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TRANSCRIPT
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CHAPTER 6
CONCLUSIONS
6.1 Introduction
The fast growth in the field of digital signal processing has resulted in the replacement
of electronic functions that were conventionally performed in analog domain by digital
domain. But there are many applications where it is absolutely not possible to replace
the analog functions with their digital counterparts irrespective of overwhelming
advances in digital technology. The analog signal processing in the form of
amplification and filtering might be needed for the processing of naturally occurring
signals obtained from the transducers. The signals picked by antenna of radio frequency
receivers and those obtained from the various transducers being weak and corrupted
with noise need amplification and filtering. Such analog functions are also required for
processing of signals in optical receivers which are inherently of low level and need
wide spectrum. Thus filtering, amplification and analog to digital conversion of analog
signals is essentially required before their application to digital processors. It has
resulted in the coexistence of both analog and digital systems on the same chip, giving
rise to the era of mixed mode design.
The usage of mixed mode circuits have become more pronounced with the
proliferation of battery powered applications where low power circuit design has
become extremely desirable for portable devices such as cellular phones, palm
computers, hearing aids, implantable cardiac pacemakers etc.. The low power
dissipation in these products results in enhanced battery life and eliminates the need of
cooling mechanism. Moreover the increasing demands of such products encourage the
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research and development efforts in the design of circuits which consume low power
and operate with low supply voltages.
The implementation of mixed circuits is feasible only in CMOS technology
because it is economical, offers high density and ensures low power dissipation. Scaling
down the supply voltage with shrinking device dimensions is the obvious technique for
reducing power consumption of mixed circuits and enhancing the available battery life.
Though scaling down the supply voltage suits the digital circuits but it poses problems
for analog part. Thus, design of analog circuits has to be optimized for low voltage and
low power applications.
Furthermore, the design of low voltage analog circuits is favored by current
mode operation where signal processing is done in current domain with least
significance of node voltages. In the presented research work, we have designed some
analog circuits based on standard QFGMOS technology and studied their applications in
analog signal processing at a nominal supply voltage of ± 0.5 V. QFGMOS is the
modification of FGMOS and offers advantages in terms of larger transconductance,
better frequency response, less chip area with no problem of trapped charges. We have
presented QFGMOS based current mirror, voltage controlled resistor and voltage
controlled current conveyors. Voltage controlled current conveyors have been further
used to realize impedance convertor and active filters in voltage and current mode. All
the investigations on these circuits have been verified through PSpice simulations using
level 7 model parameters obtained from MOSIS for 0.13 µm CMOS technology with a
supply voltage of ± 0.5 V.
It is desirable to verify the theoretical and simulated results of the studied and
proposed circuits with experimental evidences. However, the experimental verification
on actual hardware could not be carried out due to non-availability of fabrication
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facilities at the place of study and thus, the proposed circuits could not be realized in
silicon. The study of proposed circuits has been carried out using both qualitative and
quantitative analyses followed by Pspice simulations.
6.2 Summary of the Thesis Work
We have studied the design techniques for low voltage analog circuit design. We have
found that FGMOS is the right candidate for use in low voltage circuit design because
of the threshold voltage programmability. FGMOS structure has been studied and an
equivalent circuit has been developed for use in simulation and design of circuits. The
mathematical modeling and small-signal high frequency model for N-input FGMOS has
also been presented. The threshold voltage of two-input gate FGMOS has been found to
be less as compared to conventional MOSFET. As FGMOS based circuit structures
require larger silicon area, have lesser transconductance and suffer from poor frequency
response due to additional capacitances present in them, QFGMOS has been presented
as a modification over FGMOS. The equivalent circuit of QFGMOS has been presented
and compared with that of FGMOS. The mathematical modeling for two-input gate
QFGMOS as well as its simulation model have also been presented.
The current mirror (CM) which is the basic building block in analog system
design has been discussed with an emphasis on its low voltage applications using
QFGMOS. The QFGMOS CM is compared with FGMOS CM in term of input
compliance voltage and frequency response. The input compliance voltage of QFGMOS
CM has been found to 0.6 V which is less as compared to FGMOS CM (0.9 V) for input
current of 500 µA. The bandwidth of QFGMOS CM has also been found to be more
than that of the FGMOS CM. Enhancement in bandwidth of QFGMOS CM is achieved
through a novel resistive compensation technique. The compensation technique has
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been thoroughly analyzed along with the derivation of the optimal value of the
compensating resistor followed by the discussion of the results.
