Comparative Study on CMOS Full
Adder Circuits
Priyanka Rathore and Bhavna Jharia
Abstract― The Presented paper focuses on the comparison of seven full adders. The comparison is based on the power consumption & area of full adders. It also highlights on comparison of different full adder circuits which are made of various logic styles. Used in designing paper suggests the best technique of designing on the basis of performance.
Index Terms― Full adder, Power consumption, Delay XOR &XNOR.
Introduction
Energy efficiency is one of the most required
features for recent electronics system designed for high performance and small circuits .In another way the ever increasing electronic circuit demands low power small circuit equipments which can be carried easily for example mobile &laptops. Everywhere adder is the core element of complex arithmetic circuits like addition multiplication division & exponentiation .There are standard execution with different logic styles that have been used earlier to design full adder cells. By choosing appropriate (W\L) ratio we can minimize the power dissipation without decreasing the supply of voltage .To conclude some of the performance criteria are considered in the design and evolution of adder cell and some are utilized for the ease of design robustness silicon area ,delay & power consumption . The paper studies full adder circuit which is made of different techniques. It express number of transistors, area & power consumption by circuit and how to minimize number of transistors, area & power consumption through a full adder circuit.Priyanka Rathore ,PG Student ,Department of electronics & communication Engineering /R.G.P.V University/U.E. College, Ujjain, Ujjain,India/8878758487
Bhavna Jharia ,Associate professor &Head DEC,UEC, Ujjain Ujjain ,India
The Boolean expression for the sum &carry bits are as shown bellow.
Sum = (A B) Cin
Cout = A.B + Cin (AB)
Various Types of Full adder circuits:
1. HPSC Full Adder― The simultaneous generation of XOR and XNOR outputs by pass logic is beneficially exploited to a new
complementary CMOS stage to create full-swing and balanced outputs so that adder cells can be cascaded without buffer insertion [1].
Module I: XOR/XNOR -The XOR and XNOR functions are to synthesize the XOR function and to generate the XNOR function through an inverter. This type of design has the disadvantage of delaying one of the Y and Y‟ outputs. The switching speed is increased by eliminating the inverter from the critical path [2].
Module II: XOR-The cross back 6-transistor circuit can also be used. However, it suffers from insufficient driving power due to the pass transistors.
Module III: MUX-In Module III small no. of transistors is generating „Cout‟ signal but in this circuit one problem is threshold voltage drop full swing signal is generate using by 4 transistor circuit .This circuit is not provide enough driving power .The new circuit is constructed by complementary CMOS logic style.
Logic expression:
Fig.1: Schematic diagram of the HPSC full adder
As pass transistor logic has been known to implement XOR function more efficiently than the complementary CMOS, Module I and Module I1 are implemented using pass-transistor logic. For Module III, a new circuit structure is created which gives rise to the performance gain over those circuits to be compared [1] .The two complementary feedback transistors restore the weak logic caused by pass transistors. They restore the non full-swing output by either pulling it up through PMOS to the power supply
or down through NMOS to ground so that sufficient drive is provided to the successive modules. In addition, since there is no direct path between power-supply and ground, short circuit current has been eliminated. The available XOR and XNOR outputs from Module I to allow a single inverter to attached at the last stage. The output inverter guarantees sufficient drive to the cascaded cell. [3]
Cout̍ = [AB + Cin (AB)]’
This circuit has inherited the advantages of complementary CMOS, which has been proven in to be superior in performance to all pass transistor logic styles for all logic gates except XOR at high supply voltage.
2. Hybrid Full adder― The full adder is designed with hybrid logic styles. Its works at ultra-low supply voltage. The pass logic circuit that generates the intermediate XOR and XNOR. These outputs have been improved to overcome switching delay problem [4]. The increase in the transistor count of its complementary CMOS output stage is compensated by its area efficient layout. As shown in Fig. 2, the hybrid full adder circuit can be analyzed in three sub modules. The logic expressions
Y =AB Y = (AB) ̍ Sum=YCin
Cout =A.B + Cin .Y
Module I: XOR/XNOR - The functions of exclusive OR and exclusive NOR (XOR/XNOR) are to synthesize the XOR function to generate the XNOR function through an inverter. This type of circuit has the disadvantage of delaying the Y and Y‟ outputs [4] in a increase spurious switching and glitches .The pass transistor circuit with only six transistors is used to generate the balanced XOR & XNOR. The inverter is used for generating complement signal when the switching speed is increased by eliminating the inverter from the critical path.
