Nonlinear Applications of CCs - Current Conveyors: Variants, Applications and Hardware Implementations - page 214

Fig. 8.1 Precision full
wave rectifier using CCs
proposed by Toumazou
et al. [ 3 ]

D 2

Y

V in

CCII Z

A

X

D 1

V out

R 1

R 2

X D 3

CCII

Z

B

Y

D 4

Toumazou et al. [ 3 ] demonstrated the construction of a precision full wave
rectifier using current conveyors CCII01 from LTP Electronics which has band-
width f T of the order of 100 MHz. The circuit from [ 3 ] is shown in Fig. 8.1 .
The operation of the circuit can be explained as follows. The two CCs are
configured to act as voltage to current convertors. During the positive input cycle,
a current V in /R 1 flows out of the Z-terminal of CCIIA and the same amount of
current flows into the Z-terminal of CCIIB which makes diodes D 2 and D 4
conducting thereby making a voltage V out across R 2 available and ensuring
V out ¼

R 2 . During the negative cycle of the input voltage, diodes
D 3 and D 1 conduct in which case also the output current of CCIIB creates a voltage
across R 2 thereby again making V ou ¼

V in provided R 1 ¼

V in . Unfortunately, for low level input
voltages, the slewing characteristic of the input stage of the current conveyor
architecture provides a small current derive as opposed to virtually unlimited
current drive for large input signals. As a consequence of this, the output wave
form shows significant distortion because during the point at which the inputs signal
changes polarity, all the diodes are off. Thus, the output voltage of the rectifier
remains at zero volt until the voltage at the appropriate Z-terminal reaches the level
to turn the diodes on, at which point the output voltage changes sharply. Because of
the reduced value of the current conveyor s small signal dV/dT the performance of

the precision rectifier deteriorates at frequency much below the f T .
From the above discussion, it is clear that since the distortion in the output
waveform of the precision rectifier of Fig. 8.1 is due to small signal dV/dT
limitations, it can be corrected by a suitable modification of the circuit by appro-
priately biasing all the diodes. One such modification is shown in Fig. 8.2 where a
low impedance voltage difference circuit producing a voltage of the order of 0.6 V
has been appropriately added. By experiments it has been found that this modifi-
cation improves the performance of the rectifier up to the frequency of about
30 MHz.
In [ 4 ], Hayatleh et al. subsequently demonstrated that although the addition of
voltage bias (V B ) as shown in Fig. 8.2 results in improvement of the performance of
the precision rectifier, this design exhibits rather a poor temperature characteristics

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