Seminar on Cdta

8/9/2019 Seminar on Cdta

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CHAPTER 1:- INTRODUCTION

Circuits based on the current-mode approach, such as current conveyors , currentamplifiers , and operational transconductance amplifiers, seem to have a better signal dynamic

range and closed-loop bandwidth performance than conventional voltage amplifiers. Moreover,they are particularly suitable whenever the input sources and/or the outputs are current signals.Great interest has been devoted to the analysis and design of the current differencingtransconductance amplifier proposed by Biolek , mainly because it e hibits better performance, particularly higher speed and better bandwidth, than classic voltage mode operational amplifiers,which are limited by a constant gain-bandwidth product . !his device, with " current inputs and "kinds of current output, provides an easy implementation of current-mode active filters. # C$!#is a synthesis of the well-known advantages of the current differencing amplifier and atransconductance amplifier to facilitate the implementation of current-mode active filters . !hemain features of a C$!# are low gain errors %high accuracy&, high linearity, high

transconductance, and wide fre'uency response. (n addition, high output resistance at terminal )of the C$!# is re'uired to enable easy cascading without the need for additional active elementsin applications. !he conventional C$!# structure available in the literature employs a well-known current differencing amplifier followed by a standard transconductor. #s a result, theoverall performance of this circuit differs strongly from the e pected active elementcharacteristics. !he circuit e hibits low output resistance and limited linearity range, whichcauses deviations in the e pected performance of the application circuits employing the C$!#.!here are several works available in the literature on e tending the performance of current-modeactive elements, including operational transconductance amplifiers %*!#s& and secondgeneration current conveyors %CC((s&, where input stages with a wide linearity range and outputstages providing high output impedance are necessary. +owever, good linearity, high accuracy,and high output resistance can be obtained by using high performance current mirrors andade'uate input stages in the structure of the C$!#.


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CHAPTER 2:- BASIC BUILDING BLOCKS OF CURRENTDIFFERENCING TRANSCONDUCTANCE AMPLIFIER (CDTA)

2.1 Opera !"#a$ Tra#%&"#' & a#&e A p$!*!er (OTA)

!he *!#s were made commercially available for the first time in by C#. !he first publications with *!# came out in 01, when authors presented to the general public the newCM*2 *!# architectures and new filter reali3ations.

#n ideal operational transconductance amplifier is a voltage-controlled current source,with infinite input and output impedances and fre'uency independent transconductance. *!# hastwo attractive features4 & changing the e ternal dc bias current or voltage can control itstransconductance, and "& (t can work at high fre'uencies. *!# is a voltage controlled currentsource. More specifically, the term 5operational6 comes from the fact that it takes the difference

of two voltages as the input for the current conversion. !he ideal *!# is a differential-inputvoltage-controlled current source %$7CC2&. (ts symbol is shown in 8ig , and its operation isdefined by the following e'uation %9- &. Both voltagesV : and V - are with reference to ground.!he e'uivalent circuit of the ideal *!# is shown in 8ig.

F!+.2.1.1 : (a) OTA %, "$ ( ) !'ea$ e/ !0a$e# &!r& ! .

Currently, the *!# elements are supplied on the market by many manufacturers . #commercially available *!# elements are, for e ample, ;! ""0 %;inear !echnology& or M#)


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instrumentation amplifiers, wideband, high-speed 8 filters, and high-speed differential linedriver and receiver applications.

2.2 Opera !"#% %!#+ !'ea$ OTA

"$ a+e A p$!*!&a !"# %!#+ OTA

(nverting and noninverting voltage amplification can be achieved using an *!# as shownin 8ig. ".". %a& and ".". %b&, respectively . #ny desired gain can be achieved by a proper choiof gm and RL .(t should be noted that the output voltageV o is obtained from a source with outputimpedance e'ual to RL. >ero output impedance can be achieved only if such circuits arefollowed by a voltage follower.

F!+. 2.2.1: (a) I#0er !#+ a#' ( ) #"#!#0er !#+ 0"$ a+e a p$!*!er %!#+ OTA.

A "$ a+e ar!a $e Re%!% "r ( R)

# grounded voltage-variable resistor can be easily obtained using an ideal *!# as shownin 8ig. ".".". 2ince Iout ? - Iin, we have the following 4

F!+.2.2.2 : Gr" #'e' 0"$ a+e-0ar!a $e re%!% "r %!#+ OTA.


