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International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
DOI : 10.5121/vlsic.2012.3501 1
STAND-BY LEAKAGE POWER REDUCTION
IN NANOSCALE STATIC CMOS VLSI
MULTIPLIER CIRCUITS USING SELF
ADJUSTABLE VOLTAGE LEVEL CIRCUIT
Deeprose Subedi1
and Eugene John2
1
Student, Department of Electrical and Computer Engineering, University of Texas at San
Antonio, San Antonio, TX USA.
deeprose_76@hotmail.com
2
Professor, Department of Electrical and Computer Engineering, University of Texas at
San Antonio, San Antonio, TX USA.
Eugene.john@utsa.edu
ABSTRACT
In this paper, we performed the comparative analysis of stand-by leakage (when the circuit is idle), delay
and dynamic power (when the circuit switches) of the three different parallel digital multiplier circuits
implemented with two adder modules and Self Adjustable Voltage level circuit (SVL). The adder modules
chosen were 28 transistor-conventional CMOS adder and 10 transistor- Static Energy Recovery CMOS
adder (SERF) circuits. The multiplier modules chosen were 4Bits Array, 4bits Carry Save and 4Bits Baugh
Wooley multipliers. At first, the circuits were simulated with adder modules without applying the SVL
circuit. And secondly, SVL circuit was incorporated in the adder modules for simulation. In all the
multiplier architectures chosen, less standby leakage power was observed being consumed by the SERF
adder based multipliers applied with SVL circuit. The stand-by leakage power dissipation is 1.16µwatts in
Bits array multiplier with SERF Adder applied with SVL vs. 1.39µwatts in the same multiplier with CMOS
28T Adder applied with SVL circuit. It is 1.16µwatts in Carry Save multiplier with SERF Adder applied with
SVL vs. 1.4µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 1.67µwatts in
Baugh Wooley multiplier with SERF Adder applied with SVl circuit vs. 2.74µwatts in the same multiplier
with CMOS 28T Adder applied with SVL circuit.
KEYWORDS
10 Transistor SERF Adder, SVL Circuit, Stand-by Leakage Power, Dynamic Power, Delay, Low Power
Design, Sub-micron Regimes.
1. INTRODUCTION
Today’s Semiconductor device industries have been challenged with producing low power high
performing portable electronic devices due to the increasing demand of their own consumer
market. It is believed that the next generation portable electronic devices have to be developed
with ultra-low power computational units such as multipliers [4]. In order to incorporate low
power design into such computational units, Leakage power has to be minimized because leakage
power accounts for the significant portion of the total power consumption in such circuits in deep
sub-micron regimes. Research has shown that greater than 50% of the total power consumption is
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
2
due to leakage phenomenon within which stand-by leakage is another major component [6]. The
stand-by leakage is the only source of power consumption in a static circuit [8]. This means that a
fully charged device will be deficient of any charge even if it is not used for some long time. In
such case the efficiency of the device is compromised.
The CMOS technology, supply voltage and threshold voltage have been scaled down to achieve
faster performing devices [12]. However, leakage current has increased considerably and has
become a major component of the total power consumption [1]. The leakage power component
further is comprised of stand-by and active leakage currents. Most microelectronic circuits remain
for considerable amount of time in static state. Therefore, low power design approach should also
include stand-by leakage power reduction in static CMOS circuits. Stand-by leakage power
dissipation dominates the dynamic power dissipation in deep sub-micron circuits and also in
circuits that remain in idle mode for long time such as mobile phones [1]. The Stand- by leakage
power dissipation is the power dissipated by the Static or the “idle” circuit. i.e. when the circuit is
not turned ON. In the same lines, the dynamic power dissipation occurs when the circuit is
switching. The equations (1) and (2) denote leakage and dynamic power dissipation respectively.
VddIleakPleak ×= (1)
2
fCVddPdynamic α= (2)
Where α is the switching activity, f is the operating frequency, C is the load capacitance, Vdd is the
supply voltage and I leak is the cumulative leakage current due to all leakage components [1], [14].
The total power dissipation is the sum of (1) and (2). The leakage current consists of various
components, such as Pn Junction Reverse-Bias current, subthreshold leakage, Gate leakage, Gate
–induced drain leakage and punchthrough leakage [1], [3], [7], [8], [9], [10]. Among them the
subthreshold leakage is considered to be a major contributing component of the leakage power.
These leakage mechanisms are well described by literature [1].
Literature [13] includes the ideas of bulk driven voltage which generates a channel inversion and
sufficiently low gate to source voltage supply. The multi- Threshold Voltage CMOS (MTCMOS)
and Variable threshold- voltage CMOS (VTCMOS) are the two regularly used techniques for
reducing stand- by leakage power [2]. The MTCMOS technique requires additional fabrication
process for higher threshold voltage and the storage circuits are not able to retain data [2]. The
VTCMOS technique has major drawbacks as well, such as large area penalty, slow substrate-bias
controlling operation and large power overhead [2]. Therefore, in order to skip the above
mentioned drawbacks, a SVL circuit was chosen which minimizes stand-by leakage power whilst
maintaining high- speed performance.
Figure 1 shows a self-adjustable voltage level circuit with Vdd as supply and VL as output
voltages. The output voltage from this circuit is applied to the load circuit. The load circuits are the
adder modules of conventional CMOS adder and SERF adder including their half adders which all
are net-listed in the multiplier circuits. The SVL circuit hangs above these adder modules and
drizzles only minimum voltages to them as such the subthreshold leakage current of idle
MOSFETS decreases and the standby leakage power is minimized. For the stand-by mode that is
when SL= 1 it supplies less voltage to the load circuit through “weakly on” NMOS transistors [6].
The voltage supplied to the load circuit is
VnVddVl −= (3)
Where Vn is the voltage drop of all “weakly on” NMOS transistors. If Vdsn is the drain to source
voltage of the NMOS transistors, then,
VssVlVdsn −= (4)
Now, from equations (3) and (4), we have,
VssVnVddVdsn −−= (5)
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
3
Figure1. Self-Adjustable Voltage Level Circuit. Source: M. Rani etl., Leakage Power reduction
and analysis of CMOS sequential circuits (International Journal of VLSICS vol. 3 2012).
