A summary of current conveyors is presented, with focus on origin, ideal terminal behaviour, hardware implementations, parasitic elements & their effects, comparison with op amps, varieties and current research areas.

1. CURRENT
CONVEYORS
A PRESENTATION BY
SACHIN BANSAL
(2K13/EL/075)

2. CONTENTS
• ORIGIN
• BLACK-BOX BEHAVIOUR
• SOME CIRCUIT IMPLEMENTATIONS
• REAL EQUIVALENT CIRCUIT AND ITS IMPLICATIONS
• OP AMPs VS CCs : BASIC CIRCUITS
• VARIETIES AND RESEARCH HOTSPOTS
• REFERENCES

3. ORIGIN
 Sedra was designing a temperature-insensitive constant current source for
his Master’s thesis (1967)
 Used another PNP, & current mirror to cancel temperature-dependence of
current

4.  Sedra & Smith found a new building block by ungrounding Q2’s
emitter (Y), removing R0 (X), & using the current output (Z)
 Named the underlying principle current conveying and the block
first generation Current Conveyor (CCI)
 Introduced more versatile second generation Current Conveyor
(CCII) in 1970

5. BLACK-BOX BEHAVIOUR
Current conveyor as a black-box
 Characteristic equations
CCI CCII

6.  ‘+’ indicates same current directions at X & Z, ‘-’ indicates otherwise
 Accordingly, we have CCI+, CCI-, CCII+, CCII-
 Nullator-norator representations
CCI CCII

7. SOME CIRCUIT IMPLEMENTATIONS
 Using off-the-shelf ICs
Senani’s Op-Amp-OTA based implementation (1980)
CCI (R01 = R02 = 1/gm) CCII (iz = gmR01ix , R01 = 1/gm)
Advantages: Low component count, electronically controllable, both CCI (+/-),
CCII (+/-) realisable

8. Karybakas-Siskos-Laopoulos’s implementation of CCII (1992)
iz = kix
k = iz/ix = G2/G1 = IB2/IB1
Advantages: current gain tunability, temperature-independent current gain

9.  Bipolar architectures
Sedra’s implementation of CCI (1967)
Advantages: Connection of 2 complementary CCI+s to ensure bipolar
operation, another current mirror can be used to obtain CCI-

10. Fabre’s implementation (1985)
For ix << Ibias , the Q1-Q2-Q3-Q4 loop ensures vx = vy
Current mirrors ensure iz = ix as usual
Both kinds of CCI & CCII are realisable when 2 identical Z outputs are available

11.  CMOS implementations
Sedra’s implementation of CCI (1967)
(MOS counterpart of identical bipolar architecture)

12. Sedra’s ‘super-transistor’-based (1970) & Wilson’s OMA-based (1984)
CCII implementations
Super-transistor: transistor with op amp in negative feedback, simulates CCII-
2 complementary super-transistors with current mirrors lead to CCII
implementations, each capable of bipolar operation
Wilson used ‘power supply current sensing’ using Operational Mirrored
Amplifier (OMA) leading to much more precise implementations

13. Super-transistor based CCII+ OMA-based CCII+

14. REAL EQUIVALENT CIRCUIT AND ITS
IMPLICATIONS
 Parasitic elements of a real CCII become important after MHz range
 These are: Parallel RC impedance between each port Y and Z, and
ground, Thevenin resistance seen from port X, and frequency dependent
voltage and current transfers
 This changes the real characteristic equations

15. The Real Equivalent Circuit of a CCII

16. Typical values of parasitic elements
 Example of effect of parasitic elements (voltage amplifier)

17.  Ideal expression: Vout/Vin = R2/R1
 Real expression: Vout/Vin = G0 (α(s)β2(s))/(1+(R2//RY//RZ)(CY+CZ)s)
where G0 = (R2//RY//RZ)/(R1+RX) is the low frequency gain
Response is dependent on 4 poles: ωβ (double pole), ωα ,and ωz =
1/(R2//RY//RZ)(CY+CZ) ~ 1/R2(CY+CZ), if R2 << (RY//RZ)
While ωβ and ωα are at higher frequencies, ωz is at lower and hence dominant
pole frequency, if R2 >> (ωα , ωβ)/(CY+CZ)
 At input: Vin = (RY/(RY+Rg))Eg/(1+(RY//Rg)CYs)
Input attenuation at low frequency can be prevented if RY >> Rg
Pole at ωy = 1/RgCY gives another reason to keep Rg low