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@sing two such arrangements cross-connected in parallel, a floating 77 can be obtained. *nthe other hand, if the input terminals in 8ig. 9-9 are interchanged, the input resistance will be- / gm. !hus, using *!#s, both positive and negative resistors become available without actuallyhaving to build them on the chip. !hese, coupled with capacitors, lead to the creation of the socalled active-C filters.

"$ a+e % a !"# %!#+ OTA

7oltage summation can be obtained using *!#s, which in effect translate voltages tocurrents. Currents are easily summed as shown in 8ig.".".9.

F!+2.2.3. : "$ a+e % a !"# %!#+ OTA.

(t is clear that

c

(f gm ? gm" ? gm9

By changing the grounded input of one of the input *!#s, voltage subtraction can be achieved.!hese operations are useful for the reali3ation of filters which should be synthesi3ed from their transfer functions.


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I# e+ra "r %!#+ OTA

!he operation of integration can be achieved very conveniently using the *!# as isshown in 8ig. ".".


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2.4 De%!+# !%% e%

!he setup for deriving the bias conditions is as follows. !he input terminals are the same$C potential, the common-mode input voltage 7cm. =e assume that the common-mode inputvoltage is allowed to range between a minimum value 7cmAmin and ma imum value 7cm4ma ,which are given. 2imilarly, we assume that the output voltage is allowed to swing between aminimum value 7outAmin and a ma imum value 7outAma %which takes into account large signaswings in the output&.!he bias conditions are that each transistor M .M0 should remain insaturation for all possible values of the input common-mode voltage and the output voltage.

!ansistor M 4 !he lowest common-mode input voltage, 7cmA min imposes the toughestconstraint on transistor M remaining in saturation

!ransistor M"4 !he systematic offset condition makes the drain voltage of M e'ual tothe drain voltage of M". !herefore, the condition for M" being saturated is the same asthe condition for M being saturated. ote that the minimum allowable value 7cmA min

is determined by M and M" entering the linear region. !ransistor M94 2ince 7gd9?D transistor M9 is always in saturation and no additionalconstraint is necessary.

!ransistor M


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(t is sometimes desirable to operate the transistors with a minimum gate overdrivevoltage. !his ensures that they operate away from the sub threshold region, and alsoimproves matching between transistors

E. Quiescent power:

!he 'uiescent power of the op-amp is given by

P= (Vdd) (Ibias+I 5+I 7 )

2.5 C rre# C"#0e,"r

I# r"' & !"#

ecently, current mode circuits have been receiving significant attention in analog signal processing. # useful function block for high fre'ency current mode application is a currentconveyor. !he current conveyor is a three terminal device performing many useful analog signal processing functions when the device is connected with other electronic elements in specificcircuit configuration. !he current conveyor has evolved from first generation to secondgeneration. !he first generation current conveyor %CC(& was proposed by 2mith and 2edra in

0 and the more versatile second generation current conveyor %CC((& was introduced by thsame two authors in ED, as an e tension of their first generation conveyor.

8igure ".1. Block diagram of current conveyor

(n may cases, the current conveyor simplifies circuit design in much the same way as theconventional op-amp, but it presents an alternative method of implementing analog systemwhich traditionally has been based on op-amp. !his alternative approach leads to new method of


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implementing analog transfer functions, and in many cases the conveyor-based implementationoffers improved performance to the voltage op-amp based implementation in terms of accuracy, bandwidth and convenience. Circuit based on voltage op-amps are generally easy to design sincethe behaviour of a current conveyor. 2everal hundred papers have been published on the thetheory and application of current conveyor. 2ome of these applications are shown in figure.

!he second generation current controlled conveyor %CCC((& which implimented with BF!and BiCM*2 technology by 8ebre, etc in 1 and E respectively. *ne of the reasons we usethe CCC(( is that it allows implementation of electronic functions usable at high fre'uency. !heother reason to use the CCC(( is its parasitic resistance at terminal ) is controllable. !hecontrollable resistance is usable for the application for the applications of tunable circuits, suchas filters.

(n order to obtain a small-si3e low weight handheld system, a single technology schemeis preffered for ma imum integration level. !o reali3e this scheme, the low-cost high-integration

all CM*2 implementation is one of the most attractive solutions.

