Hardware Reference
In-Depth Information
I
02
I
01
Y
2
X
1
Z
1
Z
2
V
0
Y
1
2
X
2
1
CCCII+
CCCII−
C
2
C
1
Fig 9.39 An alternative form of current-controllable sinusoidal oscillator [
38
]
from 0.1 to 100
A have shown excellent correspondence between the theoretical
frequency and that obtained by SPICE simulations from about 100 Hz to 10 KHz.
While the circuits described in Figs.
9.38
and
9.39
employ only two CCCIIs
besides a canonic a number of grounded capacitors the circuits have control over
the CO through variable capacitors which is not very convenient option. A better
situation would be to have control of the condition of oscillation through an external
bias current or the gain of an active element which may be extremely programma-
ble; in the following we present two such circuits.
Kiranon et al. in [
39
] presented a current controlled oscillator based on two
translinear CCCIIs and a current mirror having current gain
μ
ʱ
as shown in Fig.
9.40
.
By straight forward analysis the CO is found to be
ʱ
¼
1+(R
x2
/R
x1
) whereas the FO
is found to be:
I
0
FO
f
0
¼
p
C
1
C
2
ð
9
:
55
Þ
:
ˀ
V
T
It is thus seen that f
0
is linearly controllable by external bias current I
0
where
I
01
¼
I
0
.
SPICE simulations based on bipolar transistor PR200N and NR 200N with the
circuit biased from
I
02
¼
2.5 V supply have demonstrated excellent linear tunability of
FO over a range of three to four decades when I
0
was varied from 0.1
A to 1 mA.
A drawback of above mentioned circuit is that the CO is fixed by setting the
required value of the current gain
μ
which in term is controlled by the emitter area
ratio of the current amplifier hence no control is possible to be exercised on the
amplitude of oscillation.
An alternative circuit which overcomes this difficulty was presented by
Abuelma
ʱ
atti and Tasadduq in [
40
] which also uses three active elements, all of
which are CCCIIs along with two grounded capacitors. This circuit is shown in
Fig.
9.41
and is governed by the following equations:
'
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