Equation (5) shows that Vdsn can be decreased by increasing Vn [6]. In other words, by increasing
the number of NMOS transistors. By decreasing Vdsn the Drain-Induced Barrier Lowering effect
is decreased which increases the threshold voltage of NMOS transistors [6], [1]. The increased
threshold voltage reduces the subthreshold leakage current of the NMOS transistors and thus, the
Stand-by leakage power is reduced [6]. It will also decrease the dynamic power dissipation
because of low supply voltage, VL.
Initially, each multiplier circuits were simulated using conventional 28 transistor adders and 10
transistor SERF adders one at a time. Their corresponding stand-by leakage power dissipation,
delay and dynamic power were noted. And then, the two adder modules were constructed with
SVL circuit and put into the multiplier architecture forming the complete circuit mesh one at a
time. Again, their corresponding stand-by leakage power dissipation, delay and dynamic power
were noted and a comparative analysis and evaluation was carried out. The SERF adder with SVL
circuit based multipliers outperformed all other combinations which suggest that this combination
(SERF adder with SVL circuit based multipliers) is suitable for ultra-low power design of
multipliers. SVL circuits have been applied by literature [6] in CMOS sequential circuits, by
Literature [2] in SRAM. This is the first time SVL circuits are being applied in CMOS multiplier
circuits.
2. ADDER MODULES
2.1. Conventional CMOS 28 Transistor (28T) Adder
Employing fast and efficient adders in multiplier circuits will aid in design of low power high
performance system [4]. Adders being the fundamental building block of the multiplier
architecture play an essential role in design of such system. Figure 2 shows the conventional
CMOS 28T adder which is constructed using same number of NMOS and PMOS transistors. The
full adder logic is as follows [4], [14]:
ACinBCinABCout ++= (6)
tCouCinBAABCinSum ′+++= )( (7)
These adder modules are constructed with functional pull up and pull down blocks of PMOS and
NMOS transistors [14]. These adders form the basic building block in our multiplier architecture
and often line in the critical signal path. Therefore, it determines the over performance of the
system.
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
4
Figure 2. Conventional CMOS full adder with 28T Source: J.M Rabaey et al,. Digital Integrated
Circuits, Prentice Hall Publication (2003).
The half adder logic can be realized setting Cin as zero [4]. During simulation Cin is set as
ground (GND=Vss=0 volts) for CMOS 28 T half adder modules. The CMOS inverters are placed
at the Carry out and Sum nodes in order to read the logic during actual verification.
2.2. 10 Transistor (10T) Adder
Figure 3. SERF full adder. Source: R Shalem et al., A novel low power energy recovery full adder cell
(Proceedings of the Great Lakes Symposium on VLSI (1999).
The 10 transistor SERF adder is considered to be efficient in both low power consumption and less
chip area [5]. In non- energy recovering circuits the charge from load capacitance during high is
directly dumped to the ground (GND) during low logic level. In contrast, SERF adder reuses
charge which the load capacitance during logic high to excite the gates rather than dumping the
charge to the GND [5].
The circuit is shown in figure 3 which consists of two exclusive NORs realized by four transistors.
The Carry Out is calculated by multiplexing inputs A and carry in (Cin). The sum is generated
from the output of second stage exclusive NOR. If there is a capacitor charging at the output node
of the first exclusive NOR and if initially, A=B=0, and A goes to high. When A and B both equal
to low the capacitor is charged by VDD. In the next stage when B goes to high keeping A fixed at
low, the capacitor discharges through A. Some charge is retained in A. Hence when A goes high it
is not required to be charged fully. So the energy consumption is well managed and low here [5].
3. MULTIPLIER ARCHITECTURES
The multipliers are the well organised array of adder cells [4]. The performance and
characteristics of the multipliers depend upon the algorithm in which they are based on [4]. Due
to the emphasis of low power design, speed is not only the criterion for design objective.
Therefore, designing multipliers with low power adder modules is essential keeping an eye on the
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
5
consumer market of portable electronic devices. In this paper, we have designed and
characterized three well-known multipliers viz. The 4 Bits Array multiplier, the 4 bits Carry Save
Multiplier and 4 bits Baugh Wooley Multipliers.
3.1. Bits Array Multiplier
Bit Array multiplier has a simple expandable structure which makes it easy to understand. In Bit
Array Multiplier, the partial product is generated by multiplying multiplicand and multiplier bits
[4]. The partial products are placed according to the correct shift in bit orders and then are added.
If there are N partial products in the Bit – Array multiplier, (N-1) bit adders are required. Figure 4
shows the schematic of 4 bit- array multiplier. There is more than just one critical path in the Bit-
Array multiplier.
Figure4. 4bits Array Multiplier. Source: J.M Rabaey Y et al., Digital Integrated Circuits, Prentice
Hall Publications (2003).
Theoretically, the approximate equation to calculate the propagation delay of these paths is shown
below.
TcarryXYTsumTandT )]2()1[( −+−++=∆ (8)
Where, Tsum is the delay between Carry in (Cin) and the sum bit of full adder, Tand is the delay
of the AND gates, Y is the width of multiplicand, X is the width of multiplier and Tcarry is the
propagation delay of the input and output carry [4].
3.2. Carry Save Multipliers
Carry-Save multiplier has the simple expandable structure like the Bits- Array multiplier. The
only difference in algorithm is that in Carry Save multiplier the output carry bits are passed
diagonally downwards instead of only to the right as the multiplication result does not change
while doing so. The literature [4] has shown that in the final stage the sums and carries are fed in
a fast carry adder usually by using fast- carry-look ahead adder. It is slightly bigger than the Bits
array in the area. However, it only has one critical path.
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
6
Figure5. 4 bits Carry-Save multiplier. Source: J.M Rabaey Y et al., Digital Integrated Circuits,
Prentice Hall Publications (2003).
The schematic of 4 bits Carry-Save multiplier is shown in figure 5. Theoretically, the
propagation delay of this multiplier is given by equation (9).
TcarryXTfinalTandT )1( −++=∆ (9)
Where Tcarry is the delay between input and output carry, X is the number of partial product
stages, Tfinal is the delay of final stage carry look ahead adder and Tand is the delay of the AND
gate [4].
