advanced applications of current conveyors - atutorial_rajput

J. of Active and Passive Electronic Devices, Vol. 2, pp. 143–164Reprints available directly from the publisherPhotocopying permitted by license only

©c©2007 Old City Publishing, Inc.Published by license under the OCP Science imprint,

a member of the Old City Publishing Group

Advanced Applications of Current Conveyors:A Tutorial

S. S. RAJPUT∗1 AND S. S. JAMUAR2

1National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-India2Department of Electrical and Electronic Engineering, Faculty of Engineering, University Putra

Malaysia, Serdang Malaysia

Current conveyors (CCs) are being increasingly employed to replaceoperational amplifiers in almost all analog signal-processing applicationsbecause their current mode architectures are particularly suitablefor today’s low-voltage high frequency applications. CCs’ uniquearchitectures can easily transform into other current mode structures.CCs’ advanced circuit and device applications are presented in thistutorial article. All these structures can be implemented in CMOS.

INTRODUCTION

Analog VLSI can address almost all real world problems and finds excitingnew information processing applications in variety of areas such as integratedsensors, image processing, speech recognition, hand writing recognition etc[1]. All conventional analog circuits viz., op amps, voltage to frequencyconverters, voltage comparators etc. are voltage mode circuits (VMCs),which suffer from low bandwidths arising due to the stray and circuitcapacitances and are not suitable in high frequency applications. Theneed for low-voltage low-power circuits is immense in portable electronicequipments like laptop computers, pace makers, cellphones etc. VMCs arerarely used in low-voltage circuits as the minimum bias voltages depend onthe threshold voltages of the MOSFETs. However, in current mode circuits(CMCs), the currents decide the circuit operation and enable the design ofthe systems that can operate over wide dynamic range. The low end of thecircuit operating range is limited by the leakage currents and noise floorlevel while the high end is decided by degradation of the trans-conductance

∗ Corresponding Author: E-mail: [email protected]; [email protected]

143

144 S. S. RAJPUT AND S. S. JAMUAR

FIGURE 1Block diagram of a CC.

per unit current available above the threshold voltage. These circuits cangive large bandwidths and are suitable for low-voltage applications. Currentfeedback amplifiers (CFAs), Operational floating Conveyors (OFCs) CurrentConveyors (CCs) etc. are the popular CMC structures and most widely usedstructure among them is the CCII structure [2]. In this tutorial article wepresent some of the emerging applications of the CCs and the classificationschemes.

A CC is a three or more port (X , Y , Z ) network. The commonly usedblock representation of a CC is shown in Figure 1, whose input-outputrelationship is given by⎡

⎣ IY

VX

IZ

⎤⎦ =

⎡⎣0 A 0

B RX 00 C 0

⎤⎦

⎡⎣VY

IX

VZ

⎤⎦ (1)

where A, B , C assume a value either 1, 0 or −1 and RX is the intrinsicresistance offered by the port X to the input currents. For an ideal CCVX = VY and the input resistance (RX ) at port X is zero (equation (1)).But in practical CCs, RX is a nonzero positive value. So the equivalentsymbol of a CC should include RX in its representation and the popularCC symbol is shown in Figure 2 [3]. The equivalent circuits are usedto analyze the complex circuits. One can understand the circuit operationbetter when the complex structures are simplified using equivalent circuits.For an analog circuit designer precise equivalent models of devices areessential for getting the near exact circuit performance and the one suchmodel is given in reference [4].

CLASSIFICATION OF CURRENT CONVEYORS

There are several schemes for classification of CCs. Most common techniquesamong them are based on the characteristics of its ports X , Y and Z .

ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL 145

FIGURE 2A CCII symbol.

CCs have also been classified similar to power amplifiers based on thequiescent current flow.

Port Y based classificationPort Y is used as input for voltage signals and it should not load theinput voltage source by drawing current. But, in some applications, it isdesirable to draw currents from the input voltage source. So, when portY draws a current equal to the current injected at port X(A = 1) and theconfiguration is termed as CCI. When port Y draws zero current, it is CCII(A = 0). Similarly, when this current equals to the current injected at portX but of opposite polarity, the configuration is known as CCIII for whichA = −1 [5–7].

Port X based classificationFor voltage signals, port Y serves as input port and now the port X servesas output port. The output voltage at port X can either have same polarityas that of the input voltage (VY ) or that of opposite polarity. CCs inwhich the polarity of the output voltage is opposite to that of the voltageapplied at port Y , are termed as inverting CCs [8] (B = −1), but whenthe polarity at port X remains same as that of input voltage, CC is calledas non-inverting CC and B = 1.