18. OP AMPs VS CCs : BASIC CIRCUITS
 CCs can outperform op amps in many applications
 Striking advantages of CCs over op amps:
Minimum no. of external passive components
No requirement of component-matching
Elimination of gain bandwidth conflict
No severe finite slew rate based effects
 So, it is widely believed that CCs are building blocks whose time has now
come

19. Inverting/Non-inverting Amplifiers
Using op amps
Using CCs
3-dB BW: 2/CPR2
if 2RP >> R2
DC gain: -R2/2R1 DC gain: 1+(R2/2R1)
Bandwidth can be fixed by R2 and the gain can be independently adjusted
using R1, hence, the two are decoupled
DC gain: -K DC gain: +K
3-dB BW: ωt/(K+1) 3-dB BW: ωt/K

20. Instrumentation Amplifier
Using op amps Using CCs
V0/(V1-V2) = (R4/R3)(1+R2/R1) V0/(V1-V2) = R2/R1
3 active devices + 7 resistors 2 active devices + 2 resistors
Requires strict component-matching No component-matching needed
Lower bandwidth (fixed GBW product) Higher bandwidth (GBW decoupled)

21. Simulation of grounded inductor
Using op amps Using CCs
Zin = sC0R2 (R1 = R3 = R4 = R5 = R, C2 = C0) Zin = sC0R2 (Z1 = Z2 = R, Z3 = C0)
2 active elements + 5 passive elements 2 active elements + 3 passive elements
Floating elements make IC Grounded elements make IC
implementation tougher implementation easier

22. Current Feedback Operational Amplifier (CFOA)- An extension of the CC concept
 CFOA is a CCII+ followed by a voltage buffer; acts as a brilliant alternative to a
conventional op amp for high-speed, high frequency applications
Simplified CFB architecture Simplified VOA architecture

23.  Advantages of CFOAs:
Very high slew rate (500-2500 V/μs against 0.5-1.0 V/μs in VFOAs)
Decoupling of gain and bandwidth
Input stage transconductance plots
For VFOA: Iout = I0 tanh[(V+-V-)/2VT] For CFOA: Iout = 2Ibias sinh[(V+-V-)/VT]

24. Generalised Function Generator
Assuming id = Isevd/vT
and matched diodes, we get
I0 = i1
m1i2
m2………in
mn
(mj = (+/-)(R0/Rj),
m1+m2+…….+mn = 1)
By judicious choice of no. &
polarity of CCs, interesting
functions can be generated

25. VARIETIES AND RESEARCH HOTSPOTS
 Varieties of CCs

27.  Current research areas:
Hardware implementation (Bi-CMOS, FG-MOS based CCs)
CC-based Field Programmable Analog Arrays (FPAA)
CC-based Logic Functions & Digital Circuits
Developing a truly universal CC building block
 Some curious phenomena:
Practicing engineers & designers are still reluctant to use CCs in their designs
Many ICs (AD844, OPA 660, OPA 2662) labelled as high-frequency and/or high
slew-rate op amps/OTAs have a CC-hardware inside
 Capable of CMOS implementation, all CC-based circuits hold the potential of
becoming part of a complete system integrated together with digital circuits
on the same chip

28. REFERENCES
• R. Senani, D.R. Bhaskar, A.K. Singh: “Current Conveyors: Variants,
Applications and Hardware Implementations”, Springer International
Publishing, Switzerland, 2015
• C. Toumazou, F.J. Lidgey, D.G. Haigh (Eds.): “Analogue IC design: the current-
mode approach”, IEE Circuits and Systems Series 2 (Peter Peregrinus, London,
1990), Chap. 3
• A. Fabre, O. Saaid, H. Barthelemy: “On the Frequency Limitations of the
Circuits Based on Second Generation Current Conveyors”, Analog Integrated
Circuits and Signal Processing, 7, 113-129, 1995
• D. Biolek, R. Senani, V. Biolkova, Z. Kolka: “Active Elements for Analog Signal
Processing: Classification, Review, and New Proposals”, Radioengineering,
17(4), 15-32, Dec. 2008
• F.J. Lidgey, K. Hayatleh: “Current-feedback operational amplifiers and
applications”, Electronics & Communication Engineering Journal, 176-182,
Aug. 1997

29. QUESTIONS?