3.3. Baugh Wooley Multiplier
Baugh Wooley multiplier has different algorithm than Bits- Array and Carry- Save multipliers. It
is used to perform 2’s complement multiplication and effectively handles the signed bits. The N x
N Baugh Wooley multiplication algorithm is given by equation (10).
∑∑∑∑
−
−+
−
−
−+
−
−
−−
+
− −
−
−−−−
−
+++++++−=×
2
0
1
1
2
0
1
.1
12
11
2
0
2
0
22
1111
12
2..2.2).(22).(2
N
Ni
N
n
Ni
ixN
N
NN
ji
ji
N N
N
NNNN
N
YiXYYXYXYXYXYX (10)
Where, X and Y are N-bits operand and their product is 2N bits number [4]. The Schematic of 4
bits Baugh Wooley Multiplier is shown in figure 6. The delay of Baugh Wooley Multiplier is
similar to that of Bit-Array multiplier as it has also more than one critical path.
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
7
Fig6. 4 bits Baugh Wooley multiplier. Source: J.M Rabaey Y et al., Digital Integrated Circuits,
Prentice Hall Publications (2003).
4. SIMULATION SET UP AND RESULTS
The CMOS sub-circuit of the CMOS AND gate, CMOS full adders and half adders and SVL
circuits were created in HSPICE decks specifying the input and output nodes. The global
variables such as supply voltage and ground were specified respectively as Vdd and Vss. These
sub-circuit programs are called each time when they are required by the multiplier architecture.
CMOS multiplier circuits’ netlists are created with different combination of adders modules. The
functionality of each circuit, CMOS AND, CMOS full adders and half adders were verified
before creating the multiplier architecture net lists. The respective logic truth tables of half adder,
full adder, CMOS AND gate and inverters were used during the verification. After extracting the
multiplier architecture net lists, each multiplier circuits with different adder modules combination
were simulated and verified to be functioning correctly before proceeding ahead for power and
delay calculations. These analyses are called the transient analysis and read on the Cscope of the
Hspice. Multipliers functionality verification can be done with number of different methods and
approaches. In multiplier function verification, we chose a typical 4bits by 4 bits multiplying
method where partial products generated are added to produce the final product in the form of
[P7….to… P0] shown by equation (11) below.
01234567
0123
0123
pppppppp
bbbXb
aaaa
−−−−−−−−−−−−−
(11)
We take the random two 4 bits numbers and find the final result of the multiplication in 8 bit
number. Subsequently, we supply the same multiplier and multiplicand two 4 bits number
through the inputs a3a2a1a0 and b3b2b1b0 in the multiplier netlist in Hspice software and check
the results in Hspice Cscope readout. The corresponding 1s and 0s of [P7…P0]are compared to
highs and lows of the Cscope output. If they match, the multiplier circuit is functioning correctly.
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
8
A total of twenty four different multiplier circuits’ netlists of different adder modules
combinations were created for the combination shown in table 2 below in Hspice decks and all of
them were tested to be functioning correctly.
Fig7. Functionality verification of CMOS AND gate.
The net lists of the circuits were extracted and simulation was performed using UC Berkeley
BSIM4 models available through Predictive Technology Model (PTM) which is a promising
model file for accurate and predictive modeling of future transistors. All the CMOS circuits were
implemented with 45nm node technology in HSPICE.
Fig8. Functionality verification of 28T CMOS full Adder with SVL circuit.
The simulations were run on a Red Hat Linux host machine. All the multipliers were compared
and analyzed for stand-by leakage power, delay and dynamic power. The delays were measured
for worst case scenarios i.e. always the worst case delay was considered for data pickup.
Fig 9. Functionality verification of 10 T SERF full adder with SVL circuit.
The delay measurements for the all combinations are tabulated for a convenient analysis. In the
same fashion, the leakage and dynamic power is also presented in tabular form.
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
9
Table1. Delay Measurements
Table2. The stand-by Leakage and dynamic power dissipation of the 3 different multipliers with
various combinations
Table3. Area/transistor count
Adders CMOS28T SERF 10T CMOS28Twith
SVL
SERF10T
With SVL
multipliers
Bits Array 512 296 548 316
Baugh
Wooley
624 354 669 399
Carry save 512 296 548 316
5. FUTURE WORK
Dual threshold CMOS technique can be applied into the multiplier circuits. There are certain
critical and non- critical paths in the multiplier circuits. A higher threshold voltage can be
assigned to the transistors in the non- critical paths and lower threshold voltage can be assigned to
Delay Measurements Bits Array Carry Save Baugh Wooley
Delay with SERF adder 1.46E-10
seconds
7.26E-09 seconds 2.02E-10 seconds
Delay with CMOS 28 T adder 1.46E-10
seconds
1.26E-08 seconds 3.66E-09 seconds
Delay with SERF applied with
SVL circuit
6.60E-09
seconds
7.27E-09 seconds 3.05E-09 seconds
Delay with CMOS 28 T applied
with SVL
9.12E-09
seconds
1.27E-08 seconds 2.74E-08 seconds
multipliers Bits Array Carry Save Baugh
Wooleypower
Leakage with 28 t 1.00E-04 watts 1.12E-04 watts 1.43E-04 watts
Dynamic with 28 t 1.10E-04 watts 1.05E-04 watts 1.36E-04 watts
Leakage with SERF 2.14E-05 watts 2.11E-05 watts 2.08E-05 watts
Dynamic with SERF 2.14E-05 watts 2.12E-05 watts 2.23E-05 watts
Leakage with 28 t
applied with SVL
1.39E-06 watts 1.40E-06 watts 2.74E-06 watts
Dynamic with 28 t
applied with SVL
5.80E-05 watts 2.25E-05 watts 1.93E-05 watts
Leakage with SERF
applied with SVL
1.16E-06 watts 1.16E-06 watts 1.67E-06 watts
Dynamic with SERF
applied with SVL
2.39E-05 watts 1.85E-05 watts 1.64E-05 watts
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
10
the transistors in the critical paths as such the leakage current is minimized without compromising
the performance [1]. Dual threshold CMOS can reduce leakage power in both stand-by and active
mode of operation without a penalty on both area and delay [1].
