Current Conveyors

Introduction

The current conveyor is a general-purpose analog building block that processes signals as currents as readily as voltages. Where the operational amplifier was conceived around the ideal of a high-gain voltage difference driven toward zero by feedback, the current conveyor is conceived around the transfer, or conveyance, of a current from one terminal to another while a voltage is simultaneously transferred in the opposite direction. This dual relationship gives the device a natural place in current-mode signal processing, the family of techniques in which information is carried by branch currents rather than node voltages.

Sedra and Smith introduced the first-generation current conveyor in 1968 and the more versatile second-generation device in 1970. For two decades the concept remained largely a theoretical curiosity, because the bipolar and early MOS processes of the period made it difficult to realize the required terminal characteristics with good accuracy. The maturation of complementary bipolar and CMOS technologies, together with growing interest in low-voltage and high-frequency analog design, revived the current conveyor in the 1980s and 1990s. Today it is recognized as a flexible primitive from which amplifiers, filters, oscillators, immittance simulators, and instrumentation front ends can be synthesized, often with fewer components and wider bandwidth than equivalent op-amp circuits.

The appeal of the current conveyor rests on a simple observation about feedback amplifiers. The gain-bandwidth product of a voltage-feedback op-amp is essentially fixed, so closed-loop bandwidth falls in proportion to closed-loop gain. Current-mode building blocks are not bound by this single constraint in the same way, and many current-conveyor circuits hold their bandwidth roughly constant as gain is varied. For designers pushing toward higher frequencies, this property alone justifies attention.

The Conveyor Concept and Terminal Definitions

A current conveyor is a three-terminal device whose ports are conventionally labeled Y, X, and Z. The defining behavior couples these terminals through a fixed set of relations among their voltages and currents. Terminal Y is a high-impedance voltage input. Terminal X is a low-impedance terminal whose voltage follows the voltage at Y. Terminal Z is a high-impedance current output that reproduces, or conveys, the current that flows into X.

The essential actions are therefore twofold. First, the voltage applied at Y is conveyed to X, so that X behaves as a voltage follower of Y but presents a low impedance. Second, whatever current is forced into X by the external circuit is conveyed to Z, so that Z behaves as a current source controlled by the X-terminal current. The combination of a voltage following one direction and a current following the other is the characteristic signature of the device, and it is what the word "conveyor" is meant to capture.

The Matrix Description

The terminal relations are compactly written as a matrix that maps the independent variables, namely the current into Y, the voltage at X, and the voltage at Z, onto the dependent variables, namely the voltage at Y, the current into X, and the current into Z. The dependent quantities are expressed as follows. The voltage at Y equals the voltage at X. The current into Y is governed by a coefficient that distinguishes the device generations, multiplied by the current into X. The current into Z equals plus or minus the current into X, with the sign distinguishing the two polarities described below.

This matrix is the cleanest statement of ideal behavior. Real devices depart from it through nonzero series resistance at X, finite resistance at Y and Z, finite bandwidth, and small errors in the voltage and current transfer ratios, which are nominally unity but in practice are slightly less than one. Good designs keep these deviations small over the intended frequency range.

First-Generation Current Conveyors

In the first-generation current conveyor, abbreviated CCI, terminals X and Y carry equal currents. When a current is forced into X, an equal current is drawn into Y, and that same current appears at Z. The current into Y is thus equal to the current into X, which in matrix terms corresponds to a coefficient of one on that entry. The voltage at X still equals the voltage at Y.

The requirement that Y sink a current equal to the X current makes Y a low-impedance terminal for current, even though it remains the voltage-defining input. This dual demand on Y, that it both set the X voltage and absorb a matching current, complicates many applications, because the signal source driving Y must supply that current. The CCI is well suited to circuits in which the input current itself is the quantity of interest, such as certain current amplifiers and current-mode instrumentation stages, but the constraint at Y limits its general convenience.

As with the second-generation device, the polarity of the conveyed current at Z defines positive and negative variants. In the positive type the Z current has the same sense as the X current; in the negative type it is inverted. The CCI saw early use but was largely superseded in general practice by the second-generation device, whose high-impedance Y terminal proved far more broadly useful.

Second-Generation Current Conveyors

The second-generation current conveyor, abbreviated CCII, is the version that dominates modern practice. It differs from the CCI in one decisive respect: no current flows into terminal Y. The corresponding matrix coefficient is zero, so Y becomes an ideal high-impedance voltage input that draws no current from the source driving it. The voltage at X still follows the voltage at Y, and the current into X is still conveyed to Z.

