FGMOS BASED LOW-VOLTAGE LOW-POWER HIGH OUTPUT IMPEDANCE REGULATED CASCODE CURRENT MIRROR

International Journal of VLSI design & Communication Systems (VLSICS) Vol.4, No.2, April 2013
DOI : 10.5121/vlsic.2013.4204 39
FGMOS BASED LOW-VOLTAGE LOW-POWER
HIGH OUTPUT IMPEDANCE REGULATED
CASCODE CURRENT MIRROR
Abhinav Anand1
, Prof. Sushanta K. Mandal2
, Anindita Dash3
, B. Shivalal Patro4
1,2,3,4
School of Electronics Engineering, KIIT University, Bhubaneswar, India
abhinavvlsi@gmail.com1
, sushantakumar@yahoo.com2
, litun.1103@gmail.com3
,
shivalal.lali@gmail.com4
ABSTRACT
Floating Gate MOS (FGMOS) transistors can be very well implemented in lieu of conventional MOSFET
for design of a low-voltage, low-power current mirror. Incredible features of flexibility, controllability and
tunability of FGMOS yields better results with respect to power, supply voltage and output swing. This
paper presents a new current mirror designed with FGMOS which exhibit high output impedance, higher
current range, very low power dissipation and higher matching accuracy. It achieves current range of up to
1500 µA, high output impedance of 1.125 TΩ, bandwidth of 4.1 MHz and dissipates power as low as 10.56
µW. The proposed design has been simulated using Cadence Design Environment in 180 nm CMOS
process technology with +1.0 Volt single power supply.
KEYWORDS
Floating Gate MOSFET, Current Mirror, Regulated Cascode, Low-Voltage & Low-Power
1. INTRODUCTION
A current mirror (CM) is a circuit designed to copy a current through one active device by
controlling the current in another active device of circuit, keeping the output current constant
regardless of loading. The responsibilities of current mirror circuit are current amplification and
to provide proper biasing to analog circuits. Due to very wide application of current mirror in low
voltage low-power analog circuits, accuracy, output impedance and power consumption play key
role in determining CM performance [6]. Their utilization in analog signal processing extends the
advantages of low-voltage operations, derivation of resistorless topologies and electronic
adjustment capability of their frequency characteristics [13].
With the increasing demand for smaller and faster products, there is an ongoing trend in
fabrication process towards smaller transistors. Reducing feature size arises potential problem
like power dissipation and dielectric breakdown due to high electric field across the feature.
These potential problems can be compensated by reducing the voltage and hence low-voltage
power supply is beneficial. In addition to this battery lifetime is crucial factor in various
applications and hence low-power device is also a compulsory requirement as per current market
trend. In the quest to achieve low-voltage and low-power, various techniques have evolved in due
course of time and Floating Gate MOS (FGMOS) technique is one amongst them.

40
FGMOS is proven technology in digital circuits but has also gained pace when it comes to its
application in the area of low-voltage, low-power analog design. FGMOS is a device in which the
second gate generally called floating gate is electrically isolated but capacitively coupled to input
gates. A control voltage present at one of the multi-input FGMOS and facility to additional
weighted inputs provides wide range of tunability to the circuit [1]. Implementation of FGMOS
allows threshold voltage (Vth) controllability without reducing the feature size, thus operates at
power supply voltage levels which are well below the intended operational limit. Also, it
consumes less power than the minimum required power for a circuit designed with conventional
MOSFET. Using regulated cascode configuration achieves high output impedance of the order of
gm
2
rout
3
where gm is device trans-conductance and rout is output resistance [5], [10]-[11].
Various FGMOS based current mirror designs have been presented in [4], [7] and [8]-[9].
FGMOS based current mirror design presented in [4] and [8] suffers from higher power
dissipation and lower current range. On the other hand [7] and [9] both presents FGMOS based
design having better results with respect to input impedance, bandwidth and supply voltage but
exhibits very low output impedance. Current mirror suggested in [12] has ultra low power
dissipation but at a very low biasing current of the order of atto ampere.
In this paper a new low-voltage, low-power regulated cascode current mirror has been proposed.
At input side of the proposed design a 3-input FGMOS device has been introduced to control the
input characteristics of CM by varying the input bias voltage at one of the floating gate. The
design has been supported by the simulated results.
The rest of the paper is organised as follows: Brief introduction of FGMOS and regulated cascode
current mirror (RCCM) is given in section 2. Section 3 describes the proposed low-voltage, low-
Power CM (LV-LPCM). Simulation results are discussed in section 4. Finally, some conclusions
are drawn in section 5.
2. FGMOS AND REGULATED CASCODE CURRENT MIRROR
In this section floating gate MOSFET (FGMOS) and regulated cascode CM (RCCM) have been
briefly introduced with their diagrammatic representation. This section also justifies the
implementation of FGMOS and RCCM into the final design.
2.1. Floating Gate MOSFET (FGMOS)
Floating Gate MOS transistors are widely used in digital world as EPROM, EEPROM, flash
memories, and neuronal computational element in neural network, digital potentiometers and
single transistor DAC’s. But, it is also gaining its popularity in analog design and has found
application in low-voltage low-power analog design and analog storage elements. A typical multi
input floating gate transistor is shown in Figure 1. It is a conventional MOSFET in which the gate
is capacitively coupled to the input using another poly-silicon layer. The equation that models the
behaviour of floating gate voltage (VFG) of FGMOS is given by equation (1) [1], [3].