Finally, we have presented the development and characteristics of an N-type
QFGMOS based low voltage current mirror (LVCM). The QFGMOS based LVCM has
been simulated using 0.13 µm technology at supply voltage ± 0.5 V. The circuit offers
input resistance of 235 Ω, output resistance of 117 kΩ and consumes 0.83 mW of
power. The input compliance voltage of the QFGMOS based LVCM has been found to
be less by 0.1 V as compared to simple QFGMOS CM for an input current of 500 µA.
The offset current is found to be 2.2 nA and current transfer ratio is 0.98 with error that
varies from 13.5 % for input currents below 100µA to −1.8% at input current of 500µA.
The bandwidth of the QFGMOS based LVCM was found to be 656 MHz. Enhancement
in bandwidth for QFGMOS based LVCM is also shown using both passive as well as
active resistors. A P-type version of the QFGMOS based LVCM has also been
developed and simulated to verify its operation.
Finally, a QFGMOS based voltage controlled resistor has been presented and
analyzed. The behaviour of QFGMOS based resistor has been compared with its
FGMOS counterpart through Pspice simulations. It has been observed that for a given
value of control voltage, the QFGMOS based resistor offers a high value of resistance
and better frequency response as compared to its FGMOS version.
QFGMOS based LVCMs have also been used in the design of CMOS translinear
CCCII. The translinear CCCII has been further modified by feeding the bias current
applied through a FGMOS based voltage controlled current source (VCCS). The
resulting configuration has been called voltage controlled current conveyor (VCCCII).
The simulated characteristics of both VCCCII+ and VCCCII– structure are
presented. It is observed that the voltage transfer ratio of VCCCII+ is 0.921with a
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bandwidth of 333 MHz. The input resistance at port Y is found to be 42.67 kΩ and
output resistance at port X is 2.09 kΩ. The bandwidth for current transfer is found to be
248 MHz with current transfer ratio of 0.964. The input resistance at port X is found to
be 2.09 kΩ and output resistance at port Z is 113 kΩ. Similar results have also been
discussed for VCCCII–.The concept of electronic tunability in VCCCII is also discussed
with the generation of intrinsic resistance at port X. It is observed that the intrinsic
resistance varies from 31 kΩ to 2.12 kΩ as bias voltage to VCCS is varied from 0V to
0.5V.
Lastly, the applications of QFGMOS based voltage controlled current conveyor
in realization of transconductors, impedance converters and active filters have been
presented. Positive and negative transconductors have been realized using VCCCIIs.
These circuits have been further employed to obtain the circuit of impedance converter
to simulate a grounded inductance. VCCCII has also been used to implement the
structures of low-pass and band-pass filters. A new topology of band-pass filter which
provides transfer function both in voltage as well as transconductance mode is also
presented. Finally, a universal filter based on VCCCII± that provides the low-pass,
high-pass, band-pass, band-reject and all-pass responses simultaneously has also been
presented.
6.3 Suggestions for Future Work and Study
We have studied and verified the working of the circuits presented in the thesis through
PSpice simulations using actual CMOS process parameters along with relevant
mathematical derivations. Though PSpice simulation results validate the behavior of
circuits but hardware implementation and experimental verification provide more
realistic results. Due to non feasibility of fabrication facilities at the place of research
we could not present the experimental results. Therefore, this work can be further
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extended by carrying out the layout of the presented circuits and their implementation
on chip.
The use of QFGMOS can also be investigated for the design of low voltage
voltage-controlled oscillators and companding filter applications. The non-linear
applications of the current mode circuits have not been studied. Many new applications
of QFGMOS can be sought in analog multipliers, mixers, frequency multiplication to
mention a few. The use of QFGMOS based voltage controlled oscillators and
multipliers can be further explored in the design of phase locked loops and frequency
modulation circuits. Further, the QFGMOS based low voltage digital circuits where
high operating speed bears much significance also needs to be investigated.