1. IJCST Vol. 4, Issue 3, July - Sept 2013
w w w.ijcst.com International Journal of Computer Science And Technology 335
ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
Analytical Description of Operational
Transconductance Amplifier
1
Arvind Singh Rawat, 2
Sudhir Jugran, 3
Krishna Chandra Mishra
1,3
Faculty of Tech., University Campus, Uttarakhand Technical University, Dehradun, UK, India
2
Dept. of ECE, Uttaranchal University, Dehradun, UK, India
Abstract
Inthispaperanalyticaldescriptionofoperationaltransconductance
amplifiers (OTA) is done. The basic properties of the Operational
Transconductance Amplifier (OTA) are discussed. In this paper
voltage-controlled amplifier, filters, and the impedances are
presented. Controllable voltage gain and first-order and second-
order active filter with controllable edge design and undertook
significant frequencies is performed.Aversatile family of voltage
controlledfilterSystematicdesignrequirementsoftheappropriate
sections described. The total number of components used circuit
is small, and the design equations and voltage control attractive
features. In this paper the limitations and practical OTA-based
filtersbyusingcommercialconsiderationsavailablebipolarOTAs
are discussed. OTAs of events in continuous-time monolithic
filters are considered.
Keywords
Transconductance, Voltage, Amplifier, Frequencies, Current etc
I. Introduction
Operational Transconductance Amplifier (OTA) is an amplifier
which produces an output of the differential input voltage is
present. Thus, a Voltage-Controlled Current Source (VCCS) is.
Trans-conductance amplifier for controlling a current is usually
an additional input. OTA is a high impedance differential input
stage and it can be used in navigation with feedback operational
amplifier that is similar to a standard.
The realization of a CMOS operational amplifier that combines
Unit with high gain DC current gain, high bandwidth was difficult.
Thereisaparticularprobleminlowvoltagecircuits.HighDCgain
cascodetransistordesignshaveledtoariseorHowever,thecurrent
low level of long channel devices biased the unit requires high-
frequency gain, a step required in short channel devices designed
highbiascurrentbiasedLevels.Isawellknownwaytoincreasethe
DC gain cascode without degrading the high frequency amplifier
yield. But it is not possible at low voltage cascode circuit.
OTA’s are perfect for a multitude of electronic music applications
because they can control a parameter, such as amplifier gain or
filter frequency, and control it very accurately over a range of
at least three decades. The gain of the amplifier is expressed as
a transconductance (current out / voltage in), and that gain is
programmable because it is proportional to the current going into
a gain programming pin.
Operational amplifiers are widely used in the analog signal
conditioning chain. Some of the examples are signal up/down
scaling, active filters, buffers and drivers. The amplifier topology
andtherequireddesigneffortdependhighlyontheapplicationand
the required specifications. Many parameters like speed, power
consumption,noise,offset,areaconsumption,input/outputvoltage
range, and load drive, supply voltage and production test time
must be considered.
II. Operations in Transconductance Amplifier
Starting with the basic Operational Transconductance Amplifier
(OTA) configurations the drawbacks and limitations of OTAs
will be discussed. More advanced circuits will be introduced
to improve the gain, input and output voltage range, Common
Mode Rejection Ratio (CMRR), Power Supply Rejection Ratio
(PSRR), load drive capability and offset of the amplifier. The
negative feedback is the most powerful technique to control the
important parameters. Most of the presented circuits use feedback
to improve the overall accuracy. Finally the design of current
and voltage references will be discussed. The biasing current of
the amplifier has significant impact on the overall performance.
Several topologies and circuits will be presented.
Animprovementistheintegrationofanoptional-useoutputbuffer
amplifier to the chip on which the OTA resides. This is actually
a convenience to a circuit designer rather than an improvement
to the OTA itself, dispensing with the need to employ a separate
buffer. It also allows the OTA to be used as a traditional op-amp,
if desired, by converting its output current to a voltage.
The Operational Transconductance Amplifier (OTA) is the most
widelyusedtypeofamplifierinanalogandswitchingapplications.
To achieve high driving capability and fast settling, much power
is consumed.Although the basic class-AB amplifier gives a better
tradeoff between power and slew rate, the driving capability of
the amplifier is reduced by decreasing both supply voltage and
input bias current to decrease short channel and power dissipation
effects. This problem is a critical issue for implementing an
amplifier with high driving capability and with very low standby
power dissipation.There are several modified class-AB amplifiers
that have been presented to achieve the mentioned approaches.
However, most of them require an additional current circuitry
based on conventional current mirrors to boost the tail current
dynamically when a large differential input voltage is applied.
This current source circuit suffers from mismatched effects due
to process variations resulting in limited output current that lacks
control. Although an increase in the ratio of output transistors
of the class-AB stage is often a possible way to overcome
the slew rate limitations; this increase affects both the power
dissipation and active area of the OTA and often results in extra
stray capacitances connected to the output nodes, which degrade
the frequency response. The topologies presented in use the two
additional current sources based on the basic current mirrors by
which currents can be dynamically increased if a large differential
voltage is applied to the input terminals of the OTAs.