Module III: MUX - In Module III small no. of transistors are generating Cout signal but in this circuit there problem of threshold voltage drop. Full swing signal is generated by using 4 transistor circuit .This circuit does not provide enough driving power .The new circuit is constructed by complementary CMOS logic style [8].
Logic expression:
Cout = A.B+C (AB)
Fig 2: Schematic Diagram of hybrid full adder
3. Hybrid CMOS Full adder― This full adder is based on a new XOR–XNOR circuit. This output stage advantage is good driving capability for enabling cascading of adders without the need of buffer [9]. This full adder is energy efficient and outperforms several standard full adders without trading off driving capability and reliability. The new full-adder circuit successfully works in a low voltage with excellent signal integrity and driving capability [5].
Fig.3: Schematic Diagram of the hybrid cmos full Adder.
The centralized full adders, XOR and XNOR circuits are presented to generate the signals H and H‟. These signals are passed on to module II and III along with the carry from the previous stage and the other inputs A and B to produce the Sum and Cout. This circuit is shown in fig. 3.
operation the cross-coupled PMOS pull-up transistors providing the intermediate signals quickly. The two modules rely heavily on the intermediate signal H and H „to produces two signals H and H‟ .The delay response of module I is critical [2].
Module II-The XOR –XNOR functions are generated by an inverter. This circuit provides good driving capability. In this circuit is free by threshold loss and has the lowest PDP amongst all circuits that are used for module II [5].
Module III-It employs the proposed hybrid-CMOS output stage with a static inverter at the output. This circuit has a lower PDP as compared to the other circuits. The static inverter provides good driving capabilities as the inputs are decoupled from the output the circuit is provide low power consumption. [3].
4. CPL Full adder― In a CPL eliminated PMOS transistor NMOS pass transistor use for logic realization .The NMOS transistor use positive feedback .This type of circuit speed is high [1].
Using by this phenomenon reduce power consumption & reduce width of transistor. The CPL adder is balanced circuit with respect to generation of sum &carries out [6].
The number of transistors is more than comparative other design. This is due requirement of seven inverters to generate complement signals [6]. This circuit design is very complex. It improves speed &minimizes area.
Fig.4: Schematic Diagram of the CPL Full Adder.
switching function. The propagation of the signal through the transistor is controlled by a signal applied to its gate. In the case of an NMOS transistor, logic „1‟ at the gate passes the input from source to drain and logic „0‟ opens the source to drain path. A PMOS transistor exhibits similar behavior with a control signal of logic level 0 [11].
Fig. 5: Schematic Diagram of the New 14T Full Adder
6. DPL Full adder― Two new full-adders cicuit is made designing by DPL logic styles [12] and SR-CPL [13]. The logic structures are presented in fig 6 & 7. A full adder circuit designing is using by DPL logic style. To made by XOR – XNOR gate & multiplexer is based on pass transistor logic &we obtain by MUX sum output. SR-CPL logic style is constructed by XOR-XNOR gates [15].In both situations the and/or gates have been built using a powerless & groundless pass transistor respectively, and a pass-transistor based multiplexer to get the „Co‟output [12].
7. SRCPL― SR-CPL logic style is constructed by XOR-XNOR gates .In both situation the and/or gates have been built using a powerless &groundless pass transistor respectively, and a pass-transistor based multiplexer to get the „Co‟ output [14].
Fig. 7: Schematic Diagram of the SRCPL Full Adder
Table:
Conclusion― The paper concludes that CPL is the best suitable design for full adder since it an NMOS pass transistor network is used for logic realization and eliminate the PMOS transistor. Due to positive feedback and use of NMOS transistors, the circuit is inherently fast. This property is used to reduce the width of the transistors to reduce power consumption without much speed degradation.The proposed hybrid-CMOS is output stage with a static inverter at the output. This circuit has a lower PDP as
compared to the other existing designs. The static inverter provides good driving capabilities as the inputs are decoupled from the output. After the analysis and comparison of seven full adders it is found that the DPL technique is the best one. The SRCPL technique stands second to the DPL technique.
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S. no.
Scheme Technology No. of Transistor
Area
µm² Power µw
1 New
14T 90nm 14 39.83 983.8
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3 Hybrid 90nm 26 55.11 1206.5
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6 DPL 90nm 28 38.87 491.3
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