Volume 3 No.4, Spring 2000

ISSN# 1523-9926

 

Novel Generations of Inverting Current Conveyor Using Universal Current Conveyor



Daniel Bev
becvar@umel.fee.vutbr.cz
Dept. of Microelectronics
Brno University of Technology
Kamil Vrba
vrbak@utko.fee.vutbr.cz
Dept. of Telecomunications
Brno University of Technology

ABSTRACT
Two novel types of current conveyor are introduced and defined in this paper. Both types are from the family of inverting current conveyors. They are the first- and the third- generation inverting current conveyors (ICCI, ICCIII) in the concrete. A mathematical definition of their behavior and a method of their realization are included. Their realizations are done with help of Universal Current Conveyor (UCC) -  novel versatile circuit building block. The proposed UCC  is a building block that is able to replace any type of existing current conveyor. The CMOS design of the universal current conveyor with reduced parasitic resistance on the X-terminal is shown. PSpice simulations of the proposed circuits are included. Finally, applications using these novel elements are presented.
UNIVERSAL CURRENT CONVEYOR
Sedra and Smith introduced the first current conveyor in 1968. It was called the first-generation current conveyor (CCI) [1] and it started the evolution of this universal building block. Since then designers have developed several variations of current conveyor. They have been called the second-generation and the third-generation current conveyor (CCII, CCIII) [2, 3]. These building blocks have found application in many fields, e.g. current-mode and mixed-mode filter design, instrumentation and wideband amplifiers, and many more. The most successful type is CCII. But, in some cases a disadvantage of CCII can be observed. Conventional CCII cannot be used in applications demanding differential or floating inputs like impedance converter circuits and current-mode instrumentation amplifiers. Then the design of such an amplifier requires two or more CCIIs. This problem has been solved with the help of special current conveyors - current conveyors with differential input (DDCC, DVCC) [4, 5].
The proposed universal current conveyor is a versatile building block that is able to replace any type of existing current conveyor. In this letter a detailed analysis of the proposed block is included inclusive of small-signal analysis and basic PSpice simulations, which take into account second-order effects. The design of UCC on the transistor level using CMOS technology AMS 1.2mm is included too.
Definition and possibilities

The universal current conveyor (UCC) is a novel eight-port building block. UCC has three high-impedance inputs (differential  Y1,Y2 and summing Y3 ), one low-impedance input X and four current outputs ( ). Outputs  Z1, Z2 are complementary to outputs . The matrix description of UCC and its symbol are shown in Fig. 1.
 
Figure 1. Matrix description and symbol of UCC

The main advantage of the UCC is its versatility. All existing types of current conveyor with single input could be realized with the help of the UCC. This goal can be reached by using suitable connection between terminals of the UCC. An example for CCI- is as follows:

CCI-
Y2, Y3 - grounded - vY2 , vY3 ,iY2 , iY3  missing
Y1, Z1 - connected - iY1=iZ1
                                 vZ1, iZ1 missing
, Z2- grounded - missing 
- output Z 

Similar procedures realizing all the other types of current conveyor can be found in Tab. 1 and Tab. 2. Moreover, a number of novel types of current conveyor can be realized in this way. This will be demonstrated in the following chapter. 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Table 1. Realization of CC with single input
Table 2. Realizations of CC with differential input

Notice: Current conveyors, which are marked as "novel", haven't published before.
 
UCC - CMOS implementation
 
The first CMOS realization of UCC is shown in Fig. 2. All transistors operate in saturation region.  The input stage is similar to the DDCC in [4]. 
 
Figure2. Basic CMOS implementation of UCC

The transconductance elements are realized with two differential stages (M1 and M2, M3 and M4). Current mirror (M5 and M6) is a classical active load, which provides a high-gain of the input stage. Moreover, it converts the differential current to single-ended output current (M9), and then the drain-currents equal

It is obvious 
The output voltage in node X can be obtained as
where A0 is the open-loop gain of amplifier (M1-M10) without negative feedback from node X back to the amplifier (gate of M3) and it is supposed that A0 is much larger than one.
One of the main advantages of this implementation is insensitivity to the body effect. All the PMOS transistors have sources connected to the positive supply rail. A similar situation exists in the case of NMOS transistors. Their sources are connected to the negative supply rail, except for M1-M4, which form differential pairs. But the error caused by body effect is cancelled because the transistors in the differential pair have the same source voltages and then the variations in threshold voltages are the same.
 