III. MOS Amplifiers
The positive feedback loop increases the tail current and improves
thedrivingcapability.Thesecurrentsourcesalsogenerateextensive
distortionforsmallinputsignals.Sincepositivefeedbackloopgain
is limited by mismatched effects, more distortion is created than
when using an amplifier without an adaptive biasing circuit. The
other topologies need more than two additional MOS amplifier
stages and higher quiescent current, which results in increased
power dissipation and requires more silicon area. In this work, a

2. IJCST Vol. 4, Issue 3, July - Sept 2013 ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
w w w.ijcst.com336 International Journal of Computer Science And Technology
new class-AB amplifier using an adaptive biasing circuit, which
eliminates the drawbacks previously specified. Analysis and
experiment results show that reducing the mismatched effects of
the positive feedback loop of the amplifiers presented in increases
the output current and decreases the distortion of small input
signalsefficiently.Furthermore,thesymmetricswitchingbehavior
of the new current sources at transients is verified. This causes the
amplifier to be very sensitive in the adaptive biasing operation
due to the reduction in mismatched effects and increment in the
positivefeedbackloopfactor,evenwhenasmall-signaldifferential
input voltage is applied and the bias current is very small. The
sensitivity becomes a critical necessity when the amplifier is used
to drive biological applications in which the amplitude of the input
signals is typically in the range of 1 μV to 100 mV. Additionally,
reducing the distortion of small input signals leads to improved
small-signal characteristics, such as CMRR, PSRR, and so on.
To enhance these characteristics, the circuit will suffer from
mismatched offset, noise, area, and power due to using additional
circuitry.
The traditional operational amplifier (op-amp) is used as the vast
majority of active filter in the active device Literature. For design
purposes, assume that the op-amp model A, R, O is the opposite
of the v in o = ∞ common this is used to create and respond to
large essentially independent of the gain of the op-amp filters.
This has given rise to a host of practical filter design approach.
It is also, however, become clear that limitations preclude the
use of the operational amplifier filters at high frequencies, these
filters are trying to integrate with the exception of a few non-
demanding or failed programs, and adaptive voltage or current
external filter control to adjust plans features do not exist. With
the realization of the BJT and MOSFET obviously the current
and transconductance amplifiers. An immediate void in the
literature type of filter structures employing an amplifier. A well-
integrated trans-resistance evolution, transconductance, and
current amplifiers have not kept pace, Although a few devices
with voltage amplifiers the alternative categories. The structures
that are shown design simplicity and programmability update
and brightness compared to AMP-based structures component
was decreased.
Many of the basic OTA-based structures using the OTAs and
capacitorsand,therefore,isattractiveforintegration.Thestructure
was often only a very small component of the e.g. second-order
filters can be created biquadratic two OTAs and two capacitors
when compared with the VCVS design. Convenient internal
or external voltage or filter features are available with current
control this design. They are attractive for reference frequency
e.g., master or slave applications. Some groups of recently, the
use of continuous-time monolithic filters OTAs structures.
The high frequency discrete bipolar OTAperformance such as CA
3080, is pretty good for a practical standpoint. Transconductance
gain, gm, can be varied over several decades, setting an external
dc bias current IABC. The main limitation of existing OTAs is
the restricted differential input voltage swing required to maintain
linearity. For the CA 3080, which 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 is
reduced as is the will discuss the design objective of filters based
on operational amplifiers, the main will emphasize the structures
in which the standard filter parameters of interest are directly
OTA proportional to gm. Therefore, gm will be a both design
parametersuchasresistorsandcapacitors.Asitisassumedthatthe
gain of the transconductance OTAproportional to an external DC
bias current, external control of filter parameters can be obtained
through the bias current. Most of the existing work on designing
a filter based on OTA addressed the problem, either focusing on
applying feedback so that the filter characteristics independent
gain or change transconductance existing structures operational
amplifier by the inclusion of some OTAS additional passive
components. In either case, component circuits were typically
intense cumbersome to tune. Some of the more practical circuit
can found in the application notes of the manufacturer.
IV. Building Blocks of OTA
The OTA basic building blocks of many analog as the data
converter’s ADC and DAC or Gm-C circuits as filters. Gm-C
filters, the performance is related to the OTA’s operations.
Transconductance of the OTA device input voltage, output
current control, it means OTA voltage-controlled current source,
while the op-amps voltage controlled voltage source. OTA is an
op-amp without default output buffer, so it can only be drive is
loaded.TheOperationalTransconductanceAmplifier(OTA)isthe
with the highest power consumption of analog block integrated
circuits in many applications. Low power consumption, handset
devices is becoming more important so it is a challenge to design
low power OTA. One for an OTA design speed, power, and gain
standoff these parameters are generally contraindicated because
parameters.The two-stage OTAs: OTAs are three types of folded-
cascode OTAs, and telescopic OTAs. The telescope compared
with other power amplifier uses the least two amplifiers, so it
is widely used in low power access applications. It also has a
high speed comparison the other two topology. An operational
transconductance amplifier (OTA) is a voltage input, current
output amplifier. Input voltage Vin and the output current Io are
related to each other by proportionality constant and continuous
proportionality of the amplifier transconductance “gm” is
V. Conclusion
In this paper, the basic concept of different OTAis described along
withitsadvantageanddis-advantage. Agroupofvoltage-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 etc. 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 OTAbased filters using commercially
available OTAs is the severely limited differential input voltage
capability inherent with conventional differential amplifier input
stages.Recentresearchresultssuggestedsignificantimprovements
in the input characteristics of OTAs can be attained
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ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
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ArvindSinghRawatispursuingMTech,
Faculty of Technology, University
Campus, Uttarakhand Technical
University, Dehradun, UK, India.
Sudhir Jugran is working as a Sr.
Lecturer, Department of Electronics
& Communication Engineering,
Uttaranchal University, Dehradun, UK,
India.
Krishna Chandra Mishra is pursuing
MTech, Faculty of Technology,
University Campus, Uttarakhand
Technical University, Dehradun, UK,
India.