Port Z based classificationPort Z is the current output port and usually, the magnitude of the outputcurrent at port Z equals to the magnitude of the current injected into portX . In some cases, however, this amplitude may be scaled version (generallyup scaled) of the input current and also the direction of the current maybe same or opposite to that of the current in port X . A CC with positivecurrent output is termed as CC+ and with negative output currents as CC−[4]. A CC can have two or more output ports, which can independentlysink or source currents. Such a CC is known as multi port CC. A multiport CC with both types of output ports (positive as well as negative), isknown as composite port CCII.

Quiescent current based classificationSimilar to the classification of power amplifiers, CCs have been classifiedas Class A, Class B and Class AB mode CCs. In a class A CC, a quiescentcurrent flows throughout the circuit operation. The bandwidth of this CCis high. Contrary to this, in class B CC, current flows through the circuitonly when the input signal is present. Such a circuit consumes negligiblepower in standby mode, but its bandwidth is much smaller compared toclass A CCs. Class AB CCs have emerged as the best alternative, where asmall amount of quiescent current flows throughout the circuit operations.Class AB CCs have higher bandwidth than that of a class B CCs and thepower consumption is much less than a class A CC [9–12].

Other CC configurationsOther CC configurations are electronically controlled CC (ECCII) differentialvoltage CC (DVCC), differential difference CC (DDCC), fully differentialCC (FDCC) and operational floating conveyor (OFC) [9–12]. There aresome other variations in the above structure. A recently introduced CCstructure [4] has negative RX (equation (1)).

CCII REALIZATIONS

CCII is the most versatile CMC structure among all CCs and can be usedin almost all analog circuit operations [13]. The conventional applicationsof CCs include amplifiers, oscillators, filters, wave shaping circuits, analogcomputers etc. [9, 12]. Low-voltage and low-power architectures of CCs areparticularly suitable in the design of voltage and power starved systems. Theneed of such systems arises in medical electronics, space instrumentationetc. where we need longer life of batteries and/or available power is limited.We take use a class AB CCII of Figure 3, to demonstrate its capability incircuit and device structures.


FIGURE 3Class AB CCII.

CCII APPLICATIONS IN MATHEMATICAL OPERATIONS

Squarer/square rooter cellMost common CCII based mathematical cell is shown in Figure 4. This cellcan perform various mathematical functions like current amplifier, voltageamplifier, log amplifier, antilog amplifier, current differentiator, currentintegrator, low pass filter and high pass filter etc by proper selection ofZ1 and Z2. Table 1 shows some of the possible functions, which can beperformed by this mathematical cell.

TABLE 1Functions performed by circuit of Figure 5 under different conditions

Z1 Z2 Transfer function Function

R1 R2 R2/R1 Current amplifierR1 C2 sC2/R1 Current differentiatorC1 R2 R1/sC2 Current integratorR1 + 1/sC1 R2 R2/(R1 + 1/sC1) Low pass filterR1/sC1/(R1 + 1/sC1) R2 R2 (R1sC1+1)/ R1 High pass filter1 R2 – Log amplifierR1 D2 – Antilog amplifier


FIGURE 4Basic CCII based circuit structure.

This cell works as squarer if Z1 is a resistance and Z2, a squaringelement [12, 14, 15]. When Z1 is a squaring element and Z2 a resistanceit serves as square rooter. MOSFETs have the desired squaring property.But only positive current can be injected if we use a NMOS and negativecurrents can be sourced if we use a PMOS and hence, bipolar signalrequires parallel combination of PMOS and NMOS transistors. A circuitshown in Figure 5 can replace this parallel combination. This circuit hastwo identical sections consisting of M1, M2 and M3, M4. Transistors M3and M4 operate when the input current (II N ) is negative and M1 andM2 operate for positive currents. We assume that M1 and M3 operate in

FIGURE 5Proposed circuit used in place of MOSFETs.


saturation region and M2 and M4 are forced to operate in sub-thresholdregions by selecting low biasing currents Ibias1 and Ibias2. Depending uponthe polarity II N , either section consisting of M1 and M2 is operating orthe section built using M3 and M4 is operating [12].

The relationship between input current (II N ) and resultant voltage (VI N )across the input terminals, is given by (assuming M1 is in saturation andM2 is in sub-threshold) [12, 14, 15]

IIN = 0.5βn

(VIN + ηVther log

(Ibias1

IDO

W2

L2

)− �VT

)2

(2)

The mismatch between the threshold voltages of NMOS and PMOS(�VT ) is usually quite small (< 50mV) and Ibias1 is chosen to be toolow (<1nA). So we can simplify equation (2) as II N = 0.5βn(Vin)2, whichshows that the output current flowing through the proposed structure is thesquare of the voltage developed at the drain terminal of the input MOSFET.

