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J. of Active and Passive Electronic Devices, Vol. 2, pp. 143–164
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Advanced Applications of Current Conveyors:
A Tutorial
S. S. RAJPUT∗1 AND S. S. JAMUAR2
2
1
National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-India
Department of Electrical and Electronic Engineering, Faculty of Engineering, University Putra
Malaysia, Serdang Malaysia
Current conveyors (CCs) are being increasingly employed to replace
operational amplifiers in almost all analog signal-processing applications
because their current mode architectures are particularly suitable
for today’s low-voltage high frequency applications. CCs’ unique
architectures can easily transform into other current mode structures.
CCs’ advanced circuit and device applications are presented in this
tutorial article. All these structures can be implemented in CMOS.
INTRODUCTION
Analog VLSI can address almost all real world problems and finds exciting
new information processing applications in variety of areas such as integrated
sensors, image processing, speech recognition, hand writing recognition etc
[1]. All conventional analog circuits viz., op amps, voltage to frequency
converters, voltage comparators etc. are voltage mode circuits (VMCs),
which suffer from low bandwidths arising due to the stray and circuit
capacitances and are not suitable in high frequency applications. The
need for low-voltage low-power circuits is immense in portable electronic
equipments like laptop computers, pace makers, cellphones etc. VMCs are
rarely used in low-voltage circuits as the minimum bias voltages depend on
the threshold voltages of the MOSFETs. However, in current mode circuits
(CMCs), the currents decide the circuit operation and enable the design of
the systems that can operate over wide dynamic range. The low end of the
circuit operating range is limited by the leakage currents and noise floor
level while the high end is decided by degradation of the trans-conductance
∗ Corresponding
Author: E-mail: [email protected]; [email protected]
143
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 1
Block diagram of a CC.
per unit current available above the threshold voltage. These circuits can
give large bandwidths and are suitable for low-voltage applications. Current
feedback amplifiers (CFAs), Operational floating Conveyors (OFCs) Current
Conveyors (CCs) etc. are the popular CMC structures and most widely used
structure among them is the CCII structure [2]. In this tutorial article we
present some of the emerging applications of the CCs and the classification
schemes.
A CC is a three or more port (X, Y , Z ) network. The commonly used
block representation of a CC is shown in Figure 1, whose input-output
relationship is given by
⎤⎡ ⎤
⎡ ⎤ ⎡
0
A 0
VY
IY
⎣VX ⎦ = ⎣ B R X 0⎦ ⎣ I X ⎦
(1)
0 C 0
IZ
VZ
where A, B, C assume a value either 1, 0 or −1 and R X is the intrinsic
resistance offered by the port X to the input currents. For an ideal CC
VX = VY and the input resistance (R X ) at port X is zero (equation (1)).
But in practical CCs, R X is a nonzero positive value. So the equivalent
symbol of a CC should include R X in its representation and the popular
CC symbol is shown in Figure 2 [3]. The equivalent circuits are used
to analyze the complex circuits. One can understand the circuit operation
better when the complex structures are simplified using equivalent circuits.
For an analog circuit designer precise equivalent models of devices are
essential for getting the near exact circuit performance and the one such
model is given in reference [4].
CLASSIFICATION OF CURRENT CONVEYORS
There are several schemes for classification of CCs. Most common techniques
among them are based on the characteristics of its ports X, Y and Z .
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
145
FIGURE 2
A CCII symbol.
CCs have also been classified similar to power amplifiers based on the
quiescent current flow.
Port Y based classification
Port Y is used as input for voltage signals and it should not load the
input voltage source by drawing current. But, in some applications, it is
desirable to draw currents from the input voltage source. So, when port
Y draws a current equal to the current injected at port X( A = 1) and the
configuration 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 port
X but of opposite polarity, the configuration is known as CCIII for which
A = −1 [5–7].
Port X based classification
For voltage signals, port Y serves as input port and now the port X serves
as output port. The output voltage at port X can either have same polarity
as that of the input voltage (VY ) or that of opposite polarity. CCs in
which the polarity of the output voltage is opposite to that of the voltage
applied at port Y , are termed as inverting CCs [8] (B = −1), but when
the polarity at port X remains same as that of input voltage, CC is called
as non-inverting CC and B = 1.