This single change removes the awkward current demand at Y and makes the device behave, at its input, like the noninverting input of an op-amp: a clean voltage sense point. The result is a building block that is far easier to embed in larger circuits, because the voltage input and the current path are cleanly separated. The high-impedance Y, the low-impedance X that mirrors the Y voltage, and the high-impedance Z that mirrors the X current together form an elegant and widely reused primitive.

Positive and Negative Variants: CCII+ and CCII-

The CCII is further classified by the direction in which the X current is conveyed to Z. In the positive conveyor, written CCII+, the current that flows into X produces a current of the same sense flowing into Z; the two currents track in direction. In the negative conveyor, written CCII-, the conveyed current is inverted, so the current at Z opposes the current at X. The choice between them is dictated by the polarity required in a given feedback or summing arrangement.

Many practical conveyors provide both a positive and a negative Z output simultaneously, a configuration sometimes called a dual-output current conveyor. Supplying current of both senses from one device is valuable in filter and oscillator synthesis, where copies of a signal current in opposite phases are frequently needed. A device offering several scaled or differently signed outputs is broadly termed a multiple-output current conveyor.

Relationship to the Op-Amp and the Voltage Follower

It is illuminating to see how familiar blocks emerge from the CCII. If terminal Z is left as a current output and loaded with a resistor to ground, the resulting voltage is proportional to the X current; the device then resembles a transresistance, or current-to-voltage, stage. If a voltage buffer is added at Z, the cascade of Y-to-X voltage following and X-to-Z current following reproduces the behavior of an operational amplifier, and indeed a voltage-feedback op-amp can be modeled as a CCII followed by a voltage buffer. The current-feedback amplifier, treated as a separate topic, is even more directly a CCII+ with an output buffer. This unifying viewpoint is one reason the conveyor is valued as a teaching and synthesis tool.

Implementations

A physical current conveyor must realize three things at once: a voltage follower from Y to X with low output impedance at X, a current-sensing path that captures the X current, and a current mirror or replication scheme that delivers that current to a high-impedance Z. Different technologies achieve this with different circuit idioms.

Bipolar Realizations

A classic bipolar CCII begins with a complementary pair of emitter followers, often a translinear arrangement of two NPN and two PNP transistors forming a mixed translinear loop, to carry the Y voltage to X at low impedance. The current drawn through this output stage at X is sensed and replicated by complementary current mirrors, which steer an equal current to the Z terminal. The well-matched, high-transconductance devices available in complementary bipolar processes give bipolar conveyors excellent speed and low X-terminal resistance, which is why many early high-performance parts were bipolar.

CMOS Realizations

In CMOS, the voltage follower from Y to X is commonly built from a differential input pair with feedback, or from a class-AB output stage that holds X near the Y potential while sourcing and sinking current. The X current is again replicated by MOS current mirrors to form Z. CMOS conveyors integrate readily alongside digital logic and support low-voltage operation, although the lower transconductance and higher mismatch of MOS devices, relative to bipolar transistors, tend to raise the X-terminal resistance and the transfer errors unless careful sizing and cascoding are used.

A particularly compact and popular approach realizes the conveyor from a second-generation block built around a differential pair and cross-coupled mirrors, an arrangement that yields a low-voltage CCII suitable for modern submicron processes. Designers also derive conveyors from the operational transconductance amplifier and from the dual-output current source, reflecting the close kinship among current-mode primitives.

Nonidealities to Manage

The performance of any implementation is judged by how nearly it approaches the ideal matrix. The most important departures are the parasitic series resistance at X, which directly degrades accuracy because it adds to any external impedance placed there; the finite shunt resistance and capacitance at Y and Z, which set the usable impedance levels and bandwidth; and the voltage and current transfer ratios, which fall slightly below unity and drift with frequency, temperature, and bias. Tracking these parameters across process and operating conditions is the central task of conveyor design.

Applications

Because the current conveyor cleanly separates a voltage-sensing input from a current-replicating output, it serves as a synthesis primitive for a wide range of analog functions. A few representative classes illustrate its versatility.

Basic Amplifiers and Impedance Functions

A resistor from X to ground sets a current determined by the Y voltage, and routing that current to Z through a load resistor yields a voltage gain set by the ratio of the two resistors, giving a simple voltage amplifier whose bandwidth does not collapse as gain rises. Placing impedances at X and Z allows the conveyor to synthesize controlled resistances, and feedback arrangements built from conveyors can simulate grounded and floating inductors and frequency-dependent negative resistances, providing inductorless realizations of functions that would otherwise require bulky coils.