Figure12. Dual threshold Voltage circuitry
6. CONCLUSIONS
The table1 show the propagation delay of different multiplier circuits. The delay of multipliers
with SERF adders applied with SVL circuit exhibit a huge difference compared to the rest two
multiplier combinations applied with CMOS 28T Adder modules. The CMOS 28T Adder applied
with SVL circuit has large penalty in area and have bad delays. The delay is 6.6nS in Bits Array
Multiplier with SERF Adder applied with SVL circuit vs. 9.12nS in the same multiplier with
CMOS 28 T Adder applied with SVL. It is 7.2nS in Carry Save Multiplier with SERF Adder
applied with SVL circuit vs. 12.7nS in the same multiplier with CMOS 28T Adder applied with
SVL circuit. It is 3.05nS in Baugh Wooley multiplier with SERF Adder applied with SVL circuit
vs. 27nS in the same multiplier with CMOS 28T Adder applied with SVL circuit. The table2
show a comparison of stand-by leakage and dynamic power dissipation of different multiplier
circuits applied with various combinations of adder modules and SVL circuit. Interestingly, the
dynamic power dissipation has decreased for combinations that used SVL circuit because of low
supply voltage, VL created by SVL circuit itself. Transistor count is proportional to area of the
chip. Table 3 suggests that multipliers with SERF adder modules applied with SVL has less area
compared to the multipliers with CMOS 28T adder modules applied with SVL circuit. The table2
shows that stand-by leakage power dissipation with SERF adder applied with SVL circuit in all
three multipliers circuits is less compared to the same three multipliers with other combinations.
The stand-by leakage power dissipation of multipliers with SERF adders applied with SVL circuit
exhibit a significant difference compared to the rest two multiplier combinations applied with
CMOS 28T Adder modules. The CMOS 28T Adder applied with SVL circuit has large penalty in
area. The stand-by leakage power dissipation is 1.16µwatts in Bits array multiplier with SERF
Adder applied with SVL vs. 1.39µwatts in the same multiplier with CMOS 28T Adder applied
with SVL circuit. It is 1.16µwatts in Carry Save multiplier with SERF Adder applied with SVL vs.
1.4µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 1.67µwatts
in Baugh Wooley multiplier with SERF Adder applied with SVl circuit vs. 2.74µwatts in the same
multiplier with CMOS 28T Adder applied with SVL circuit. Therefore, the multiplier circuits with
SERF Adder applied with SVL circuit outperform the multiplier circuits with CMOS 28T Adder
applied with SVL circuit in terms of stand-by leakage power dissipation, area and delay. In other
words, the SERF adders applied with SVL circuit are suited for ultra-low power design of CMOS
multipliers circuits.
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
11
REFERENCES
[1] K. Roy, S. Mukhopadhyay and H. Mahmoodi-Meimand “Leakage Current Mechanisms and Leakage
Reduction techniques in Deep-Submicrometer CMOS Circuits” Proceedings of the IEEE, Vol. 91,
No.2, February, 2003.
[2] T. Enomoto, Y. Oka and H. Shikano, “A Slef-Controllable –Voltage –Level (SVL) Circuit for Low-
Power, High-Speed CMOS Circuits", IEEE Journal of Solid –State Circuits, Vol. 38, No. 7, July
2003, Pages 1220-1226.
[3] A. Agarwal, S. Mukhopadhyay, A. Raychowdhury, K. Roy “ Leakage Power Analyis And Reduction
for Nanoscale Circuits” IEEE Computer Society, Vol. 26, April 2006, pages 68-80.
[4] D. Kudithipudi and E. John, “Implementation of Low Power Digital Multipliers using 10 Transistor
Adder Blocks” Journal of Low Power Electronics, Vol. 1 2005, pages 1-11.
[5] R. Shalem, L.K John and E. John, “A Novel Low Power Energy Recovery Full Adder Cell”,
Proceedings of the Great Lakes Symposium on VLSI (1999), Pages 380-383.
[6] M. J. Rani and S. Malarkann, “Leakage Power Reduction and Analysis of CMOS sequential
Circuits”, International Journal of VLSI design and Communication Systems, Vol. 3, No. 1 February
2012.
[7] K. Kim, N. Park, Y. Kim and M. Choi, “Leakage Minimization Technique for Nanoscale CMOS
VLSI based on Macro-Cell Modelling”, IEEE Design & Test of Computers, Vol. 24 July 2007 pages
322-330.
[8] F. Fallah and Massoud Pedram “ Standby and Active Leakage Current Control and minimization in
CMOS VLSI Circuits”, IEICE Leakage- Review-Journal.
[9] M. Singh, S. Akhase, S. Sharma “ Leakage power reduction techniques of 45nm of Static Radom
Access Memory ( SRAM) cells”, International Journal of Physical Sciences, vol. 6, December 2011,
pages 7341-7353.
[10] J. Deshmukh, K. Khare “Standby leakage Reduction in nanoscale CMOS VLSI circuits”, Proceedings
of the International Conference and workshop on Emerging trends in Technology, 2010 pages 911-
914.
[11] J.M Rabaey Y et al., Digital Integrated Circuits, Prentice Hall Publications 2003.
[12] Ratul kr. Baruah “ Design of a low power low voltage CMOS opamp”, International Journal of VLSI
design and Communication system, Vol. 1, No. 1 March 2010.
[13] Neha Gupta etl., “Low power low voltage bulk driven balanced OTA”, International Journal of VLSI
design and Communication system, Vol. 2, No. 4 December2011.
[14] Y. Sunil Gavaskar Reddy and V.V.G.S Rajendra Prasad “ Power comparision of CMOS and
adiabiatic full adder circuits”, International Journal of VLSI design and Communication system, Vol.
2, No. 3 September 2011
International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012
12
Authors
Deeprose Subedi received his Bachelor of Science in Physics from Tribhuvan
University, Kathmandu, Nepal and is currently pursuing Bachelor of Science in
Electrical Enginering from University of Texas at San Antonio. His research
interests include, low power vlsi, Intregrated Circuit design, CMOS analog and
mixed signal systems design and power electronics. This paper is the spin out
result of a NSF funded research entitled “Experimental Study on Computer
Architecture and Performance Evaluation” where he participated in summer
2012 at University of Texas at San Antonio. He is a Student member of IEEE.