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S. S. RAJPUT AND S. S. JAMUAR
Port Z based classification
Port Z is the current output port and usually, the magnitude of the output
current at port Z equals to the magnitude of the current injected into port
X. In some cases, however, this amplitude may be scaled version (generally
up scaled) of the input current and also the direction of the current may
be same or opposite to that of the current in port X. A CC with positive
current output is termed as CC+ and with negative output currents as CC−
[4]. A CC can have two or more output ports, which can independently
sink or source currents. Such a CC is known as multi port CC. A multi
port CC with both types of output ports (positive as well as negative), is
known as composite port CCII.
Quiescent current based classification
Similar to the classification of power amplifiers, CCs have been classified
as Class A, Class B and Class AB mode CCs. In a class A CC, a quiescent
current flows throughout the circuit operation. The bandwidth of this CC
is high. Contrary to this, in class B CC, current flows through the circuit
only when the input signal is present. Such a circuit consumes negligible
power in standby mode, but its bandwidth is much smaller compared to
class A CCs. Class AB CCs have emerged as the best alternative, where a
small amount of quiescent current flows throughout the circuit operations.
Class AB CCs have higher bandwidth than that of a class B CCs and the
power consumption is much less than a class A CC [9–12].
Other CC configurations
Other CC configurations are electronically controlled CC (ECCII) differential
voltage CC (DVCC), differential difference CC (DDCC), fully differential
CC (FDCC) and operational floating conveyor (OFC) [9–12]. There are
some other variations in the above structure. A recently introduced CC
structure [4] has negative R X (equation (1)).
CCII REALIZATIONS
CCII is the most versatile CMC structure among all CCs and can be used
in almost all analog circuit operations [13]. The conventional applications
of CCs include amplifiers, oscillators, filters, wave shaping circuits, analog
computers etc. [9, 12]. Low-voltage and low-power architectures of CCs are
particularly suitable in the design of voltage and power starved systems. The
need of such systems arises in medical electronics, space instrumentation
etc. 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 in
circuit and device structures.
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
147
FIGURE 3
Class AB CCII.
CCII APPLICATIONS IN MATHEMATICAL OPERATIONS
Squarer/square rooter cell
Most common CCII based mathematical cell is shown in Figure 4. This cell
can perform various mathematical functions like current amplifier, voltage
amplifier, log amplifier, antilog amplifier, current differentiator, current
integrator, low pass filter and high pass filter etc by proper selection of
Z 1 and Z 2 . Table 1 shows some of the possible functions, which can be
performed by this mathematical cell.
TABLE 1
Functions performed by circuit of Figure 5 under different conditions
Z1
R1
R1
C1
R1 + 1/sC1
R1 /sC1 /(R1 + 1/sC1 )
1
R1
Z2
R2
C2
R2
R2
R2
R2
D2
Transfer function
R2 /R1
sC2 /R1
R1 /sC2
R2 /(R1 + 1/sC1 )
R2 (R1 sC1 +1)/ R1
–
–
Function
Current amplifier
Current differentiator
Current integrator
Low pass filter
High pass filter
Log amplifier
Antilog amplifier
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 4
Basic CCII based circuit structure.
This cell works as squarer if Z 1 is a resistance and Z 2 , a squaring
element [12, 14, 15]. When Z 1 is a squaring element and Z 2 a resistance
it serves as square rooter. MOSFETs have the desired squaring property.
But only positive current can be injected if we use a NMOS and negative
currents can be sourced if we use a PMOS and hence, bipolar signal
requires parallel combination of PMOS and NMOS transistors. A circuit
shown in Figure 5 can replace this parallel combination. This circuit has
two identical sections consisting of M1, M2 and M3, M4. Transistors M3
and M4 operate when the input current (I I N ) is negative and M1 and
M2 operate for positive currents. We assume that M1 and M3 operate in
FIGURE 5
Proposed circuit used in place of MOSFETs.
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
149
saturation region and M2 and M4 are forced to operate in sub-threshold
regions by selecting low biasing currents Ibias1 and Ibias2 . Depending upon
the polarity I I N , either section consisting of M1 and M2 is operating or
the section built using M3 and M4 is operating [12].
