A short primer on analog current conveyor circuits
Article By : Jean-Jaques (JJ) DeLisle
Current conveyor (CC) circuits allow a relatively small variation in voltage and are used in a variety of complex electronic applications.
Current conveyor (CC) circuits are among the most common current-mode circuits and are used as critical building blocks in a variety of complex circuits. Unlike voltage-mode circuits, current-mode circuits allow for only a relatively small variation in voltage while allowing a large variation in current.
In 1968, K. C. Smith and Adel Sedra introduced the first generation of CCs (CCI) and later in 1970 introduced the second generation of CCs (CCII), as shown in Figure 1. Since then, there has been steady development and generation of new current-mode circuits, including current feedback op-amp (CFOA), inverting CC (ICC), operational floating conveyor (OFC), current differencing buffered amplifier (CDBA), four-terminal floating nuller (FTFN), and operational trans-resistance amplifier (OTRA).
Figure 1 Here is a view of positive first-generation current conveyor or CCI+ (top left), positive second-generation current conveyor or CCII+ (top right), negative first-generation current conveyor or CCI- (bottom left), and negative second-generation current conveyor or CCII- (bottom right). Source: Springer
This blog focuses on the basics of current conveyor (CC) generations 1 and 2, as well as some common CC applications.
The basic concept of a current conveyor is that it’s a three-terminal open-loop current-mode amplifier with low and fixed current gain. With these devices, the gain is set by a transistor property or by the control of impedance levels at the input or output of the device. The result is that current conveyors can convey current between the input and output terminals with different impedances at each port.
Another outcome of this is that current conveyors tend to exhibit higher gain over a larger signal bandwidth than voltage-mode operational amplifiers. Theoretically, a CC amplifier is limited only by the transit frequency (ft) of the transistors used in the circuit. Additionally, current conveyors also tend to exhibit better common-mode rejection ratio (CMRR) than instrumentation amplifiers.
First– and second-generation current conveyors
With the first-generation current conveyor (CCI), it can be observed from the equation matrix—shown in Figure 2 and Figure 3—that a voltage applied to X terminal will result in equivalent potential at the Y terminal. It’s also observable that an input current at the Y terminal will result in the equivalent current also being forced in the X terminal.
Moreover, the current stimulus at Y that results in the current at terminal X will result in a negative of that current at terminal Z. It’s also important to note that the potential at terminal X that is set by Y is independent of the current forced into port X and that the current through port Y that is summarily fixed by X is independent of the voltage applied to Y.
Figure 2 A matrix representation of first-generation current conveyors is shown for both positive and negative. Source: Springer
Figure 3 The functional diagram highlights a first-generation current conveyor (CCI).
Early current conveyors were designed using bipolar junction transistors (BJTs), mainly due to the transconductance and because early voltage ranges of BJTs were higher than complementary metal-oxide semiconductor (CMOS) devices at the time.
The difference between first- and second-generation current conveyors is that for second generations the current in terminal Y is zero (see Figure 4 and Figure 5). The voltage seen on terminal Y still forces the same potential at terminal X. Also, the current that flows in terminal X is still forced on terminal Z, with the voltage potential at terminal X being independent of the current in terminal X. In this way, the current conveyed to the Z terminal has the characteristics of a current source with a high output impedance. Lastly, terminal Y exhibits an infinite input impedance.
Figure 4 The functional diagram highlights a second-generation current conveyor.
Figure 5 Here is a view of the second-generation current conveyor equation matrix.
An example circuit can be seen in Figure 6, where terminal Y is the base/gate, X is the emitter/source, and Z is the collector/drain of the ideal transistor model of a CCII.
Figure 6 The circuit diagram shows a second-generation bipolar current conveyor. Source: Analog Devices
These current conveyors operate well as current followers. Furthermore, with the X/Y gain close to 1, the Z terminal exhibits a high-output impedance that is generally superior to what can be achieved with CMOS current conveyors.
Current conveyor applications
There are many applications for current conveyor circuits spanning from network synthesis to analog computation. These include current conveyor current amplifier (CCCS), voltage to current converter (VCCS), current buffers/current follower, voltage amplifier, differential variations of voltage-to-current converters, instrumentation amplifier, current/voltage summing/integration/differentiation, impedance converter, and gyrator. Modern CC designs include both BJT and CMOS realizations.
This article was originally published on Planet Analog.
Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.