Figure 3. Block structure of UCC
Transistors M13-M21 form an auxiliary current conveyor, which is connected like a negative feedback to the current conveyor (M1-M12). This structure is shown in Fig. 3 and described in [6]. The main goal of this connection is lowering the parasitic resistance RX of the X-terminal. The circuitry can be described by following equations
where Zt is transfer impedance which is created by parallel combination of R'y2 and C
Now vx (when Y1, Y2 and Y3 are grounded) is can be obtain as 
and parasitic impedance can be obtained as
where Rx1,2 is the parasitic resistance of appropriate current conveyor and R'Y2 is input resistance of the second current conveyor. The input resistance is close to Rx1Rx2/R'Y2 and it is very low (mW) due to R'Y2>> Rx1,2. The circuit stability is ensured by compensation capacitor C. In proposed circuitry C is the parasitic capacitance belong to CMOS structure and dominant pole p1 is then
Output terminal Z1 is formed by transistor M21 (it operates like current source) and transistor M20, which duplicates the current of transistor M18. Output Z2 is the same as output Z1 and they form the positive types of current conveyor. Outputs  and operate on the same principle, but their output currents have opposite direction to Z1 and Z2. The small signal output resistance of the current output terminals is determined by parallel connection of output resistances of the transistors, which form each current output. Generally it is given by
where gdi, gdj are output conductance's of transistor Mi, Mj, respectively.
The biasing currents through transistors M7, M8 and M10, M12, M16, M19, M21, M23, M25, M27, M29, M31 are equal to 150 mA and 300 mA, respectively. 
Simulations and macromodel
Basic simulations of the UCC, which take into account the second-order effects, are included in this section. Simulations were carried out by standard PSpice circuit simulator. The described macromodel of UCC is based on these simulations and was applied in the design of the following applications using the UCC.
Model level which was used is level 2. Next  rows show list of some important parameters of used models:

nmos vth=0.736V,
cgso =0.290e-09   cgdo =0.290e-09   cgbo =0.170e-09

pmos vth=-0.751V 
cgso =0.290e-09   cgdo =0.290e-09   cgbo =0.170e-09

The capacitances are per unit gate area.
The second order effects are effects like channel length modulation, mobility degradation and bulk charge effects on current.
 
 

Figure 4. AC Voltage Transfer Vx/Vy

Figure 5. AC Current Transfer Iz/Ix

Figure 6. Parasitic Input Impedance Zx

Figure 7. DC Current Transfer Izbar/Ix

AC voltage transfer between Y- and X-terminal is shown in Fig. 4. The input voltage was applied on Y1-terminal and its value was 1 V. Other high-impedance input terminals and current-output terminals  were grounded. The cut-off frequency is 64 MHz. 
Characteristics in Fig. 5 show ac current transfer from X- to Z1-terminal. High-impedance Y-terminals and Z2,, -terminals were grounded. The cut-off frequency of the Z1-terminal is 73.5 MHz. The value of applied input ac current was 100 mA.
Fig. 6 shows the parasitic impedance of the X-terminal and its dependence on frequency. The value of frequency independent contribution of this impedance (pure resistive contribution) is 4 mW. It can be observe that the value of parasitic impedance on 1 MHz is still relatively small (about 0.3 W). It can be achieved due to described UCC structure.
The last characteristic in Fig. 7 shows the dc current transfer (from X-terminal to -terminal). High-impedance Y-terminals and Z1,Z2, -terminals were grounded. The value of the applied current source (on X-terminal) was changing from -250 mA to 250 mA.
Important properties of the UCC, including voltage and current offsets, are summarized in Tab. 3.
 

Table 3. Properties of the UCC
The previously simulation results are carried out by standard Pspice simulator. The simulated file was a netlist file extracted from the schematic (Figure 2). The most accuracy simulations can be done with netlist extracted from the layout of the proposed circuit (parasitic capacitances of MOS transistors convergate more to real values). 
Fig. 8 shows the macromodel of the UCC. It is composed of ideal current-followers and differential voltage-followers. Parasitic elements model the frequency behavior of a real UCC. The values of these components were derived from the above PSpice simulations. This macromodel is simple but it is useful, especially for simplified filter design using the UCC. The multifunction filter included later in this paper was designed with the help of this macromodel.
Figure 8. Macromodel of the proposed UCC
NEW TYPES OF INVERTING CURRENT CONVEYORS
The last current conveyor that was published is the second-generation inverting current conveyor (ICCII) [7]. ICCII differs from CCII in the negative voltage transfer between the Y- and the X-terminal. Next two missing types of inverting current conveyor are introduced in this paper. A mathematical definition of their behavior, the method of their realization with the help of universal current conveyor (UCC) and applications using these novel elements are presented.
Definition of ICCI and ICCIII
Generally, a current conveyor (CC) is three-terminal device (except special types of CC) and the relations between its ports can be described by the matrix equation given in Fig. 9 with the general symbol of current conveyor. Coefficients a, b, c determine each type of current conveyor (coefficients can acquire values 0, -1 and 1).
 