The relationship between input current (I I N ) and resultant voltage (VI N )
across the input terminals, is given by (assuming M1 is in saturation and
M2 is in sub-threshold) [12, 14, 15]
2
Ibias1 W2
− VT
IIN = 0.5βn VIN + ηVther log
ID O L 2
(2)
The mismatch between the threshold voltages of NMOS and PMOS
(VT ) is usually quite small (< 50mV) and Ibias1 is chosen to be too
low (<1nA). So we can simplify equation (2) as I I N = 0.5βn (Vin )2 , which
shows that the output current flowing through the proposed structure is the
square of the voltage developed at the drain terminal of the input MOSFET.
Square rooting structure
The current square rooting circuit is shown in Figure 6. I I N is injected
into port Y which results in to voltage (VI N ). This volatge is proportional
to the square root of the input current I I N . This voltage gets transferred
to port Xfrom the portY . The current (I X ) through port X is decided by
the voltage present at port X (VI N ) and the resistance R X This current
then gets transferred to high impedance port Z [12, 14, 15]. The resultant
FIGURE 6
CCII based current square rooter.
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 7
Characteristics of current square rooter for DC currents.
current (I X = I Z ) equals
1
IX = IZ ≈
RX
2I I N
βn
(3)
The current I Z is the square root of the injected current I I N . A simulated
performance of the CCII based current square rooter is shown in Figure 7.
Current squaring structure
CC based current squaring structure can be implemented if Z 1 is the
resistance and Z 2 is the squaring circuit (proposed circuit of Figure 5).
The resultant circuit is shown in Figure 8. The input current develops a
voltage I I N RY at port Y . This voltage (VY ) transfers to port X and the
current I X will now be 0.5βn (I I N RY )2 . This current is available at the high
impedance output port Z for further processing and is the square of the
input current I I N . Simulated current squaring performance of this circuit
is shown in Figure 9.
CC APPLICATIONS IN DEVICE STRUCTURES
A CC is the hybrid structure of voltage buffer and current buffer (CMCs
and VMCs). This structure is used to derive other current mode structures
[12, 16–18]. CMCs operate at low voltages and have wide bandwidths. In
this section we describe various schemes used to derive other CMCs from
the CC structiures.
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
FIGURE 8
CCII based current squarer.
FIGURE 9
Response of current squarer for DC currents.
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 10
CCII based OFC.
OFC structures
Operational Floating Conveyor (OFC) is another current mode analog
signal-processing (CMASP) cell. An OFC is more versatile analog building
block than an op amp and CCII [19]. The class AB realization of the
OFC can be obtained by suitable modifications in class AB CCII of Figure
4. The proposed modifications are very simple and it requires that the
connection between drain of M3 and gate of M2 be removed. The gate of
M2 is renamed as port X and the drain terminal of M3 now functions as
port 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 voltage
amplifiers. The circuit of a current amplifier suitable for an OFC structure
is shown in Figure 11 [12, 13, 20]. Rin is connected between port X
and port W . From port W , a resistance R F is connected to ground. If a
current Iin is injected, the resulting structure behaves as a current amplifier
and the amplified current is available at port Z . The current gain (Ai ) is
(1 + Rin /R F ). 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 at
port X. The voltage gain (Av ) is (1 + R F /Rin ) [12, 13, 20].
CFA structures
A current feedback amplifier (CFA) is the most widely used CMASP
structure. The CCs can be used to get CFAs as well [12,16–18]. Figure
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
FIGURE 11
OFC based current amplifier.
FIGURE 12
OFC based voltage amplifier.
153
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 13
CCII 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 a
voltage buffer. The port Y acts as non-inverting input while port X serves
as inverting port for this resultant CFA. A high input impedance voltage
buffer follows this. The non-inverting port (+Iin ) exhibits high impedance
to voltage signals where as the inverting port present low impedance to
the input current signals.
Since high input impedance section consisting of 2-transistors M11 and
M12, is placed at the output port Z of CCII-; only a small current flow
through port Z . This current develops high voltage at the gates of M11
and M12. Now we may conclude that the characteristics of the resultant
structure are similar to that of an op amp structure. The port X and port
Y are virtually shorted. Any current injected into port X will flow through
the feedback resistance R F and an equivalent voltage will develop at the
output port.
The frequency response for the inverting, CFA based amplifier has a
bandwidth of 80 MHz, 100 MHz and 120 MHz for a gain of 10, 2 and 1
respectively [12].