41
Figure 1. Equivalent schematic of N-input n-channel FGMOS
(1)
Where and QFG accounts for the amount of charges that is being
trapped in FG during fabrication.
FGMOS device provides impressive features relevant to low-voltage, low-power context. It
shows flexibility in implementing both linear as well as non-linear functions. Independent control
of threshold voltage (Vth) accounts for controllability of the device. Also, since it is a multi-input
device, FGMOS allows addition of extra inputs as per designer’s requirement and hence provide
tunability to the circuit.
In order to simulate FGMOS in Cadence design environment, a simulation model is needed which
is shown in Figure 2 [1], [3]. In the model, the voltages V1, V2, ..., Vn are the input voltages which
are coupled to gate by resistor-capacitor parallel combination. VF and V0 are floating gate voltage
and substrate voltage respectively. M is a conventional MOSFET.
Figure 2. Simulation Model for N-input n-channel FGMOS

42
Figure 3. Test circuit for comparison between N-type FGMOS and NMOS
Figure 3 shows the test circuit for comparison between 2-input N-type FGMOS and a
conventional NMOS. The simulated output characteristics and power dissipation on comparative
basis is shown in Figure 4 and Figure 5 respectively.
Figure 4. DC response of N-type FGMOS and conventional NMOS
Power dissipation of 2-input N-type FGMOS is 673.9 µW for the test circuit, which is nearly 40%
less than that of conventional NMOS transistor. This suffices for one of the reason why FGMOS
device is used in low-voltage, low-power design.

43
Figure 5. Power Dissipation of N-type FGMOS and conventional NMOS
2.2. Regulated Cascode Current Mirror (RCCM)
Regulated cascode CM is improved version of simple CM and it uses negative feedback concept
for output current stabilization [2]. A typical regulated cascode configuration is shown in Figure
6. The primary reason behind considering RCCM configuration as a basis for the proposed design
is its high output impedance of the order of gm
2
rout
3
. Also, current gain matching for regulated
cascode configuration is good as long as primary transistors are properly biased.
Figure 6. Regulated Cascode Current Mirror configuration

44
3. PROPOSED FGMOS BASED LV-LPCM
The proposed LV-LP RCCM is based on FGMOS technique and is shown in Figure 7. It allows
the design to work at a voltage as low as +0.8 Volts but works best at a single supply voltage of
+1.0 Volts. It exhibits good matching accuracy and high output impedance of the order of 1.125
TΩ.
Table 1. Transistor Sizes of the Proposed CM
Transistors W(µm) L(nm) W/L
3-input N-
Type
FGMOS
18 180 100
NM0 36 180 200
NM1 18 180 100
NM2 5.4 180 30
NM3 18 180 100
NM4 18 180 100
PM0 18 180 100
PM1 18 180 100
Figure 7. Proposed LV-LP Regulated Cascode CM

