Comparative study on transistor based full adder designs

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WSN 53(3) (2016) 404-416
EISSN 2392-2192
Comparative study on transistor based full adder
designs
R. Anitha
School of Electronics Engineering, VIT University, Vellore, Tamil Nadu, India
E-mail address: ranitha@vit.ac.in
ABSTRACT
Recently in the generic systems the load on the processor is much heavy. The ability and the
challenging process have ended with larger core operations are in the core processor. This paper is
basically given special importance on different methodologies having been proposed for Adder, which
is the basic operation of the Arithmetic unit. The wide research on the digital adders has been covered
so many applications like designs of ALU, RISC, CISC processors, DSP used for data path arithmetic,
low power CMOS, optical computing, Nanotechnology and so on. This paper gives greater knowledge
and understanding about the various techniques that have amply used of Adder from the earlier years.
In this paper we analyzed the implementation of different types of full Adders implemented using
CMOS logic (Static CMOS and Dynamic CMOS), CMOS Transmission Gates, Pass Transistor Gates
(CPL and DPL).
Keywords: Full Adder; CPL; DPL; CMOS; Transmission gates; Arithmetic Unit
1. INTRODUCTION
In the ALU, the most important arithmetic is the Full Adder block. Full adder is the
basic unit in almost all computational circuits. Low power techniques applied on Full Adder
block can reduce the computational power consumption. Several full adder circuits have been
proposed targeting on design accents such as power, delay, speed and area [1].
World Scientific News 53(3) (2016) 404-416
Digital Logic Circuit implementation can be done by using so many various designs
Techniques. Recent years there are lot of literature has appeared to have the basic logic
functions in the innovative digital designs. The innovative designs includes gates built using
CMOS logic (Static CMOS and Dynamic CMOS), CMOS Transmission Gates, Pass
Transistor Gates (CPL and DPL). In the static CMOS circuits, the pMOS and nMOS
transistors are in duality principle. Transmission gate is built with a parallel connected pMOS
and nMOS, where complementary input signals will be given to the gate of both the
transistors. So the transmission gate will be working as a bidirectional switch. But in
transmission gate we have the disadvantage of time-skew problems can lead to short circuits.
So that logic can be reduced by using the CPL. It consists of pure nMOS or only pMOS
transistors involved in the logic operations. But threshold voltage drop will be very high in
case of using only nMOS transistor. So for low voltage applications instead of using CPL, we
can use the DPL to design the logics. [2]
The implementation of full adders can have several categories. The conventional full
adder uses the static CMOS and complementary Pass transistor logics (CPL). In last few
decades, so many new designs have been evolved for the deep submicron technology. Based
on this the review on full adder is done and presented in this paper. There are several
improvements had been made in the parameters such as Area, power, delay and speed.
In this review paper we started analyzing with the conventional 28T full adder circuit
using Static CMOS logic and then gradually moving towards the full adder implemented with
less than 8T is summarized.
2. LITERATURE REVIEW
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“In 1992, N. Zhuang and H. Wu, “A New Design of the CMOS Full Adder”, IEEE
Journal of Solid-State Circuits”.
Full adder circuit is designed using CMOS Transmission gates with and without driving
outputs. And the design is compared with the conventional Full Adder design using Static
CMOS gates which has 28T shown in Fig. 1. Using the transmission gate design 4T has been
reduced.
In 1997, R. Zimmermann and W. Fichtner , “Low-Power Logic Styles: CMOS Vs PassTransistor Logic” IEEE Journal of Solid-State Circuits
In this paper, full adder designed in complementary MOS logic is compared with passtransistor logic as shown in Fig.1 and fig. 2. With only few exceptions Complementary
CMOS, however, proves to be superior to CPL with respect to speed, area, power
consumption, and power-delay products. CMOS to be 20% faster and superior to CPL, even
though CPL was found to be the most efficient. An implemented 32-bit adder using
complementary CMOS has a power-delay product of less than half that of the CPL version.
The paper shows that complementary CMOS is the logic style of choice for the
implementation of arbitrary combinational circuits, if low voltage, low power, and small
power-delay products are of concern. Complementary static CMOS performs much better
than CPL and other pass-transistor logic styles if low power is of concern.
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In the year 1999, R. Shalem, E. John, and L. K. John, “A Novel Low-power Energy
Recovery Full Adder Cell”
SERF – Static Energy Recovery Full Adder with 10T circuit is designed and
implemented in this paper. Energy efficient circuit is based on the energy recovering logic is
used. By using this logic energy consumption is very less than the non energy recovering
circuit. It takes approximately 26% to 55% less energy than the other designs and is shown to
be 19% faster. So SERF circuit fig. 3 is more superior than the other circuits.