Current-Mode Filters

Current conveyors are workhorses of continuous-time filter design. Combined with grounded capacitors and resistors, a small number of conveyors realize first- and second-order sections, and cascades or feedback loops of such sections build higher-order low-pass, high-pass, band-pass, and notch responses. True current-mode realizations process the signal entirely as currents through the conveyors, exploiting the wide bandwidth of the current path. So-called universal biquads provide several standard responses, such as low-pass, band-pass, and notch, from a single topology by selecting different output nodes. Designs that use only grounded capacitors are favored for integration, because a grounded capacitor absorbs parasitic substrate capacitance more gracefully than a floating one.

Oscillators

Sinusoidal oscillators follow naturally from the same building blocks. A conveyor-based integrator or gyrator loop, arranged so that the loop gain satisfies the Barkhausen condition at the desired frequency, produces sustained oscillation. Quadrature oscillators, which deliver two sine waves ninety degrees apart, are readily built because dual-output conveyors supply the inverted and noninverted currents that such loops require. The oscillation frequency typically depends on capacitor and resistor values that can be tuned electronically, which suits the conveyor to voltage- or current-controlled oscillator applications.

Instrumentation and Interface Circuits

The high-impedance Y input and the accurate current transfer make the conveyor a strong candidate for instrumentation amplifiers, where a precise, well-defined gain must be applied to a small differential signal from a sensor. Conveyor-based instrumentation stages can achieve high common-mode rejection with a single gain-setting resistor and without the tight resistor matching that the classic three-op-amp instrumentation amplifier demands. Current conveyors also serve as current-to-voltage and voltage-to-current converters, as active probes, and as front ends for photodiodes and other current-output transducers, where sensing a current directly is more natural than first converting it to a voltage.

Advantages over Operational Amplifiers at High Frequency

The practical case for the current conveyor is sharpest at high frequencies, where the limitations of the classical voltage-feedback op-amp become acute. Several distinct advantages reinforce one another.

The first is the relaxation of the gain-bandwidth tradeoff. In a voltage-feedback op-amp the product of closed-loop gain and bandwidth is approximately constant, so a stage configured for high gain is necessarily slow. Conveyor-based amplifiers, and the closely related current-feedback amplifiers derived from them, can maintain nearly constant bandwidth as the gain-setting resistor is changed, decoupling the two specifications over a useful range.

The second is improved large-signal speed. The low-impedance X terminal can source and sink substantial current on demand, so the capacitances that limit slewing are charged and discharged quickly. Conveyor-derived amplifiers are therefore known for high slew rates and for a slew rate that does not impose the same hard limit on usable bandwidth that it does in conventional op-amps. The signal path being a current path, rather than a high-gain voltage node, also tends to reduce the dominant-pole compensation that slows general-purpose op-amps.

The third advantage is architectural simplicity at the system level. Currents sum effortlessly at a node by Kirchhoff's current law, so summing, weighting, and distributing signals often require no additional active stages. Filters and oscillators built from conveyors can use fewer components than their voltage-mode counterparts, which reduces parasitic loading and pushes usable frequencies higher. These benefits come with costs, chiefly greater sensitivity to the parasitic resistance at X and, in many implementations, higher noise and offset than a well-trimmed precision op-amp, so the conveyor is chosen where speed and current-mode convenience matter more than dc precision.

Summary

The current conveyor is an analog building block defined by the simultaneous conveyance of a voltage from its Y terminal to its X terminal and of the X-terminal current to its Z terminal. The first-generation device (CCI) forces equal currents in X and Y, while the second-generation device (CCII) makes Y a high-impedance voltage input that draws no current, and the CCII has become the standard form. The positive and negative variants, CCII+ and CCII-, differ in whether the conveyed current preserves or inverts its sense, and dual- and multiple-output devices supply several copies at once.

Realized in complementary bipolar or CMOS technology from voltage followers and current mirrors, the conveyor synthesizes amplifiers, simulated impedances, continuous-time filters, sinusoidal and quadrature oscillators, and instrumentation front ends, frequently with fewer parts than op-amp equivalents. Its decisive advantages appear at high frequency, where it relaxes the gain-bandwidth tradeoff, delivers high slew rate, and exploits the natural summation of currents. Where dc precision is paramount the operational amplifier may remain preferable, but for wideband, current-mode, and electronically tunable analog functions the current conveyor is a powerful and unifying primitive.