118

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

119

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

120

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

121

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

122

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

123

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.