Square rooting structureThe current square rooting circuit is shown in Figure 6. II N is injectedinto port Y which results in to voltage (VI N ). This volatge is proportionalto the square root of the input current II N . This voltage gets transferredto port Xfrom the portY . The current (IX ) through port X is decided bythe voltage present at port X (VI N ) and the resistance RX This currentthen gets transferred to high impedance port Z [12, 14, 15]. The resultant

FIGURE 6CCII based current square rooter.


FIGURE 7Characteristics of current square rooter for DC currents.

current (IX = IZ ) equals

IX = IZ ≈ 1

RX

√2II N

βn(3)

The current IZ is the square root of the injected current II N . A simulatedperformance of the CCII based current square rooter is shown in Figure 7.

Current squaring structureCC based current squaring structure can be implemented if Z1 is theresistance and Z2 is the squaring circuit (proposed circuit of Figure 5).The resultant circuit is shown in Figure 8. The input current develops avoltage II N RY at port Y . This voltage (VY ) transfers to port X and thecurrent IX will now be 0.5βn(II N RY )2. This current is available at the highimpedance output port Z for further processing and is the square of theinput current II N . Simulated current squaring performance of this circuitis shown in Figure 9.

CC APPLICATIONS IN DEVICE STRUCTURES

A CC is the hybrid structure of voltage buffer and current buffer (CMCsand VMCs). This structure is used to derive other current mode structures[12, 16–18]. CMCs operate at low voltages and have wide bandwidths. Inthis section we describe various schemes used to derive other CMCs fromthe CC structiures.


FIGURE 8CCII based current squarer.

FIGURE 9Response of current squarer for DC currents.


FIGURE 10CCII based OFC.

OFC structuresOperational Floating Conveyor (OFC) is another current mode analogsignal-processing (CMASP) cell. An OFC is more versatile analog buildingblock than an op amp and CCII [19]. The class AB realization of theOFC can be obtained by suitable modifications in class AB CCII of Figure4. The proposed modifications are very simple and it requires that theconnection between drain of M3 and gate of M2 be removed. The gate ofM2 is renamed as port X and the drain terminal of M3 now functions asport W . The resultant OFC is a 4-port network as shown in Figure 10.

The resultant OFC circuit can be used to build current and voltageamplifiers. The circuit of a current amplifier suitable for an OFC structureis shown in Figure 11 [12, 13, 20]. Rin is connected between port Xand port W . From port W , a resistance RF is connected to ground. If acurrent Iin is injected, the resulting structure behaves as a current amplifierand the amplified current is available at port Z . The current gain (Ai) is(1 + Rin/RF ). The OFC based voltage amplifier is shown in Figure 12,where and the input voltage is applied at port Y and the output is taken atport X . The voltage gain (Av) is (1 + RF/Rin) [12, 13, 20].

CFA structuresA current feedback amplifier (CFA) is the most widely used CMASPstructure. The CCs can be used to get CFAs as well [12,16–18]. Figure


FIGURE 11OFC based current amplifier.

FIGURE 12OFC based voltage amplifier.


FIGURE 13CCII based CFA.

13 shows the circuit of CFA, which was derived through a CCII-structure.This circuit uses a CCII- whose output port Z is buffered through avoltage buffer. The port Y acts as non-inverting input while port X servesas inverting port for this resultant CFA. A high input impedance voltagebuffer follows this. The non-inverting port (+Iin) exhibits high impedanceto voltage signals where as the inverting port present low impedance tothe input current signals.

Since high input impedance section consisting of 2-transistors M11 andM12, is placed at the output port Zof CCII-; only a small current flowthrough port Z . This current develops high voltage at the gates of M11and M12. Now we may conclude that the characteristics of the resultantstructure are similar to that of an op amp structure. The port X and portY are virtually shorted. Any current injected into port X will flow throughthe feedback resistance RF and an equivalent voltage will develop at theoutput port.

The frequency response for the inverting, CFA based amplifier has abandwidth of 80 MHz, 100 MHz and 120 MHz for a gain of 10, 2 and 1respectively [12].

CC IN BUILT IN SELF TEST STRUCTURES

CCs can be used in built-in self-test structures to monitor the supply currentand/or currents in various branches of the circuits. This current gives thesignatures of the faults through which the location of these faults may alsobe determined.


FIGURE 14CCII based current sensor schematic.