Dr. Eugene John is the Professor of electrical and computer engineering at the
University of Texas at San Antonio. His research interests include low power
techniques, power efficient circuits and systems, multimedia and embedded
systems and performance evaluation. He has a Ph.D. in electrical engineering
from the Pennsylvania State University. He is a senior member of the IEEE.

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STAND-BY LEAKAGE POWER REDUCTION IN NANOSCALE STATIC CMOS VLSI MULTIPLIER CIRCUITS USING SELF ADJUSTABLE VOLTAGE LEVEL CIRCUIT

  • 1. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 DOI : 10.5121/vlsic.2012.3501 1 STAND-BY LEAKAGE POWER REDUCTION IN NANOSCALE STATIC CMOS VLSI MULTIPLIER CIRCUITS USING SELF ADJUSTABLE VOLTAGE LEVEL CIRCUIT Deeprose Subedi1 and Eugene John2 1 Student, Department of Electrical and Computer Engineering, University of Texas at San Antonio, San Antonio, TX USA. deeprose_76@hotmail.com 2 Professor, Department of Electrical and Computer Engineering, University of Texas at San Antonio, San Antonio, TX USA. Eugene.john@utsa.edu ABSTRACT In this paper, we performed the comparative analysis of stand-by leakage (when the circuit is idle), delay and dynamic power (when the circuit switches) of the three different parallel digital multiplier circuits implemented with two adder modules and Self Adjustable Voltage level circuit (SVL). The adder modules chosen were 28 transistor-conventional CMOS adder and 10 transistor- Static Energy Recovery CMOS adder (SERF) circuits. The multiplier modules chosen were 4Bits Array, 4bits Carry Save and 4Bits Baugh Wooley multipliers. At first, the circuits were simulated with adder modules without applying the SVL circuit. And secondly, SVL circuit was incorporated in the adder modules for simulation. In all the multiplier architectures chosen, less standby leakage power was observed being consumed by the SERF adder based multipliers applied with SVL circuit. The stand-by leakage power dissipation is 1.16µwatts in Bits array multiplier with SERF Adder applied with SVL vs. 1.39µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 1.16µwatts in Carry Save multiplier with SERF Adder applied with SVL vs. 1.4µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 1.67µwatts in Baugh Wooley multiplier with SERF Adder applied with SVl circuit vs. 2.74µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. KEYWORDS 10 Transistor SERF Adder, SVL Circuit, Stand-by Leakage Power, Dynamic Power, Delay, Low Power Design, Sub-micron Regimes. 1. INTRODUCTION Today’s Semiconductor device industries have been challenged with producing low power high performing portable electronic devices due to the increasing demand of their own consumer market. It is believed that the next generation portable electronic devices have to be developed with ultra-low power computational units such as multipliers [4]. In order to incorporate low power design into such computational units, Leakage power has to be minimized because leakage power accounts for the significant portion of the total power consumption in such circuits in deep sub-micron regimes. Research has shown that greater than 50% of the total power consumption is
  • 2. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 2 due to leakage phenomenon within which stand-by leakage is another major component [6]. The stand-by leakage is the only source of power consumption in a static circuit [8]. This means that a fully charged device will be deficient of any charge even if it is not used for some long time. In such case the efficiency of the device is compromised. The CMOS technology, supply voltage and threshold voltage have been scaled down to achieve faster performing devices [12]. However, leakage current has increased considerably and has become a major component of the total power consumption [1]. The leakage power component further is comprised of stand-by and active leakage currents. Most microelectronic circuits remain for considerable amount of time in static state. Therefore, low power design approach should also include stand-by leakage power reduction in static CMOS circuits. Stand-by leakage power dissipation dominates the dynamic power dissipation in deep sub-micron circuits and also in circuits that remain in idle mode for long time such as mobile phones [1]. The Stand- by leakage power dissipation is the power dissipated by the Static or the “idle” circuit. i.e. when the circuit is not turned ON. In the same lines, the dynamic power dissipation occurs when the circuit is switching. The equations (1) and (2) denote leakage and dynamic power dissipation respectively. VddIleakPleak ×= (1) 2 fCVddPdynamic α= (2) Where α is the switching activity, f is the operating frequency, C is the load capacitance, Vdd is the supply voltage and I leak is the cumulative leakage current due to all leakage components [1], [14]. The total power dissipation is the sum of (1) and (2). The leakage current consists of various components, such as Pn Junction Reverse-Bias current, subthreshold leakage, Gate leakage, Gate –induced drain leakage and punchthrough leakage [1], [3], [7], [8], [9], [10]. Among them the subthreshold leakage is considered to be a major contributing component of the leakage power. These leakage mechanisms are well described by literature [1]. Literature [13] includes the ideas of bulk driven voltage which generates a channel inversion and sufficiently low gate to source voltage supply. The multi- Threshold Voltage CMOS (MTCMOS) and Variable threshold- voltage CMOS (VTCMOS) are the two regularly used techniques for reducing stand- by leakage power [2]. The MTCMOS technique requires additional fabrication process for higher threshold voltage and the storage circuits are not able to retain data [2]. The VTCMOS technique has major drawbacks as well, such as large area penalty, slow substrate-bias controlling operation and large power overhead [2]. Therefore, in order to skip the above mentioned drawbacks, a SVL circuit was chosen which minimizes stand-by leakage power whilst maintaining high- speed performance. Figure 1 shows a self-adjustable voltage level circuit with Vdd as supply and VL as output voltages. The output voltage from this circuit is applied to the load circuit. The load circuits are the adder modules of conventional CMOS adder and SERF adder including their half adders which all are net-listed in the multiplier circuits. The SVL circuit hangs above these adder modules and drizzles only minimum voltages to them as such the subthreshold leakage current of idle MOSFETS decreases and the standby leakage power is minimized. For the stand-by mode that is when SL= 1 it supplies less voltage to the load circuit through “weakly on” NMOS transistors [6]. The voltage supplied to the load circuit is VnVddVl −= (3) Where Vn is the voltage drop of all “weakly on” NMOS transistors. If Vdsn is the drain to source voltage of the NMOS transistors, then, VssVlVdsn −= (4) Now, from equations (3) and (4), we have, VssVnVddVdsn −−= (5)