Figure 9. Mathematical definition and symbol of general current conveyor
Table 4. Definition of current conveyor with help of a, b, c
Concrete matrix description for each subtype of ICCI and ICCIII can be derived from Tab. 4. The difference between CC and ICC of the same subtype is only in the sign of voltage transfer between the Y- and the X-terminal.
Realisations of ICCI and ICCIII
Novel types of inverting current conveyor, which were defined in the previous chapter, have no special circuit implementation but they can be realized with the help of universal current conveyor. This versatile building block can replace all types of existing current conveyor with single input including the novel types ICCI and ICCIII. UCC is a eight-port element and the realization of each type of current conveyor can be obtained by using a suitable connection between its terminals. Procedures realizing all the existing types of current conveyor can be found in Tab.1 and Tab.2. The PSpice simulations of these novel conveyor exhibit resemblance to UCC simulations. The UCC determines properties of the ICCI or ICCIII.
 
Figure 10. Realisation of ICCI and ICCIII with the help of UCC
Application based on ICCI and ICCIII

Synthetic grounded inductor (generally gyrator) is an application which shows the usability of the novel types of inverting current conveyor. Two possible realizations with the help of novel types of ICC are given in Fig. 11. One realization consists of CCI+ and ICCIII+. The CCI+ is replaced by ICCIII- in the other realization of gyrator. Both circuitries can be described by the equation:
 Figure11. Synthetic inductor realizations and simulated results
(R1=R3=10 W, C=1 nF, R2 was a parameter)
Another application using one of the novel types of ICC is the multifunction network in Fig. 12. The input quantities are Vi1, Vi2, Vi3 and Vi4 while the output quantities are Vo1 and Vo2. The complete circuit can be described by the following equations:
For Vi3 = 0 we have
and if Vi1 = 0 then

Figure 12. Multifunction network and possible filter
realizations

   

It is evident from the above relations that the six-port can be applied in the voltage mode (Vout/Vin) as filters, which are described in Fig. 12. The simulations in Fig. 13 show the transfer characteristics of these filters (left-sided) and right-sided characteristics compare response of lowpass filter with ideal UCC and real UCC.
Figure 13.  Simulations of multifunction network (C1 =C2=10 nF R1=1.7 kW, R2=2.9 kW,)
CONCLUSIONS

Two novel types of current conveyor, the first- and the second-generation inverting current conveyor (ICCI and ICCIII), were presented in this paper. Their behavior and circuit realizations with the help of universal current conveyor were described. Applications which demonstrated the usage of the novel elements were introduced. The application with ICCI was presented in the case of multifunction six-port network, which realizes filter functions. Simulations of these applications showed the practical usability of the novel ICC. The novel ICCI and ICCIII can be inserted in the family of current conveyors as novel and useful building blocks.
Next, a novel universal building block UCC was presented too. UCC is able to replace all existing types of current conveyor with single input. The versatility of the proposed block, which is the main advantage, was demonstrated on the realizations of new types of inverting current conveyors (ICCI and ICCIII). The UCC is useful as a generalization tool and design concept for filtering applications and multifunctional circuits. 
The versatility of UCC is paid by worse frequency behavior. Each special designed current conveyor will be better from this point of view.
The future goal is the design and practical realization of UCC in BiCMOS and bipolar technology, which will allow achieving more practical useful values of the output currents, and realizing the UCC as universal device. Then the practical usage of UCC as very universal discrete device will be able.
   
ACKNOWLEDGEMENTS

This paper was carried out by project CEZ: J22/98: 262200011.
REFERENCES

 
[1] SEDRA, A., and SMITH, K.C.: "The current conveyor: A new circuit building block", Proc. IEEE, Vol.56, pp. 1368-1369, Aug. 1968
[2] SEDRA, A.S., and SMITH, K.C.: "A second generation current conveyor and its application", IEEE Trans., 1970, CT-17, pp. 132-134
[3] FABRE, A.: "Third-generation current conveyor: A new helpful active element", Elec. Letters, Vol. 31, No.5, pp. 338-339, Mar. 1995
[4] CHIU, W. and LIU, S.I.: "CMOS differential difference current conveyors and their applications", IEE Proc.-Circuit Devices Syst., Vol. 143, No.2, pp.91-96, 1996
[5] ELWAN, H.O. and SOLIMAN, A.M.: "Novel CMOS differential voltage current conveyor and its applications.", IEE Proc.-Circuits Devices Syst., 1997, Vol. 144, No.3, pp. 195-200
[6] PAYNE A. and TOUMAZOU C., "Practical integrated current conveyors.", ISCAS'94 Tutorials, chap. 11.2.6, pp.595-596
[7] AWAD, I.A. and SOLIMAN, A.M.: "Inverting second generation current conveyors: the missing building blocks, CMOS realizations and applications", Int.J.Electronics, 1999, Vol. 86, No.4, pp. 413-432

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