Oscillator uses dual-output current-controlled conveyors
Varying the bias current of dual-output current-controlled conveyors controls the frequency of oscillation of the circuits.
Abhirup Lahiri, Netaji Subhas Institute of Technology, New Delhi, India; Edited by Martin Rowe and Fran Granville -- EDN, November 13, 2008
In the last decade, engineers have done much work in designing and implementing current-mode circuits using second-generation current conveyors, which have higher signal bandwidth, greater linearity, larger dynamic range, simpler circuitry, and lower power consumption than their predecessors. Recently, a second-generation dual-output, current-controlled conveyor has emerged. The device is an active building block (Figure 1), and the following equations characterize it: IY=0, VX=VY+IXRX, and IZ+=IX; IZ–=–IX.The parasitic resistance at terminal X is RX=(VT/2IB), where VT is the thermal voltage and IB is the bias current of the conveyor that is tunable over several decades.
Figure 2 shows current-controlled oscillators with few components, employing only two dual-output current-controlled conveyors and two grounded capacitors. The devices use no external resistors, and the parasitic resistance at terminal X realizes resistance. The proposed design for the circuit provides electronic controllability of frequency of oscillation.
The characteristic equation for both of the circuits in Figure 2 is s2C1C2RX1RX2+sC2RX1–sC1RX1+1=0. Satisfying Barkhausen’s criteria—that the loop gain is unity or greater and that the feedback signal arriving back at the input is phase-shifted 360°—the required condition for oscillation is C1=C2, and the frequency of oscillation is f=1/(2π
).
Assuming that C1=C2=C and taking RX1=RX2=VT/2IB yield a frequency of oscillation: f=(IB/πCVT). Clearly, the dc-bias current, IB, can vary the frequency of the current conveyors, and the frequency is, therefore, electronically controllable.
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Comment on “Oscillator uses dual-output current-controlled conveyors” and creating quadrature oscillator by a simple modification
The circuits exemplified in this design-idea have been created using the proposed circuits in [1]. But although in [1], the circuits used either CCII (+)/CCCII (+) or CCII (-)/CCCII (-) to create different circuits, the proposals in this design idea use a dual-output active device. Hence, instead of using different active devices to create circuits, the same active building block (ABB), and viz. dual-output CCCII has been used to create two different oscillator circuits.
Since, a DO-CCCII can implement both CCCII (+) and CCCII (-) by appropriately grounding the z- or z+ terminals respectively and hence, the proposed methodology is a unified and a more versatile method in creating such current-controlled oscillators.
Remarkably, another similar circuit had already been proposed by Neeta Pandey et al in [2] and the authors argue that their circuit offers a lesser total harmonic distortion (THD), which indeed is true. Using a DO-CCCII as in this design-idea, would help in creating two different circuits from [2] -a regular circuit exercise.
Investigating the availability of ECOs
While creating current-mode (CM) oscillators, the availability of explicit-current-outputs (ECOs) is quite desirable. This feature had not been investigated either in [1] or [2] and could only be provided by use of a dual or multiple-output device. The author believes that un-grounding the unused z (+/-) terminals of the two DO-CCCIIs would help creating two quadrature phase-shifted ECOs [3].
For example: Consider the Fig. 2(a) of the design-idea and un-grounding z- of the first DO-CCCII and z+ of the second DO-CCCII would create two ECOs which are phase-shifted by 90 degrees. Hence, the circuit creates a quadrature current-controlled oscillator, which could be used in quadrature mixers in communication systems.
It is expected that this design-idea and comment, along with the below mentioned references would be of help to the analog designers.
Reference:
[1] J. W. Horng, “A sinusoidal oscillator using current-controlled current conveyor,” International Journal of Electronics, vol. 88, pp. 659 – 664, 2001
[2] N. Pandey, S.K. Paul, A.K. Bhattacharya, “Sinusoidal Oscillator –A new configuration based on current conveyor,” URSI Proceedings, 2005.
[3] A. Lahiri, "New current-mode quadrature oscillators using CDTA," IEICE Electronics Express, 2009, in Press, Online, Doi: 10.1587/elex.6.1
Abhirup Lahiri - 2009-26-1 22:28:00 PST -
There are many commercially available current feedback op-amps that could be used as current conveyors
Example: AD844 by Analog Devices. The device would work as a CCII+ between y,x and z.
Hope this helps.
Abhirup Lahiri - 2009-11-1 06:04:00 PST -
Hi,
Tnanks for this article.
I''m also looking for commercial ICs implementing conveyors, as I''d like also to simulate floating inductors.
Can you give me references to commercially available conveyors?
Thank you!
Enrique V. - 2008-30-12 07:07:00 PST -
Well infact i did provide the schematic in the draft version of the article, but it was edited by the design staff and then not included. Anyways the previously posted reference (Sagabas et al) contains the required schematic.
Abhirup Lahiri - 2008-6-12 00:53:00 PST -
Perhaps Mr. Lahiri could post a schematic of his implementation of the circuit? It sounded interesting from the design idea, but a bit vague, as has been pointed out.
Brpooks Lyman - 2008-19-11 18:06:00 PST