Fig. 3. SERF adder circuit with 10T
In the same year 1999, M. Vesterbacka, “Signal Processing Systems”, IEEE Conference
Publications”
In this paper its explained how XOR and XNOR circuits are used to realize a 1-bit full
adder circuit based on transmission gates. A 6T CMOS XOR circuit that also produces a
complementary XNOR output is introduced in the full adder. The resulting full adder circuit
is realized using only 14T and compared with existing 16T full adder circuit [2]. Both circuits
are shown in Fig.4 and fig.5 respectively. Also the layout of 14T and 16T full adder cells have
been designed and simulated for comparison. Both adders yield similar performance in terms
of power consumption, propagation delay and power delay product, but 14T adder cell has the
advantage of full voltage swing at all circuit nodes.
The area is somewhat lower for the proposed adder on account of reduced device count.
However, due to two feedback MOSFETs in the proposed adder that need to be ratioed, there
is a higher cost in terms of design effort.
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Fig. 4. 14T Full Adder.
Fig. 5. 16T full adder using Transmission gates.
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Further in 2001, D. Radhakrishnan, “Low-Voltage Low-Power CMOS Full Adder”, IEE
Proc.- Circuits Devices Systems
XOR – XNOR gate using CMOS Pass transistor logic is designed, which fully
compensated the threshold voltage drop. When the power supply is scale down for certain
bounds, as long as the sizing of CMOS transistor in the initial stage of design. So with less
number of transistor count leads to the low power and high speed. In this paper they designed
14T, 16T and fully restored CMOS full adder with 14T and simulated in the .35µm
technology.
In 2002, H.T.Bui, Y.Wang, and Y.Jiang, “Design and Analysis of Low-Power 10transistor Full Adders using XOR-XNOR gates”
In this paper, proposed a technique along with the existing ones to build a total of 41
new 10-transistor full adders using novel XOR and XNOR gates in combination. Simulation
has been carried out with different adders and different input test patterns, load capacitance
and frequencies using HSPICE. When compare to the previous circuits this new adder
consumes on average of 10% less power and have higher speed. One con in this circuit is the
threshold-voltage loss of the pass transistors.
In 2004, Y. Jiang, A. A. Sheraidah, Y. Wang, E. Sha, and J. G.Chung, “A Novel
Multiplexer based Low-Power Full Adder”
This paper constitute based on the multiplexer based Full adder, which as designed
using 12T. This MUX based FA has reduced number of logic transition and low short circuit
current when compare to the previous FA. They compared the 12T MUX based FA with the
conventional 28T FA and four other 10T FA [4] – [7] using HSPICE. This designed has
achieved 26% less power than the conventional and 23% than 10T and 64% speed is
increased.
In 2005, F. Vasefi and Z. Abid, “Low Power N-bit Adders and Multiplier Using Lowest
Number of Transistors 1-bit Adders”
In this they proposed two designs of 10T adder as shown in Fig. 6 and one 12T full
adder by adding two extra stack transistors to 10T adder. 10T adder suffers from threshold
loss while 12T eliminated this problem completely. The 10T adder achieved a 43.68%
improvement in power consumption. Both proposed adders were further used to implement 4bit RCA and 4×4 array multiplier. The schematic of 12T adder is shown in Fig. 6 (b).
Fig. 6. 10T Full Adder
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In 2006, A. Saberkari and S. B. Shokouhi, “A Novel Low-Power Low-Voltage CMOS
1-Bit Full Adder Cell with the GDI Technique”
XOR – XNOR gates has been implemented using GDI techniques and a new design is
drawn for Full Adder. Even though the area is very large, it has low power – delay product
and energy efficient over a wide range of voltages among all the other logics.
In 2007, P. Lee, C Hsu and H.Y. Hung, “Novel 10T Full Adders realized by GDI
Structure”.
Full Adder is implemented using GDI techniques with 4T GDI XOR – XNOR gates
shown in fig. 7. Using this 4 different types 10T adder has been implemented and compared.
The proposed designs possess the advantage of flexibility, less transistor counts and improved
power consumption as well as delay making it a better alternative.
Fig. 7. 4T GDI XOR – XNOR gates
In 2008, K. Navi and O. Kavehei, “Low-Power and High-Performance 1-Bit CMOS
Full- Adder Cell”
1 bit Adder is designed using bridge style and compared with conventional CMOS
Adder. At room Temperature Bridge style CMOS Adder is simulated using HSPICE in 90 nm
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technology, and under given conditions, an improvement of 41.5% in speed over conventional
CMOS adder is achieved. In addition, the suggested adder shows 13.8 to 31.5% degradation
in terms of power consumption than conventional CMOS design with different power supply.
In addition, the delay and average power consumption results depict impressive improvement
of 60% over conventional CMOS counterpart in power-delay product.