45
Power dissipation of the circuit has been reduced to a value as low as 10.56 µW. It achieves
higher current range of up to 1500 µA. The aspect ratios for various transistors have been
summarized in Table1.
Transistors NM1, 3-input N-type FGMOS, NM4 and NM3 accounts for conventional regulated
cascode CM. Transistors PM0 and PM1 are biasing transistors and provides accurate matching at
the output. Transistor NM2 is connected in negative feedback to increase the output resistance
and stabilize the output current. NM0 is specifically introduced in common gate configuration to
achieve higher output resistance up to TΩ range. The circuit however suffers from low bandwidth
which is overshadowed by other satisfactory results viz. high output resistance, large current
range and very low power dissipation.
3. SIMULATION RESULTS
The proposed circuit is simulated with Cadence Spectre simulator in Virtuoso ADEL
environment. Various simulated results corresponding to its respective analysis viz. transient, DC
or AC analysis with appropriate test circuits has been done. DC analysis is carried out to obtain
current range. Figure 8 shows that the design achieves a current range of upto 1500 µA. Input and
output characteristics are shown in Figure 9 and Figure 10 respectively.
Figure 8. Transfer Characteristics

46
Figure 9. Input Characteristics
Figure 10. Output Characteristics

47
Figure 11 illustrates the variation in input resistances with input biasing current. It can be
observed from the graph that input impedance of the circuit is 24.5 KΩ for biasing current of 20
µA and as low as 540 Ω for a input biasing current of 500 µA.
Figure 11. Input Impedance versus Input biasing current
Transient analysis is carried out to evaluate power dissipation of the circuit. It is observed to be
10.56 µW as shown in Figure 12, which makes the circuit a low power design.
Figure 12. Power dissipation
AC analysis is performed to obtain the output impedance and bandwidth of the proposed FGMOS
based LV-LP RCCM. The output impedance comes out to be 1.125 TΩ and bandwidth of 4.1
MHz at a biasing current of 10 µA and is shown in Figure 13 and Figure 14 respectively.

48
Figure 13. Output Impedance
Figure 14. Frequency Response
The performance of the proposed design has been compared with previous works and is being
presented in Table 2.

49
Table 2. Comparison with previous works
4. CONCLUSION
This paper demonstrates a floating gate MOSFET (FGMOS) based low-voltage, low-power
regulated cascode current mirror which exhibits a very high output impedance of 1.125 TΩ and
dissipates power as low as 10.56 µW at an input biasing current of 10 µA. The current range for
the circuit is also at par with respect to other design. The proposed design can work well with
low-power, low-voltage analog signal processing & bio-medical application. Measured
performance of proposed current mirror is comparable with other proposals reported to date, as
shown in Table 2. The simulation is done in Cadence design environment for 180 nm CMOS
technology with a single power supply of +1.0 Volts.
REFERENCES
[1] Esther Rodriguez-Villegas, “Low Power and Low Voltage Circuit Design Techniques with FGMOS
Transistor”, IET Circuit and Device System Series 20.
[2] Hitesh, Anuj Goel, “Advancement in Current Mirror Technique”, International Journal of Advance
Research in Computer Science & Software Engineering, Vol. 2, Issue 1, January 2012.
[3] Yin, L., Embabi, S. H. K., and Sánchez-Sinencio, E.: “A floating-gate MOSFET D/A converter”,
Proceedings of the IEEE International Symposium on Circuits and Systems, 1997, pp. 409–12.
[4] Parshotam S. Manhas, Susheel Sharma, K. Pal, L.K.Mangotra & K.S. Jamwal, “High performance
FGMOS-based low volage current mirror”, Indian journal of Pure & Applied Physics, Vol. 46, May
2008, pp. 355-358
[5] Behzad Razavi, Design of analog integrated circuits, McGraw Hill, 2001.
[6] S.S Rajput and S.S Jamuar, “ A high performance current mirror for low voltage designs”, Asia-
Pacific Conference on Circuits and Systems, pp. 170-173, Dec 2000.
[7] Susheel Sharma, S.S. Rajput, L.K. Mangotra and S.S. Jamuar, “FGMOS based wide range low
voltage current mirror and its application”, Asia-pacific conference on Circuit & Systems. Vol. 2 pp.
331-334, Oct 2002.
[8] Rockey Gupta, Shusheel Sharma, S.S. Jamuar, “A low voltage current mirror based on quasi-floating
gate MOSFET”, IEEE Conference, 2010.
[9] Bhawna Aggarwal, Maneesha Gupta, “Low-voltage cascode current mirror based on bulk-driven
MOSFET and FGMOS Techniques”, International Conference on Advances in Recent Technologies
in Communication and Computing, 2009.
[10] D.Johns and K.Martin, “Analog Integrated Circuit Design”, John Wiley & Sons, 1997.
Parameters
Proposed
Work
REF[12] REF[7] REF[9] REF[8] REF[4]
Process
Technology (µm)
0.18 0.18 0.50 0.25 0.50 0.50
Supply Voltage
(Volts)
+1.0(single
supply)
±1.8 ±0.75 ±0.45 ±0.75 ±0.75
Current
Range(µA)
0-1500 - 0.1-500 0-150 0-500 ~2-500
Output
Impedance(TΩ)
1.125 0.045 0.00135 0.0153 0.00167 0.7854
Input
Impedance(KΩ)
3.73 (Ibias=
50 µA)
20000 0.2 1.99 0.48 -
Power
Dissipation(µW)
10.56 1.5 - 96.5 1500 250
Bandwidth(MHz) 4.1 15.8 500 - 640 631