CCII as current sensorsThe power supply current is an important parameter through which thehealth of any circuit can be determined. The change in power supplycurrent under quiescent condition could be due to the fault in any part ofthe circuit [12]. Thus by monitoring the quiescent power supply current, afaulty system could be detected. A CCII based current measuring structureis shown in Figure 14 in which one can measure the current in the circuitunder test (CUT) and compare it with the current flowing through a goodcircuit.

CCIII as current sensorsThe CCII based current sensor circuits require that CUT should have floatingpower supply and the current mirror should be of high performance. TheCCIII based circuit can be used without such constraints [3, 12]. However,


FIGURE 15CCIII based current sensor schematic.

CCII based sensors require that the voltages at current tapping point shouldbe small compared to the bias voltages so that the CCIII can safely operatein its operating range.

The block schematic of CCIII based current sensor is shown in Figure15. The current enters into port X and comes out of port Y . Since the inputports X and Y present virtual short circuit, no voltage drop is required forcurrent tapping. The current flowing into the CUT is available at outputport Z for further processing, where one can process the signal throughneural computing techniques.

APPLICATIONS IN SPACE EXPLORATION

Aboard a space vehicle, space, power and weight constraints are at thepremium. In such applications, the measuring and control systems arerequired to operate at low voltage and low power levels. In this sectionwe will deal with two emerging applications of CCII in the instruments,which are must for in-situ measurements of space plasma.

Current ElectrometersThe need for measuring very low currents that lie in the range of microamperes down to pico-amperes and even femto-amperes exists in many diverseareas of research such as mass spectroscopy, particle accelerators, ultra highvacuum technology, photo-metric measurements and atmospheric research


FIGURE 16Single stage multi output port CCII.

[12, 21–26]. An alternative to the conventional electrometer design, CCIIbased current electrometer is described. It has very low power consumptionand can be used on board a spacecraft, where power consumption, spaceand supply voltage levels are crucial. This circuit has large bandwidth (>10MHz) and can be used as a multi gain range electrometers. A single stagemulti port CCII is shown in Figure 16. Current gain at port Z1 and Z2are −1 and −10 respectively. The current gain of −10 has been obtainedusing larger aspect ratio of the output transistor at port Z2. A voltage−Iin R f develops at port Z1. This voltage is used for the measurement ofthe current Iin .

A multi range current electrometers is obtained by cascading CCIIstructures. Three stage CEM is shown in Figure 18 where the output atdifferent ports are given as [12, 21–25]⎡

⎢⎢⎣V1

V2

V3

V4

⎤⎥⎥⎦ =

⎡⎢⎢⎣

100 R1

101 R2

102 R3

103 R4

⎤⎥⎥⎦ Iin (4)

where R1 = R2 = R3 = R4= 20 k�.The CEM has many output voltage ports (Figure 17). The output

voltage signals are available simultaneously at all these output ports. Theoutput voltage from the proper port is selected for further processing. Thetask of the selection of proper output port from all these ports can be


FIGURE 17Three stage CCII based current electrometer.

accomplished using an analog multiplexer. An analog switch can be usedto select the appropriate output for further processing. The control signalsfor the multiplexer have been obtained by monitoring the voltages at theoutput ports of different CCIIs. The simulated output characteristics of thedifferent ports are shown in Figure 18.

Electronic simulation of plasmaIn space experiments atmospheric parameters are measured by samplingthe plasma particles through a probe, which converts them into equivalentcurrents [12, 26]. The charged particle densities (N), their temperatures (T )and their density distribution can be obtained from the current collected

FIGURE 18Output voltage characteristics at different ports of the CEM.


FIGURE 19Portable electronic plasma simulation source in BiCMOS technology.

through plasma transducers, for example the current of an electron RPAhas an exponential characteristics.

Earlier plasma simulation sources use p − n junction diodes. Limitationsof diode based sources forced one to use a transistor. In these sourcesforward bias voltage across the base emitter junction has been manipulatedto achieve the required temperature variations and spacecraft chargingeffects. This voltage have been decreased or increased according to higheror lower temperatures.

The inclusion of spacecraft charging effects into plasma simulationsource requires a shift in the I − V characteristics. A voltage equal to thespacecraft potential is subtracted from the virtual bias voltage.

Portable electronic plasma simulation source (PEPSS) in BiCMOStechnology is shown in Figure 19. Six blocks of CCIIs are used inconjunction with two n − p − n transistors and six resistors. Block 1functions as a voltage amplifier and transfers the input voltage to portX where a resistance RX is placed. Blocks 2, 3 and 6 act as a voltagebuffer and transfer the voltage applied at its port Y to port X . Block 4 isused in current conveyor mode which transfer the current flowing throughtransistor Q1 to the output port Z . Block 5 is used as a current amplifierand scales the current available at the output port Z .