CC IN BUILT IN SELF TEST STRUCTURES
CCs can be used in built-in self-test structures to monitor the supply current
and/or currents in various branches of the circuits. This current gives the
signatures of the faults through which the location of these faults may also
be determined.
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
155
FIGURE 14
CCII based current sensor schematic.
CCII as current sensors
The power supply current is an important parameter through which the
health of any circuit can be determined. The change in power supply
current under quiescent condition could be due to the fault in any part of
the circuit [12]. Thus by monitoring the quiescent power supply current, a
faulty system could be detected. A CCII based current measuring structure
is shown in Figure 14 in which one can measure the current in the circuit
under test (CUT) and compare it with the current flowing through a good
circuit.
CCIII as current sensors
The CCII based current sensor circuits require that CUT should have floating
power supply and the current mirror should be of high performance. The
CCIII based circuit can be used without such constraints [3, 12]. However,
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 15
CCIII based current sensor schematic.
CCII based sensors require that the voltages at current tapping point should
be small compared to the bias voltages so that the CCIII can safely operate
in its operating range.
The block schematic of CCIII based current sensor is shown in Figure
15. The current enters into port X and comes out of port Y . Since the input
ports X and Y present virtual short circuit, no voltage drop is required for
current tapping. The current flowing into the CUT is available at output
port Z for further processing, where one can process the signal through
neural computing techniques.
APPLICATIONS IN SPACE EXPLORATION
Aboard a space vehicle, space, power and weight constraints are at the
premium. In such applications, the measuring and control systems are
required to operate at low voltage and low power levels. In this section
we will deal with two emerging applications of CCII in the instruments,
which are must for in-situ measurements of space plasma.
Current Electrometers
The need for measuring very low currents that lie in the range of micro
amperes down to pico-amperes and even femto-amperes exists in many diverse
areas of research such as mass spectroscopy, particle accelerators, ultra high
vacuum technology, photo-metric measurements and atmospheric research
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
157
FIGURE 16
Single stage multi output port CCII.
[12, 21–26]. An alternative to the conventional electrometer design, CCII
based current electrometer is described. It has very low power consumption
and can be used on board a spacecraft, where power consumption, space
and supply voltage levels are crucial. This circuit has large bandwidth (>10
MHz) and can be used as a multi gain range electrometers. A single stage
multi port CCII is shown in Figure 16. Current gain at port Z 1 and Z 2
are −1 and −10 respectively. The current gain of −10 has been obtained
using larger aspect ratio of the output transistor at port Z 2. A voltage
−Iin R f develops at port Z 1. This voltage is used for the measurement of
the current Iin .
A multi range current electrometers is obtained by cascading CCII
structures. Three stage CEM is shown in Figure 18 where the output at
different ports are given as [12, 21–25]
⎡ ⎤ ⎡ 0 ⎤
V1
10 R1
⎢V2 ⎥ ⎢101 R2 ⎥
⎢ ⎥ = ⎢ 2 ⎥ Iin
(4)
⎣V3 ⎦ ⎣10 R3 ⎦
3
V4
10 R4
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. The
output voltage from the proper port is selected for further processing. The
task of the selection of proper output port from all these ports can be
158
S. S. RAJPUT AND S. S. JAMUAR
FIGURE 17
Three stage CCII based current electrometer.
accomplished using an analog multiplexer. An analog switch can be used
to select the appropriate output for further processing. The control signals
for the multiplexer have been obtained by monitoring the voltages at the
output ports of different CCIIs. The simulated output characteristics of the
different ports are shown in Figure 18.
Electronic simulation of plasma
In space experiments atmospheric parameters are measured by sampling
the plasma particles through a probe, which converts them into equivalent
currents [12, 26]. The charged particle densities (N), their temperatures (T )
and their density distribution can be obtained from the current collected
FIGURE 18
Output voltage characteristics at different ports of the CEM.
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
159
FIGURE 19
Portable electronic plasma simulation source in BiCMOS technology.
through plasma transducers, for example the current of an electron RPA
has an exponential characteristics.
Earlier plasma simulation sources use p − n junction diodes. Limitations
of diode based sources forced one to use a transistor. In these sources
forward bias voltage across the base emitter junction has been manipulated
to achieve the required temperature variations and spacecraft charging
effects. This voltage have been decreased or increased according to higher
or lower temperatures.
