A current conveyor is an abstraction for a three-terminal analogue electronic device. It is a form of

electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...

with unity gain. There are three versions of generations of the idealised device, CCI, CCII and CCIII. When configured with other circuit elements, real current conveyors can perform many analogue signal processing functions, in a similar manner to the way

op-amp An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...

s and the ideal concept of the op-amp are used.

History

When Sedra and Smith first introduced the current conveyor in 1968, it was not clear what the benefits of the concept would be. The idea of the op-amp had been well known since the 1940s, and

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manufacturers were better able to capitalise on this widespread knowledge within the

electronics industry The electronics industry is the economic sector that produces electronic devices. It emerged in the 20th century and is today one of the largest global industries. Contemporary society uses a vast array of electronic devices built-in automated or ...

. Monolithic current conveyor implementations were not introduced, and the op-amp became widely implemented. Since the early 2000s, implementations of the current conveyor concept, especially within larger VLSI projects such as mobile phones, have proved worthwhile.

Advantages

Current conveyors can provide better gain-bandwidth products than comparable op-amps, under both

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and large signal conditions. In instrumentation amplifiers, their gain does not depend on matching pairs of external components, only on the absolute value of a single circuit element.

First generation (CCI)

The CCI is a three-terminal device with the terminals designated ''X'', ''Y'', and ''Z''. The potential at ''X'' equals whatever voltage is applied to ''Y''. Whatever current flows into ''Y'' also flows into ''X'', and is mirrored at ''Z'' with a high output impedance, as a variable constant current source. In sub-type CCI+, current into ''Y'' produces current into ''Z''; in a CCI-, current into ''Y'' results in an equivalent current flowing ''out'' of ''Z''.

Second generation (CCII)

In a more versatile later design, no current flows through terminal ''Y''. The ideal CCII can be seen as an ideal transistor with perfected characteristics. No current flows into the

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or base which is represented by ''Y''. There is no base-emitter or gate-source voltage drop, so the emitter or source voltage (at ''X'') follows the voltage at ''Y''. The gate or base has an infinite input impedance (''Y''), while the emitter or source has a zero input impedance (''X''). Any current out of the emitter or source (''X'') is reflected at the collector or drain (''Z'') as a current in, but with an infinite output impedance. Because of this reversal of sense between ''X'' and ''Z'' currents, this ideal bipolar or field-effect transistor represents a CCII−. If current flowing out of ''X'' resulted in the same high-impedance current flowing ''out'' of ''Z'', it would be a CCII+.

Third generation (CCIII)

The third configuration of the current conveyor is similar to the CCI except that the current in ''X'' is reversed, so in a CCIII whatever current flows into ''Y'' also flows out of ''X''.

References