Entering the year 2009, M.Hosseinghadiry, H. Mohammadi and M.Nadisenejani, “Two
New Low-Power High-Performance Full Adders with Minimum Gates”
XOR is designed using 3T and XNOR with 4T designed and by using this design 10T
and 9T full adder is implemented and evaluated the performance with the 12T. 10T circuit
shown in Fig. 7 (b) has threshold loss problem due to degraded XNOR output and output
multiplexers. 9T adder shown below in Fig. 8 showed 24% improvement in term of power
consumption and has better PDP for different operating voltages as well as frequencies but
10T adder is 12% faster than earlier reported full adder cells.
Fig. 8. 9T Full Adder
In 2010, Tripti Sharma, K.G.Sharma, Prof.B.P.Singh and Neha Arora, “A Novel CMOS
1-bit 8T Full Adder Cell”
Did the modification by changing aspect ratios of the transistors of earlier proposed 8T
adder shown in Fig. 9. By reducing the transistor size the threshold loss remains the same
while reducing the power consumption. This effort results into low power adder design.
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Fig. 9. 8T Full Adder [12]
Step to 2011, Shivshankar Mishra, V. Narendar and Dr. R. A. Mishra, “On the Design
of High-Performance CMOS 1-Bit Full Adder Circuits
Using Cadence VIRTUOSO environment in 180 nm technology with 9 transistor and
10T Adder circuit at different supply voltages. And it was concluded that have a great signal
level, less power and high speed. The designed circuit is 19% to 47% faster than the
conventional Full Adder and 6% to 67% less power. Both the proposed circuit is given in the
Fig. 10.
Fig. 10. 9T Full adders using XOR and XNOR cells
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Further in 2011, Deepa Sinha, Tripti Sharma, K. G. Sharma and Prof. B. P. Singh “
Ultra Low Power 1-Bit Full Adder”
The 8T adder due to simultaneous enabling of two transistors by adding an extra
transistor and gives full voltage swing at low supply voltage. This circuit has eliminated
serious threshold problem in This circuit shows improvement in power, delay and powerdelay product with varying operating voltages, frequencies under different temperature
ranges. The proposed circuit is shown in Fig. 11.
Fig. 11. 9T Full adder with extra added nMOS [ 14]
Then in the same year, M. Kumar, S. K. Arya and S. Pandey, “Single bit full adder
design using 8 transistors with novel 3 transistors XNOR gate”
In present work a new XNOR gate using three transistors has been presented, which
shows power dissipation of 550.7272 µW in 0.35 µm technology with supply voltage of 3.3V.
Minimum level for high output of 2.05V and maximum level for low output of 0.084V have
been obtained. A single bit full adder using eight transistors has been designed using proposed
XNOR cell, which shows power dissipation of 581.542 µW.
Minimum level for high output of 1.97V and maximum level for low output of 0.24V is
obtained for sum output signal. For carry signal maximum level for low output of 0.32V and
minimum level for high output of 3.2V have been achieved.
Power consumption of proposed XNOR gate and full adder has been compared with
earlier reported circuits and proposed circuit’s shows better performance in terms of power
consumption and transistor count. The proposed Full adder with XNOR and MUX is shown
in Fig. 12.
In 2012, A. Bazzazi, A. Mahini and J. Jelini, “Low Power Full Adder Using 8T
Structure”
In this paper 8T adder circuit is implemented with sizing of the transistors. FA operates
in 100 MHz range. The 5% variation of power supply and also several temperatures are
measured. In this paper author has changed the substrate biasing of CMOS and also
compatible with transistor size 10% variation. The power consumption of this adder is 200nw.
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Fig. 12. Full adder using two XNOR gates and multiplexer
In 2013, M.Geetha Priya, K.Baskaran,” Low Power Full Adder With Reduced
Transistor Count”
This paper presents the 3T XOR gate with sufficient area and power. New 8T full adder
is designed using this 3T XOR gate since conventional Full adder circuit increases the overall
computational delay. Low power consumption and high speed has achieved when it was
simulated in 90 nm CMOS technology using HSPICE at room temperature. 89% average
power consumption has improved in this method.
The simulation result is shown in the given Table. 1.
Table 1. Simulation results for the proposed full adder circuits in 90nm Process technology
at 50 MHz frequency.
Adder
No. of
Transistors
Power
(nW)
Delay
(ps)
PDP
(E -21 J)
CMOS
28
54.72
375.2
20530
CPL
32
76.82
462.8
35552
TGA
20
42.99
282.5
12144
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TFA
16
41.02
297.1
12187
HPSC
22
38.83
539.8
20960
Hybrid
CMOS
24
34.68
372.9
12932
14T
14
45.29
427.6
19366
Proposed
8T
8
5.87
288.4
1692
3. CONCLUSION
Concisely, the above discussion on literature reveals that all the modifications and size
shrinking of single bit full adders are done to improve the power consumption of the cell so
that it can be better useful for portable applications. The delay got reduced for a certain
designs and the speed of the full adder circuit is increased. So the arithmetic unit of the
processor will give the result very faster as the technology develops.
Reference
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[9]
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( Received 30 July 2016; accepted 19 August 2016 )
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