50
[11] P.E. Allen and D.R. Holberg, “CMOS Analog Circuit Design”, 2nd ed. Oxford, U.K. : Oxford, 2002.
[12] Seyed Javad Azhari, Khalil Monfaredi, and Hassan Faraji Baghtash, “A Novel Ultra Low Power High
Performance Atto-Ampere CMOS Current Mirror with Enhanced Bandwidth”, Journal of Electronics
Science and Technology, Vol. 8, No. 3, September 2010.
[13] Costas Laoudias and Costas Psychalinos, “Application of current mirrors in analog signal
processing”, Physics Department, Electronics Laboratory, University of Patras, Rio Patras, Greece.
Authors
Mr. Abhinav Anand : He received his B.Tech in Electronics & Telecommunication
Engineering from College of Engineering Roorkee, Uttarakhand. He has worked as
Lecturer with Uttaranchal Institute of Technology, Dehradun. Currently he is an
M.Tech student in VLSI design & Embedded System in School of Electronics
Engineering at KIIT University, Bhubaneswar, Odisha. His main researches focus on
Low-Voltage, Low-Power analog circuit design.
Dr. Sushanta K. Mandal : He received his B.E. degree in Electrical Engineering from
Jalpaiguri Govt. Engineering College, West Bengal, India in 1993, MS and Ph.D degree
from Indian Institute of Technology (IIT), Kharagpur, India in 2002 and 2008
respectively. He has 12 years of teaching and industry experience. He was actively
associated with Advanced VLSI Design Laboratory at IIT. Kharagpur from 2002-2007.
From August 2007 to June 2010, he worked as an Assistant Professor at Dhirubhai
Ambani Institute of Information and Communication Technology (DA-IICT),
Gandhinagar. Currently he is a Professor at KIIT University, Orissa since July 2010. He has published
number of research papers in reputed journals and conferences. His research interests include modelling
and synthesis of on-chip passive devices, digital, analog and mixed-signal CMOS VLSI design, high-
level synthesis and optimization of analog circuits, soft computing applications in VLSI.
Mrs. Anindita Dash : She received her B.Tech in Electronics & Telecommunication
Engineering from Ghanashyam Hemalata Institute Of Technology and Management,
Odisha in 2008. She has worked as embedded engineer in Larsen and Toubro Embedded
Division , Mysore from June 2008 to September 2010. Currently She is pursuing
M.Tech in VLSI design & Embedded System in School of Electronics Engineering at
KIIT University, Bhubaneswar, Odisha. Her area of research includes low-voltage, low-
power, high speed analog and mixed signal circuit design.
Mr. B. Shivalal Patro : He has completed his B.Tech from Trident Academy of
Technology in Electronics & Telecommunication Engineering in 2010, Odisha. He then
completed M.Tech in Communication Systems from KIIT University in 2012. Currently,
he is a Ph.D scholar at KIIT University, Bhubaneswar, Odisha. His area of research
includes high speed, low power analog and mixed signal IC design and optimization.

FGMOS BASED LOW-VOLTAGE LOW-POWER HIGH OUTPUT IMPEDANCE REGULATED CASCODE CURRENT MIRROR

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