8igure ".1." #pplications of current conveyor

2.6 Ba%!& 7e"r, "* & rre# &"#0e,"r%


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Current conveyors are four terminal devices. (t is *pen-loop current-mode amplifier withlow and fi ed current gain. !he gain is set by transistor aspects or by controlling the impedancelevels at the output or input. (t is Capable to convey current between two terminals %) and >&with very different impedance levels. 2ome advantages of current conveyors compared to op-amp is it can provide a higher voltage gain over a larger signal bandwidth than corresponding op-amp and better CM in instrumentation amplifiers.

2.8 F!r% +e#era !"# & rre# &"#0e,"r CCI

8igure ".E. first generation current conveyor

!he above figure shows a first generation current conveyor CC(. !he operation of thiscurrent conveyor may be e plained as

(f a voltage is applied to terminal , an e'ual potential will appear on the input terminal)

#n input current ( being forced into terminal ) will result an e'ual amount of currentflowing into terminal

!he current ( will be conveyed to output terminal > such that terminal > has thecharacteristics of a current source, of value (, with high output impedance

Hotential of ) being set by that of , is independent of the current being forced into port

) Current through port being fi ed by ) is independent of the voltage applied to

2.9 Ma r! repre%e# a !"# "* *!r% +e#era !"# & rre# &"#0e,"r


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2.; MOS ! p$e e# a !"# "* *!r% +e#era !"# & rre# &"#0e,"r

M*2 transistors M and M" form a current mirror that forces the drain currents of theHM*2 transistors M9 and M< to be e'ual and hence the voltages at the terminals ) and areforced to be e'ual

8igure ". . M*2 representation of first generation current conveyor

2.1< Se&"#' +e#era !"# & rre# &"#0e,"r% CCII


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(t is published by 2edra in ED. !he basic mechanism can be e plained as below4-

(f a voltage is applied to terminal , an e'ual potential will appear on the input terminal)

!he current in node ?D !he current ( will be conveyed to output terminal > such that terminal > has the

characteristics of a current source, of value (, with high output impedance Hotential of ) being set by that of , is independent of the current being forced into port

) !erminal e hibits an infinite input impedance

8igure ". D. second generation current conveyor CC((

2.11 Ma r! repre%e# a !"# "* %e&"#' +e#era !"# & rre# &"#0e,"r CCII

2.12 App$!&a !"# "* & rre# &"#0e,"r%


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1. C rre# a p$!*!er% (CCCS)

(a)

2. " I &"#0er er

%b&3. C rre# **er "r & rre# *"$$"=er

%c&4. "$ a+e a p$!*!er


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%d&5. D!**ere# !a$ " I &"#0er er

%e&

6. I#% r e# a !"# a p$!*!er

%f&

8. C rre# !# e+ra "r


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%g&9. C rre# '!**ere# !a "r

%h&

8igure ". ". #pplications of current conveyors %a& to %h&

CHAPTER 3:-CURRENT DIFFERENCINGTRANSCONDUCTANCE AMPLIFIER (CDTA)


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3.1 I# r"' & !"#

!he C$!# element with its schematic symbol in 8ig. has a pair of low-impedance currentinputs p and n, and an au iliary terminal 3, whose outgoing current is the difference of inputcurrents. #lso in 8ig. is given a possible implementation of C$!# using the *!# components.+ere, output terminal currents are e'ual in magnitude, but flow in opposite directions, and the product of transconductance gm and the voltage at the 3 terminal gives their magnitudes.!herefore, this active element can be characteri3ed with the following e'uations4

V p=V n=0, I z =I p I n

I !+ =g mV z , I ! = g mV z

=here V 3 ? Iz . "z and "z is the e ternal impedance connected to " terminal of the C$!#. C$!#

can be thought as a combination of a current differencing unit followed by a dual-outputoperational transconductance amplifier, $*-*!#. (deally, the *!# is assumed as an idealvoltage-controlled current source and can be described by I ? gm%V : I V I&, where I is outputcurrent,V : and V I denote non-inverting and inverting input voltage of the *!#, respectively.C$!# applications do not re'uire the use of e ternal resistors, which are substituted by internaltransconductors. #nalogously to the well-known 5 gmC J applications, the 5C$!#-CJ circuits areformed by C$!# elements and grounded capacitors. 2uch structures are well-suited for on-chipimplementation.

