Realization and implementation of current conveyor
based linear oscillator
P. K. Sinha, Assistant Professor (praweenrsinha@rediffmail.com)
ABSTRACT
Voltage Controlled Oscillators (VCO) with linear tuning laws employing Current Conveyor (CC) in
conjuction with two analog multipliers (AMs) have been highlighted. VCO has been realized by
devising an RC-active oscillator configuration with two analog multipliers (AM). Appropriately
embedded AM enable independent control of the oscillation frequency through an external
control voltage Vc applied as a common input to both AMs. The workability of the presented
VCOs has been demonstrated by experimental results.
1. INTRODUCTION
Recently a number of new current- mode
circuits have been investigated [1-3]. The
use of current conveyors as an alternative
to
current
feedback
operational
amplifiers(CFOAs),
has
attracted
considerable attention in various high
speed communication systems, signal
processing, signal generation, various
instrumentation and most importantly in
medical applications due to their
commercial availability as well as its known
advantages. The use of CFOAs has been
extensively investigated in realizing
oscillators[4] and the references cited
therein, this study now investigates the use
of CCs as an alternative to CFOAs. AD844
which contains a CCII+ followed by a voltage
buffer is used to implement current
conveyor.
Voltage-controlled oscillators (VCOs) are
important building blocks in several
instrumentation,
electronic
and
communication systems, such as function
generators, in production of electronic
music to generate variable tones, in phase
locked
loop
and
in
frequency
synthesizers[3, 6-8].
VCO is realized by an RC-active oscillator
configuration with two analog multipliers
(AM) in conjuction to enable independent
control of the oscillation frequencythrough
an external control voltage Vc applied as a
common multiplicative input to both the
multipliers. This technique gives rise to a
linear tuning law of the form
foα Vc(1)
Based on this approach, a number of VCO
configurations have been proposed by
various researchers in the past employing
traditional VOAs/CFOAs and AMs.
Generally the circuits implementing VCO
requires two or more CFOAs along with two
AMs [5]. The main objective of this study is
to highlight two simple linear VCOs which
are realizable with only a single CC along
with two AMs. Experimental results using
AD844 have been given and the advantages
offered have been highlighted.
2. VCO Configurations Based on CCI
A CCI is characterized by the terminal
equations Ix=Iy, Vx=Vy and Iz=Ix, On the other
hand, the output of an AM with two inputs
V1 and V2 is of the form Vo = K
,
whereVref is the reference voltage of the
multiplier set internally (usually 15 volts in
case of AD734) and K can be set up to +1 or
-1 by grounding the appropriate terminals
of AD734.
The proposed circuits are shown in Figure
1(a) and figure 1(b):
The frequency of oscillation (FO) thus given
by:
FO :fo=
(3)
Thus, it is seen that fois linearly controllable
by the external control voltage Vc , as
desired.
AD844 is CCII+ followed by a voltage buffer.
It is converted to CCI by the circuit
arrangement shown in figure 2. Because of
low impedance at the input terminalCCI
circuit can be used as an accurate
currentamplifier. Moreover, in CCII
arrangement y-terminal is grounded and
this grounding must be done carefully
sincea poorly grounded input terminal may
cause unwantednegative impedance at the
other input terminal.
Figure 1(a): Proposed circuit 1
Figure 2: CCII to CCI conversion
3. Experimental Results
Figure 1(b): Proposed circuit 2
Choosing the same Vref for both the
multipliers in the circuits gives the condition
of oscillation (CO):
CO: R1 = R2 = R (say)
(2)
Both the VCOs have been experimentally
studied using AD844 type CCI and AD734
type AMs biased with
15V DC power
supplies and it has been possible to
generate oscillation frequencies from few
kHz to several hundreds of kHz.
Experimental results of the proposed VCOs
are shown in figures 3 and 4. The
component values chosen are as under: For
circuit of figure 1(a), R= 1kΩ, C1 = 0.1uF, C2 =
0.1uF ; For circuit of figure 1(b), R = 1kΩ, C1
= 0.1nF, C2 = 0.1nF.
Figure 3(a) and figure 4(a) shows the
variation of oscillation frequency with
control voltage for figure 1(a) and 1(b)
respectively. The practical results show a
reasonably good correspondence between
the theoretical and experimental results to
confirm the linear tuning of the two VCOs.
Figure 4. Experimental results for circuit 2 (a)
Variation of frequency with Vc (b) A typical
waveform generated( f= 516 kHz at Vc= 4V, V(peak
to peak)= 30 mV)
4. Concluding Remarks
In this study, we have highlighted two
simple CCI-AM-based VCOs, providing linear
tuning laws which are derived from
previously known works. The workability of
new VCOs has been verified and proved by
experimental results.
Figure 3. Experimental results for circuit 1 (a)
Variation of frequency with Vc (b) A typical
waveform generated( f= 127 kHz at Vc= 4V, V(peak
to peak)= 250 mV)
The VCOs presented here offer better gain
bandwidth products than the comparable
op-amps under both small and large signal
conditions. It also overcomes the limitations
of
CFOAs
in
component
matching.Moreover, this current-mode
approach is not just restricted to current
processing, but also offers certain
important advantages when interfaced to
voltage-mode circuits.
5. References
[1] “Linear VCO with Sine Wave Output,”
IEEE Transactions on Instrumentation and
Measurement, Vol. 35, No.2, by S. K. Saha,
1986.
Feedback Op-Amp,” by D. R. Bhaskar, R.
Senani, A. K. Singh and S. S. Gupta, 2010.
[2] “Linear Voltage Controlled oscillator,”
IEEE Transactions on Instrumentation and
Measurement, Vol. 37, No.1, by S. K. Saha
and L. C. Jain, 1988.
[6] “On the realization of Linear Sinusoidal
VCOs,” by D. R. Bhaskar, R. Senani and M.
P. Tripathi, International Journal of
Electronics, 1993.
[3] “Linear Sinusoidal VCO,” International
Journal of Electronics, Vol. 65, No. 2,by V. P.
Singh and S. K. Saha, 1988.
[7] “New active-R Sinusoidal VCOs with
Linear Tuning Laws,” by D. R. Bhaskar and R.
Senani, 1996.
[4] “Linear Sinusoidal VCOs: New
Configurations Using Current Feedback OpAmps,” International Journal of Electronics,
Vol. 97, No. 3, by D. R. Bhaskar, R. Senani
and A. K. Singh, 2010.
[8] “Realization of Novel Linear Sinusoidal
VCOs,” by D. R. Bhaskar and M.P. Tripathi,
Analog Integrated Circuits and Signal
Processing, Vol. 24, 2000.
[5] “Two Simple Analog Multiplier Based
Linear VCOs Using A Single Current