The inclusion of spacecraft charging effects into plasma simulation
source requires a shift in the I − V characteristics. A voltage equal to the
spacecraft potential is subtracted from the virtual bias voltage.
Portable electronic plasma simulation source (PEPSS) in BiCMOS
technology is shown in Figure 19. Six blocks of CCIIs are used in
conjunction with two n − p − n transistors and six resistors. Block 1
functions as a voltage amplifier and transfers the input voltage to port
X where a resistance R X is placed. Blocks 2, 3 and 6 act as a voltage
buffer and transfer the voltage applied at its port Y to port X. Block 4 is
used in current conveyor mode which transfer the current flowing through
transistor Q1 to the output port Z . Block 5 is used as a current amplifier
and scales the current available at the output port Z .
The current (Iout ) available at the output port Z of block 5 is given as
⎤
⎡
R2 V S
q
V
−
in
R1
R3
RZ
⎦
Iout =
I S exp ⎣
(5)
R4
RX
kT
The performance parameters of a plasma simulation source include the
obtainable current range, voltage shift capability and temperature variability
range.
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 20
Simulated current characteristics at various temperatures.
All CCIIs used in the plasma simulation source operate at ±1.0 V. The
output current (Iout ) varies between 5 pA to 40 µA. It has an output
impedance of 30 M and consumes 2.7 mW power. Figure 20 depicts Iout
when Rin /R Z 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 the
I − V curve increases. Iout can be attenuated or amplified to a desired level
to 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 in
Figure 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 the
electrometer for measurement. Corresponding to the different slopes, for
different temperatures, ac current signals of varying amplitudes are available
from the PEPSS. Thus, the proposed PEPSS can simulate the requisite
current signals for testing the instrument for its temperature measuring
capabilities as well.
CMOS plasma simulator
It may be desirable to have MOSFET only current source structure.
This will avoid the use of BiCMOS technology and/or hybrid circuit
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
161
FIGURE 21
Simulation of effects of plasma potentials.
structure. A MOSFET has exponential I − V characteristics when operated
in sub-threshold region [12, 26]. The drain current (I DS ) of a MOSFET
having channel length L, channel width W , oxide capacitance Cox , mobility
of electrons in channel µn , threshold voltage VT N biased with gate to source
voltage of VG S , in sub-threshold regime at temperature T is given as
IDS
µn COX W
=
L
nkT
qe
2
q(VGS − VTN )
exp
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 source
has features similar to a BiCMOS plasma source. Operation of the MOSFET
in the sub-threshold region is ensured by the application of gate bias, which
is within few mV (≈ kT/q) of the threshold voltages. The CCII based
CMOS circuit structure, given in Figure 22 is similar to Figure 19, where
transistors have been replaced by MOSFETs. Hence, the output current of
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S. S. RAJPUT AND S. S. JAMUAR
FIGURE 22
PEPSS in CMOS technology.
the CMOS source is
Iout =
⎛
R3 W1
I D O1 exp ⎝
R4 L 1
C R Z Vin −
R2 V S
R1
R X nVther
⎞
⎠
(7)
where C is a constant.
Equation (7) shows that proper selection of R Z and R X can simulate
temperature effects. Influences of spacecraft charging have been incorporated
by the use of VS , R1 and R2 . R3 and R4 selection can be made to scale
the current outputs to desired levels. An ac signal superimposed, over Vin
is used for dI e /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 ranges
from few pA to few microamperes. The I − V characteristics of current
source are similar to the one shown in Figure 20 for simulation of various
temperatures (300 K, 400 K, 600 K and 1200 K). Similarly, the I − V
characteristics of the source depicting the effect of spacecraft potential
are similar as shown in Figure 21. The spacecraft potential have been
assumed to be varying between −0.2 V, and 0.2 V with a step of
0.2 V.
CONCLUSIONS
In this tutorial article, we have presented classification and some advanced
applications of CCs. The concept of modularity has been introduced in
analog circuit design through reconfiguring a current conveyor as CFAs
ADVANCED APPLICATIONS OF CURRENT CONVEYORS: A TUTORIAL
163
and OFCs. Some of the performance parameters are bandwidth, power
dissipation etc. We have seen that these cells can perform better than op amp
based circuits in almost all signal-processing applications. These circuits
are going to find immediate applications in custom-built analog ICs. All
the 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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