table for a 2 bit comparator is given below: 

From the above truth table K map for each output can be drawn as follows: 
 

From the above K-maps logical expressions for each output can be expressed as follows: 
 

A>B:A1B1’ + A0B1’B0’ + A1A0B0’
A=B: A1’A0’B1’B0’ + A1’A0B1’B0 + A1A0B1B0 + A1A0’B1B0’
   : A1’B1’ (A0’B0’ + A0B0) + A1B1 (A0B0 + A0’B0’)
   : (A0B0 + A0’B0’) (A1B1 + A1’B1’)
   : (A0 Ex-Nor B0) (A1 Ex-Nor B1)
A<B:A1’B1 + A0’B1B0 + A1’A0’B0

By using these Boolean expressions, we can implement a logic circuit for this comparator as given below: 

4 Bit Magnitude Comparator :

A comparator used to compare two binary numbers each of four bits is called a 4 bit magnitude comparator. It consists of eight inputs each for two four-bit numbers and three outputs to generate less than, equal to, and greater than between two binary numbers. 

In a 4-bit comparator the condition of A>B can be possible in the following four cases: 
 

1. If A3 = 1 and B3 = 0
2. If A3 = B3 and A2 = 1 and B2 = 0
3. If A3 = B3, A2 = B2 and A1 = 1 and B1 = 0
4. If A3 = B3, A2 = B2, A1 = B1 and A0 = 1 and B0 = 0

Similarly the condition for A<B can be possible in the following four cases: 

1. If A3 = 0 and B3 = 1
2. If A3 = B3 and A2 = 0 and B2 = 1
3. If A3 = B3, A2 = B2 and A1 = 0 and B1 = 1
4. If A3 = B3, A2 = B2, A1 = B1 and A0 = 0 and B0 = 1

The condition of A=B is possible only when all the individual bits of one number exactly coincide with corresponding bits of another number. 

From the above statements logical expressions for each output can be expressed as follows: 

AA, 831331 r: (A3 EioNor 33)A2132′ a (A3 Ex-Nor 133) (A2 Ex-Nor 132)A131′ a (A3 Ex-Nor 33) (A2 ENor132) (Al Ex-Nor 31)A01301 
,13: A3’03 a (A3 Ex-Nor 33)A211:12 a (A3 Ex-Nor 83) (A2 Ex-Nor 132)Ar131 a (A3 Ex-Nor 33) (A2 Ex-Nor32) (Al Ex-Nor 131)A0N30 
A=B: (A3 Ex-Nor B3) (A2 Ex-Nor 82) (Al Ex-Nor BI) (AO Ex-Nor BO) 

By using these Boolean expressions, we can implement a logic circuit for this comparator as given below: 
 

NOTE:

For n- the bit comparator then, the number of combinations for which 

•  A = B  is 2ⁿ
• A > B or A < B is (2²ⁿ – 2ⁿ)/2  

Cascading Comparator :

A comparator performing the comparison operation to more than four bits by cascading two or more 4 bit comparators is called a cascading comparator. When two comparators are to be cascaded, the outputs of the lower-order comparator are connected to corresponding inputs of the higher-order comparator. 

Applications of Comparators:

1. Comparators are used in central processing units (CPUs) and microcontrollers (MCUs).
2. These are used in control applications in which the binary numbers representing physical variables such as temperature, position, etc. are compared with a reference value.
3. Comparators are also used as process controllers and for Servo motor control.
4. Used in password verification and biometric applications.