  • 3. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 3 Figure1. Self-Adjustable Voltage Level Circuit. Source: M. Rani etl., Leakage Power reduction and analysis of CMOS sequential circuits (International Journal of VLSICS vol. 3 2012). Equation (5) shows that Vdsn can be decreased by increasing Vn [6]. In other words, by increasing the number of NMOS transistors. By decreasing Vdsn the Drain-Induced Barrier Lowering effect is decreased which increases the threshold voltage of NMOS transistors [6], [1]. The increased threshold voltage reduces the subthreshold leakage current of the NMOS transistors and thus, the Stand-by leakage power is reduced [6]. It will also decrease the dynamic power dissipation because of low supply voltage, VL. Initially, each multiplier circuits were simulated using conventional 28 transistor adders and 10 transistor SERF adders one at a time. Their corresponding stand-by leakage power dissipation, delay and dynamic power were noted. And then, the two adder modules were constructed with SVL circuit and put into the multiplier architecture forming the complete circuit mesh one at a time. Again, their corresponding stand-by leakage power dissipation, delay and dynamic power were noted and a comparative analysis and evaluation was carried out. The SERF adder with SVL circuit based multipliers outperformed all other combinations which suggest that this combination (SERF adder with SVL circuit based multipliers) is suitable for ultra-low power design of multipliers. SVL circuits have been applied by literature [6] in CMOS sequential circuits, by Literature [2] in SRAM. This is the first time SVL circuits are being applied in CMOS multiplier circuits. 2. ADDER MODULES 2.1. Conventional CMOS 28 Transistor (28T) Adder Employing fast and efficient adders in multiplier circuits will aid in design of low power high performance system [4]. Adders being the fundamental building block of the multiplier architecture play an essential role in design of such system. Figure 2 shows the conventional CMOS 28T adder which is constructed using same number of NMOS and PMOS transistors. The full adder logic is as follows [4], [14]: ACinBCinABCout ++= (6) tCouCinBAABCinSum ′+++= )( (7) These adder modules are constructed with functional pull up and pull down blocks of PMOS and NMOS transistors [14]. These adders form the basic building block in our multiplier architecture and often line in the critical signal path. Therefore, it determines the over performance of the system.
  • 4. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 4 Figure 2. Conventional CMOS full adder with 28T Source: J.M Rabaey et al,. Digital Integrated Circuits, Prentice Hall Publication (2003). The half adder logic can be realized setting Cin as zero [4]. During simulation Cin is set as ground (GND=Vss=0 volts) for CMOS 28 T half adder modules. The CMOS inverters are placed at the Carry out and Sum nodes in order to read the logic during actual verification. 2.2. 10 Transistor (10T) Adder Figure 3. SERF full adder. Source: R Shalem et al., A novel low power energy recovery full adder cell (Proceedings of the Great Lakes Symposium on VLSI (1999). The 10 transistor SERF adder is considered to be efficient in both low power consumption and less chip area [5]. In non- energy recovering circuits the charge from load capacitance during high is directly dumped to the ground (GND) during low logic level. In contrast, SERF adder reuses charge which the load capacitance during logic high to excite the gates rather than dumping the charge to the GND [5]. The circuit is shown in figure 3 which consists of two exclusive NORs realized by four transistors. The Carry Out is calculated by multiplexing inputs A and carry in (Cin). The sum is generated from the output of second stage exclusive NOR. If there is a capacitor charging at the output node of the first exclusive NOR and if initially, A=B=0, and A goes to high. When A and B both equal to low the capacitor is charged by VDD. In the next stage when B goes to high keeping A fixed at low, the capacitor discharges through A. Some charge is retained in A. Hence when A goes high it is not required to be charged fully. So the energy consumption is well managed and low here [5]. 3. MULTIPLIER ARCHITECTURES The multipliers are the well organised array of adder cells [4]. The performance and characteristics of the multipliers depend upon the algorithm in which they are based on [4]. Due to the emphasis of low power design, speed is not only the criterion for design objective. Therefore, designing multipliers with low power adder modules is essential keeping an eye on the
  • 5. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 5 consumer market of portable electronic devices. In this paper, we have designed and characterized three well-known multipliers viz. The 4 Bits Array multiplier, the 4 bits Carry Save Multiplier and 4 bits Baugh Wooley Multipliers. 3.1. Bits Array Multiplier Bit Array multiplier has a simple expandable structure which makes it easy to understand. In Bit Array Multiplier, the partial product is generated by multiplying multiplicand and multiplier bits [4]. The partial products are placed according to the correct shift in bit orders and then are added. If there are N partial products in the Bit – Array multiplier, (N-1) bit adders are required. Figure 4 shows the schematic of 4 bit- array multiplier. There is more than just one critical path in the Bit- Array multiplier. Figure4. 4bits Array Multiplier. Source: J.M Rabaey Y et al., Digital Integrated Circuits, Prentice Hall Publications (2003). Theoretically, the approximate equation to calculate the propagation delay of these paths is shown below. TcarryXYTsumTandT )]2()1[( −+−++=∆ (8) Where, Tsum is the delay between Carry in (Cin) and the sum bit of full adder, Tand is the delay of the AND gates, Y is the width of multiplicand, X is the width of multiplier and Tcarry is the propagation delay of the input and output carry [4]. 3.2. Carry Save Multipliers Carry-Save multiplier has the simple expandable structure like the Bits- Array multiplier. The only difference in algorithm is that in Carry Save multiplier the output carry bits are passed diagonally downwards instead of only to the right as the multiplication result does not change while doing so. The literature [4] has shown that in the final stage the sums and carries are fed in a fast carry adder usually by using fast- carry-look ahead adder. It is slightly bigger than the Bits array in the area. However, it only has one critical path.