%a&


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%b&

%c&

8ig. 9. . 4 %a& 2ymbol of the C$!#, %b& its e'uivalent circuit, %c& implementation by bulk-driven*!#s

Marking the voltages of p, n, ! , and 3 terminals in 8ig. 9-"9 %a& with symbolsVp, Vn, V! , and Vz ,then for the C$!#:- element the following e'uations are true4


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8igure 9. ." possible implementation of C$!# using commercial (Cs available


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3.2 CMOS I p$e e# a !"# "* CDTA

F!+. 3.2.1 : CMOS ! p$e e# a !"# "* CDTA.


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CHAPTER 4 APPLICATIONS OF CDTA

4.1 I# e+ra "r %!#+ CDTA

!he operation of integration can be achieved very conveniently using the C$!# as shownin 8ig.

8igure


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4.3 C rre# - "'e Se&"#'-"r'er U#!0er%a$ F!$ er

!he e ample of the proposed C$!# is a second-order universal filter using C$!#s,grounded capacitors are reali3ed. +ere, one of the C$!#s involved in the circuit is chosen withthree ports. 2upposing the natural angular fre'uency$ o of the filter is "KL9D rad/sec, 'ualityfactor% is , according to the value of g mi % g m ? g m"?


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(t is easy to know that the sensitivities of this universal filter are shown as follows4

F!+. 4.3.2 ;H, BH, +H fre'uency responses of the universal filter.


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F!+. 4.3.3 B2, #H fre'uency responses of the universal filter.4.4 C rre# - "'e *" r-p7a%e / a'ra re "%&!$$a "r

!he 'uadrature mode of operation is a well know of waveform generation in the field of sinusoidal oscillators. Generally, a 'uadrature sinusoidal oscillator provides output signals of identical fre'uency and amplitude but of different phase angle. (t can be widely found of different phase angle. (t can be widely found in many applications such as, in telecommunicationfor 'uadrature mi ers and single sideband generators, or in instrumentations for vector generator or selective voltmeters. !herefore a 'uadrature oscillator plays an important role in manycommunication, measurement ant instrumentation system.

8igure


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the characteristic e'uation of the 'uatrature oscillator in figure can be e pressed as 4

!he oscillation condition and the oscillation fre'uency %ND& can resprectively be obtained as

gm Ogm9 % &

and

%"&

8rom e'uation % & and %"&, it is clear that the transconductance gain gm9 controls thecondition of oscillation without disturbing ND, which is controlled electronically by gm" withoutaffecting the condition of oscillation. !herefore, both the condition of oscillation and thefre'uency of oscillation can be orthogonally adPusted. #dditionally, owing to all the outputcurrents io , io", io9 and io< are obtained at high output impedences, the oscillator circuit can beconnected directly to the ne t stage.

8rom the circuit of figure, the current transfer from io to io" is

!he phase diffence %Q& between io and io" is e'ual to

Q? DD

!his results in two-'uadrature outputs. #lso, by based on the multiple output C$!#, thecircuit provides an inverted version of the output currents io and io". !hus, the relation of alloutput currents can be e pressed as 4

io&= io' and io = io

!his means that the circuit can provide the four 'uadrature current outputs.


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8igure


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E!E E"CES

R S 8abian Thateb, $alibor Biolek, U Bulk-$riven Current $ifferencing !ransconductance#mplifier U Circuit 2yst. 2ignal Hrocess %"D & 9D4 DE - D0 .

R"S $. Biolek, C$!#Vbuilding block for current-mode analog signal processing, in Hroc.WCC!$XD9, Trakow, Holand %"DD9&, pp. 9 EY. Tolka, #ctive elements for analog signal processing4 classification, review, and new proposals. adioengineering E, 1Y9" %"DD0

R


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R0S #. 2edra, T.C. 2mith, # second generation current conveyor and its application. (WWW!rans. Circuit !heory C!- E, 9"Y 9< % ED&

R S ikhil aP, anitesh Gupta and 7ikram Chopra, U Bulk driven *!# in D. 0 micron withhigh linearity.

R DS ;.+. 8erreira, #n ultra low-voltage ultra low power rail-to-rail CM*2 *!# Miller, inHroc. "DD< (WWW #sia-Hacific Conference on Circuits and 2ystems, !aiwan %"DD

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