The current (Iout ) available at the output port Z of block 5 is given as

Iout = R3

R4IS exp

⎡⎣ RZ

RX

q(

Vin − R2VSR1

)kT

⎤⎦ (5)

The performance parameters of a plasma simulation source include theobtainable current range, voltage shift capability and temperature variabilityrange.


FIGURE 20Simulated current characteristics at various temperatures.

All CCIIs used in the plasma simulation source operate at ±1.0 V. Theoutput current (Iout ) varies between 5 pA to 40 µA. It has an outputimpedance of 30 M� and consumes 2.7 mW power. Figure 20 depicts Iout

when Rin/RZ ratio corresponds to the electron temperature values of 300 K,400 K, 600 K and 1200 K. The ratios of resistances R1/R2 and R3/R4

have been selected as unity. As temperature decreases the slope of theI − V curve increases. Iout can be attenuated or amplified to a desired levelto suit the specific requirement of the test instrument by choosing R3/R4.Iout for three different voltage shifts (−0.20 V, 0.0 V, 0.20 V) is shown inFigure 21. Other levels of voltage shifts can be selected through R1/R2

ratios.For Te measurement an ac signal is super imposed at bias sweep

and its effect is reflected in PEPSS current. This current is fed to theelectrometer for measurement. Corresponding to the different slopes, fordifferent temperatures, ac current signals of varying amplitudes are availablefrom the PEPSS. Thus, the proposed PEPSS can simulate the requisitecurrent signals for testing the instrument for its temperature measuringcapabilities as well.

CMOS plasma simulatorIt may be desirable to have MOSFET only current source structure.This will avoid the use of BiCMOS technology and/or hybrid circuit


FIGURE 21Simulation of effects of plasma potentials.

structure. A MOSFET has exponential I − V characteristics when operatedin sub-threshold region [12, 26]. The drain current (IDS) of a MOSFEThaving channel length L, channel width W , oxide capacitance Cox , mobilityof electrons in channel µn , threshold voltage VT N biased with gate to sourcevoltage of VGS, in sub-threshold regime at temperature T is given as

IDS = µnCOXW

L

(nkT

qe

)2

exp

(q(VGS − VTN)

nkT

)(6)

where n lies between 1.2 and 2.These characteristics can be compared with the characteristics of a

forward biased p − n junction diode. The CMOS plasma simulation sourcehas features similar to a BiCMOS plasma source. Operation of the MOSFETin the sub-threshold region is ensured by the application of gate bias, whichis within few mV (≈ kT/q) of the threshold voltages. The CCII basedCMOS circuit structure, given in Figure 22 is similar to Figure 19, wheretransistors have been replaced by MOSFETs. Hence, the output current of


FIGURE 22PEPSS in CMOS technology.

the CMOS source is

Iout = R3

R4

W1

L1IDO1 exp

⎛⎝C RZ

(Vin − R2VS

R1

)RX nVther

⎞⎠ (7)

where C is a constant.Equation (7) shows that proper selection of RZ and RX can simulate

temperature effects. Influences of spacecraft charging have been incorporatedby the use of VS , R1 and R2. R3 and R4 selection can be made to scalethe current outputs to desired levels. An ac signal superimposed, over Vin

is used for dIe/dV measurements needed for Te determination.Simulation results show that the proposed source consumes 2.6 mW

power and has output impedance of 40 M�. The output current rangesfrom few pA to few microamperes. The I − V characteristics of currentsource are similar to the one shown in Figure 20 for simulation of varioustemperatures (300 K, 400 K, 600 K and 1200 K). Similarly, the I − Vcharacteristics of the source depicting the effect of spacecraft potentialare similar as shown in Figure 21. The spacecraft potential have beenassumed to be varying between −0.2 V, and 0.2 V with a step of0.2 V.

CONCLUSIONS

In this tutorial article, we have presented classification and some advancedapplications of CCs. The concept of modularity has been introduced inanalog circuit design through reconfiguring a current conveyor as CFAs


and OFCs. Some of the performance parameters are bandwidth, powerdissipation etc. We have seen that these cells can perform better than op ampbased circuits in almost all signal-processing applications. These circuitsare going to find immediate applications in custom-built analog ICs. Allthe result presented here has been verified using the P-Spice simulations.It is possible to explore new applications for such circuits

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advanced applications of current conveyors - atutorial_rajput

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