ijetae_icadet_14_57

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459 (Online), Volume 4, Special Issue 1, February 2014)

International Conference on Advanced Developments in Engineering and Technology (ICADET-14), INDIA.

Lord Krishna College of Engineering (An ISO 9001:2008 Certified Institute) Ghaziabad, Uttar Pradesh, INDIA. Page 339

"Sharpening Skills..... Serving Nation"

Active Filter Design Using OTA Realization Garima

1, Priya Banga

2, Akshita Singh

3

1Electrical Department, DTU

2,3Electronics and Communication Department

[email protected], [email protected]

2, [email protected]

Abstract The OTA is an amplifier whose differential input voltage produces an output current. Thus, it is a voltage

controlled current source. Operational transconductance

amplifier is one of the most significant building-blocks in

integrated continuous-time filters. We show that the

operational transconductance amplifier (OTA), as the active

element in basic building blocks. In this paper Low pass filter

is realized using OTA realization. SPICE simulation showed

that they are suitable for real time application.

Keywords- OTA, CMOS IC Design, OTA-C Low Pass filter,

cadence, VCCS.

I. INTRODUCTION

With the realization that the BJT and MOSFET are

inherently current and transconductance amplifiers,

respectively, the following question naturally arises. Can

any improvements in filter characteristics, performance, or

flexibility be obtained by using one of the other basic types

of amplifiers (e.g., transconductance, current, or

transresistance) in place of a voltage amplifier (or

specifically the operational amplifier) as the basic active

device in a filter structure? A few devices in these alternate

categories are commercially available (e.g.,

transconductance amplifiers such as the CA 3080 and LM

13600 and transresistance amplifiers such as the LM 3900)

which offer improvement in filter characteristics.

Many of the basic OTA based structures use only OTAs

and capacitors and, hence, are attractive for integration.

Component count of these structures is often very low (e.g.,

second-order biquadratic filters can be constructed with

two OTAs and two capacitors) when compared to VCVS

designs. Convenient internal or external voltage or current

control of filter characteristics is attainable with these

designs. They are attractive for frequency referenced (e.g.,

master/slave) applications. Several groups have recently

utilized OTAs in continuous-time monolithic filter

structures.

From a practical viewpoint, the high-frequency

performance of discrete bipolar OTAs, such as the CA

3080, is quite good.

The transconductance gain, gm, can be varied over

several decades by adjusting an external dc bias current,

IABC. The major limitation of existing OTAs is the

restricted differential input voltage swing required to

maintain linearity.

For the CA 3080, it is limited to about 30 mV p-p to

maintain a reasonable degree of linearity. Although

feedback structures in which the sensitivity of the filter

parameters are reduced (as is the goal in op amp based

filter design) will be discussed, major emphasis will be

placed upon those structures in which the standard filter

parameters of interest are directly proportional to gm of

the OTA. Thus, the gm will be a design parameter much as

are resistors and capacitors. Since the transconductance

gain of the OTA is assumed proportional to an external dc

bias current, external control of the filter parameters via the

bias current can be obtained. Most existing work on OTA

based filter design approached the problem by either

concentrating upon applying feedback to make the filter

characteristics independent of the transconductance gain or

modifying existing op amp structures by the inclusion of

some additional passive components and OTAS.

II. THE PROPOSED OTA ARCHITECTURE

The block diagram of proposed OTA is shown at Figure.

It is builded in 4 stages. All of these stages described

as follows.

Block Diagram of Proposed OTA






The operational transconductance amplifier (OTA) is an

amplifier whose differential input voltage produces an

output current. Thus, it is a voltage controlled current

source (VCCS). There is usually an additional input for a

current to control the amplifier's transconductance. The

OTA is similar to a standard operational amplifier in that it

has a high impedance differential input stage and that it

may be used with negative feedback.

Schematic symbol for the OTA

Pin Diagram

In the ideal OTA, the output current is a linear function

of the differential input voltage, calculated as follows:

Where Vin+ is the voltage at the non-inverting input, Vin

is the voltage at the inverting input and gm is the

transconductance of the amplifier.

The amplifier's output voltage is the product of its output

current and its load resistance:

The OTA is not as useful by itself in the vast majority of

standard op-amp functions as the ordinary op-amp because

its output is a current. One of its principal uses is in

implementing electronically controlled applications such as

variable frequency oscillators and filters and variable gain

amplifier stages which are more difficult to implement with

standard op-amps.

III. LOW PASS FILTER REALIZATION USING SINGLE OTA






SIMULATION RESULTS

Low Pass Filter Realization using OTA Realization






SIMULATION RESULTS

IV. CONCLUSION

A group of voltage-controlled circuits using the OTA as

the basic active element have been presented. The

characteristics of these circuits are adjusted with the

externally accessible dc amplifier bias current. Most of

these circuits utilize a very small number of components.

Applications include amplifiers, controlled impedances,

and filters. Higher-order continuous-time voltage-

controlled filters such as the common Butterworth,

Chebyschev, and Elliptic types can be obtained. In

addition to the voltage-control characteristics, the OTA

based circuits show promise for high-frequency

applications where conventional op amp based circuits

become bandwidth limited.

The major factor limiting the performance of OTA based

filters using commercially available OTAs is the severely

limited differential input voltage capability inherent with

conventional differential amplifier input stages.

Recent research results suggested significant

improvements in the input characteristics of OTAs can be

attained.

The design process that was followed resulted in a

CMOS operational amplifier design that at least met and, in

a few cases, exceeded the design objectives by a large margin. The notable performance areas were the DC open

loop gain of 145 dB, and the power consumption of 180

uW. Also, the settling time was quite low as can be seen by the transient response of the circuit, which means the

circuit is relatively fast. A great deal was learned in the design process, including how to approach a design project,

the tradeoffs involved in a CMOS op-amp design, patience,

and how to stay up late. There could still be a lot improved

in this circuit, but requires knowledge that is beyond the

scope of this course, mainly in the field of VLSI.






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