  • 6. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 6 Figure5. 4 bits Carry-Save multiplier. Source: J.M Rabaey Y et al., Digital Integrated Circuits, Prentice Hall Publications (2003). The schematic of 4 bits Carry-Save multiplier is shown in figure 5. Theoretically, the propagation delay of this multiplier is given by equation (9). TcarryXTfinalTandT )1( −++=∆ (9) Where Tcarry is the delay between input and output carry, X is the number of partial product stages, Tfinal is the delay of final stage carry look ahead adder and Tand is the delay of the AND gate [4]. 3.3. Baugh Wooley Multiplier Baugh Wooley multiplier has different algorithm than Bits- Array and Carry- Save multipliers. It is used to perform 2’s complement multiplication and effectively handles the signed bits. The N x N Baugh Wooley multiplication algorithm is given by equation (10). ∑∑∑∑ − −+ − − −+ − − −− + − − − −−−− − +++++++−=× 2 0 1 1 2 0 1 .1 12 11 2 0 2 0 22 1111 12 2..2.2).(22).(2 N Ni N n Ni ixN N NN ji ji N N N NNNN N YiXYYXYXYXYXYX (10) Where, X and Y are N-bits operand and their product is 2N bits number [4]. The Schematic of 4 bits Baugh Wooley Multiplier is shown in figure 6. The delay of Baugh Wooley Multiplier is similar to that of Bit-Array multiplier as it has also more than one critical path.
  • 7. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 7 Fig6. 4 bits Baugh Wooley multiplier. Source: J.M Rabaey Y et al., Digital Integrated Circuits, Prentice Hall Publications (2003). 4. SIMULATION SET UP AND RESULTS The CMOS sub-circuit of the CMOS AND gate, CMOS full adders and half adders and SVL circuits were created in HSPICE decks specifying the input and output nodes. The global variables such as supply voltage and ground were specified respectively as Vdd and Vss. These sub-circuit programs are called each time when they are required by the multiplier architecture. CMOS multiplier circuits’ netlists are created with different combination of adders modules. The functionality of each circuit, CMOS AND, CMOS full adders and half adders were verified before creating the multiplier architecture net lists. The respective logic truth tables of half adder, full adder, CMOS AND gate and inverters were used during the verification. After extracting the multiplier architecture net lists, each multiplier circuits with different adder modules combination were simulated and verified to be functioning correctly before proceeding ahead for power and delay calculations. These analyses are called the transient analysis and read on the Cscope of the Hspice. Multipliers functionality verification can be done with number of different methods and approaches. In multiplier function verification, we chose a typical 4bits by 4 bits multiplying method where partial products generated are added to produce the final product in the form of [P7….to… P0] shown by equation (11) below. 01234567 0123 0123 pppppppp bbbXb aaaa −−−−−−−−−−−−− (11) We take the random two 4 bits numbers and find the final result of the multiplication in 8 bit number. Subsequently, we supply the same multiplier and multiplicand two 4 bits number through the inputs a3a2a1a0 and b3b2b1b0 in the multiplier netlist in Hspice software and check the results in Hspice Cscope readout. The corresponding 1s and 0s of [P7…P0]are compared to highs and lows of the Cscope output. If they match, the multiplier circuit is functioning correctly.
  • 8. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 8 A total of twenty four different multiplier circuits’ netlists of different adder modules combinations were created for the combination shown in table 2 below in Hspice decks and all of them were tested to be functioning correctly. Fig7. Functionality verification of CMOS AND gate. The net lists of the circuits were extracted and simulation was performed using UC Berkeley BSIM4 models available through Predictive Technology Model (PTM) which is a promising model file for accurate and predictive modeling of future transistors. All the CMOS circuits were implemented with 45nm node technology in HSPICE. Fig8. Functionality verification of 28T CMOS full Adder with SVL circuit. The simulations were run on a Red Hat Linux host machine. All the multipliers were compared and analyzed for stand-by leakage power, delay and dynamic power. The delays were measured for worst case scenarios i.e. always the worst case delay was considered for data pickup. Fig 9. Functionality verification of 10 T SERF full adder with SVL circuit. The delay measurements for the all combinations are tabulated for a convenient analysis. In the same fashion, the leakage and dynamic power is also presented in tabular form.
  • 9. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 9 Table1. Delay Measurements Table2. The stand-by Leakage and dynamic power dissipation of the 3 different multipliers with various combinations Table3. Area/transistor count Adders CMOS28T SERF 10T CMOS28Twith SVL SERF10T With SVL multipliers Bits Array 512 296 548 316 Baugh Wooley 624 354 669 399 Carry save 512 296 548 316 5. FUTURE WORK Dual threshold CMOS technique can be applied into the multiplier circuits. There are certain critical and non- critical paths in the multiplier circuits. A higher threshold voltage can be assigned to the transistors in the non- critical paths and lower threshold voltage can be assigned to Delay Measurements Bits Array Carry Save Baugh Wooley Delay with SERF adder 1.46E-10 seconds 7.26E-09 seconds 2.02E-10 seconds Delay with CMOS 28 T adder 1.46E-10 seconds 1.26E-08 seconds 3.66E-09 seconds Delay with SERF applied with SVL circuit 6.60E-09 seconds 7.27E-09 seconds 3.05E-09 seconds Delay with CMOS 28 T applied with SVL 9.12E-09 seconds 1.27E-08 seconds 2.74E-08 seconds multipliers Bits Array Carry Save Baugh Wooleypower Leakage with 28 t 1.00E-04 watts 1.12E-04 watts 1.43E-04 watts Dynamic with 28 t 1.10E-04 watts 1.05E-04 watts 1.36E-04 watts Leakage with SERF 2.14E-05 watts 2.11E-05 watts 2.08E-05 watts Dynamic with SERF 2.14E-05 watts 2.12E-05 watts 2.23E-05 watts Leakage with 28 t applied with SVL 1.39E-06 watts 1.40E-06 watts 2.74E-06 watts Dynamic with 28 t applied with SVL 5.80E-05 watts 2.25E-05 watts 1.93E-05 watts Leakage with SERF applied with SVL 1.16E-06 watts 1.16E-06 watts 1.67E-06 watts Dynamic with SERF applied with SVL 2.39E-05 watts 1.85E-05 watts 1.64E-05 watts
  • 10. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 10 the transistors in the critical paths as such the leakage current is minimized without compromising the performance [1]. Dual threshold CMOS can reduce leakage power in both stand-by and active mode of operation without a penalty on both area and delay [1]. Figure12. Dual threshold Voltage circuitry 6. CONCLUSIONS The table1 show the propagation delay of different multiplier circuits. The delay of multipliers with SERF adders applied with SVL circuit exhibit a huge difference compared to the rest two multiplier combinations applied with CMOS 28T Adder modules. The CMOS 28T Adder applied with SVL circuit has large penalty in area and have bad delays. The delay is 6.6nS in Bits Array Multiplier with SERF Adder applied with SVL circuit vs. 9.12nS in the same multiplier with CMOS 28 T Adder applied with SVL. It is 7.2nS in Carry Save Multiplier with SERF Adder applied with SVL circuit vs. 12.7nS in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 3.05nS in Baugh Wooley multiplier with SERF Adder applied with SVL circuit vs. 27nS in the same multiplier with CMOS 28T Adder applied with SVL circuit. The table2 show a comparison of stand-by leakage and dynamic power dissipation of different multiplier circuits applied with various combinations of adder modules and SVL circuit. Interestingly, the dynamic power dissipation has decreased for combinations that used SVL circuit because of low supply voltage, VL created by SVL circuit itself. Transistor count is proportional to area of the chip. Table 3 suggests that multipliers with SERF adder modules applied with SVL has less area compared to the multipliers with CMOS 28T adder modules applied with SVL circuit. The table2 shows that stand-by leakage power dissipation with SERF adder applied with SVL circuit in all three multipliers circuits is less compared to the same three multipliers with other combinations. The stand-by leakage power dissipation of multipliers with SERF adders applied with SVL circuit exhibit a significant difference compared to the rest two multiplier combinations applied with CMOS 28T Adder modules. The CMOS 28T Adder applied with SVL circuit has large penalty in area. The stand-by leakage power dissipation is 1.16µwatts in Bits array multiplier with SERF Adder applied with SVL vs. 1.39µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 1.16µwatts in Carry Save multiplier with SERF Adder applied with SVL vs. 1.4µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. It is 1.67µwatts in Baugh Wooley multiplier with SERF Adder applied with SVl circuit vs. 2.74µwatts in the same multiplier with CMOS 28T Adder applied with SVL circuit. Therefore, the multiplier circuits with SERF Adder applied with SVL circuit outperform the multiplier circuits with CMOS 28T Adder applied with SVL circuit in terms of stand-by leakage power dissipation, area and delay. In other words, the SERF adders applied with SVL circuit are suited for ultra-low power design of CMOS multipliers circuits.
  • 11. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 11 REFERENCES [1] K. Roy, S. Mukhopadhyay and H. Mahmoodi-Meimand “Leakage Current Mechanisms and Leakage Reduction techniques in Deep-Submicrometer CMOS Circuits” Proceedings of the IEEE, Vol. 91, No.2, February, 2003. [2] T. Enomoto, Y. Oka and H. Shikano, “A Slef-Controllable –Voltage –Level (SVL) Circuit for Low- Power, High-Speed CMOS Circuits", IEEE Journal of Solid –State Circuits, Vol. 38, No. 7, July 2003, Pages 1220-1226. [3] A. Agarwal, S. Mukhopadhyay, A. Raychowdhury, K. Roy “ Leakage Power Analyis And Reduction for Nanoscale Circuits” IEEE Computer Society, Vol. 26, April 2006, pages 68-80. [4] D. Kudithipudi and E. John, “Implementation of Low Power Digital Multipliers using 10 Transistor Adder Blocks” Journal of Low Power Electronics, Vol. 1 2005, pages 1-11. [5] R. Shalem, L.K John and E. John, “A Novel Low Power Energy Recovery Full Adder Cell”, Proceedings of the Great Lakes Symposium on VLSI (1999), Pages 380-383. [6] M. J. Rani and S. Malarkann, “Leakage Power Reduction and Analysis of CMOS sequential Circuits”, International Journal of VLSI design and Communication Systems, Vol. 3, No. 1 February 2012. [7] K. Kim, N. Park, Y. Kim and M. Choi, “Leakage Minimization Technique for Nanoscale CMOS VLSI based on Macro-Cell Modelling”, IEEE Design & Test of Computers, Vol. 24 July 2007 pages 322-330. [8] F. Fallah and Massoud Pedram “ Standby and Active Leakage Current Control and minimization in CMOS VLSI Circuits”, IEICE Leakage- Review-Journal. [9] M. Singh, S. Akhase, S. Sharma “ Leakage power reduction techniques of 45nm of Static Radom Access Memory ( SRAM) cells”, International Journal of Physical Sciences, vol. 6, December 2011, pages 7341-7353. [10] J. Deshmukh, K. Khare “Standby leakage Reduction in nanoscale CMOS VLSI circuits”, Proceedings of the International Conference and workshop on Emerging trends in Technology, 2010 pages 911- 914. [11] J.M Rabaey Y et al., Digital Integrated Circuits, Prentice Hall Publications 2003. [12] Ratul kr. Baruah “ Design of a low power low voltage CMOS opamp”, International Journal of VLSI design and Communication system, Vol. 1, No. 1 March 2010. [13] Neha Gupta etl., “Low power low voltage bulk driven balanced OTA”, International Journal of VLSI design and Communication system, Vol. 2, No. 4 December2011. [14] Y. Sunil Gavaskar Reddy and V.V.G.S Rajendra Prasad “ Power comparision of CMOS and adiabiatic full adder circuits”, International Journal of VLSI design and Communication system, Vol. 2, No. 3 September 2011
  • 12. International Journal of VLSI design & Communication Systems (VLSICS) Vol.3, No.5, October 2012 12 Authors Deeprose Subedi received his Bachelor of Science in Physics from Tribhuvan University, Kathmandu, Nepal and is currently pursuing Bachelor of Science in Electrical Enginering from University of Texas at San Antonio. His research interests include, low power vlsi, Intregrated Circuit design, CMOS analog and mixed signal systems design and power electronics. This paper is the spin out result of a NSF funded research entitled “Experimental Study on Computer Architecture and Performance Evaluation” where he participated in summer 2012 at University of Texas at San Antonio. He is a Student member of IEEE. Dr. Eugene John is the Professor of electrical and computer engineering at the University of Texas at San Antonio. His research interests include low power techniques, power efficient circuits and systems, multimedia and embedded systems and performance evaluation. He has a Ph.D. in electrical engineering from the Pennsylvania State University. He is a senior member of the IEEE.