Datasheet
F
(
p
)
+
V
OUT
V
IN
+
s
2
C
1
C
2
R
1M
R
2M
R
3
) sC
1
R
1M
R
2
)
1
R
1
s
2
C
1
C
2
R
1M
R
2M
R
3S
) sC
1
R
1M
R
2S
)
1
R
1S
C
2
R
2M
V
OUT
R
3
R
2S
R
2
R
1S
C
1
V
IN
R
1
R
1M
R
3S
R
B1
R
B2
R
B3
BUF602
+
−
µ
V
µ
I
I
C C
R R
+
−
V
OUT
V
2
2
2
2
3
4
1
1
1
1
2
2
2
2
3
4
1
1
1
1
1
1
Controlled
Sources
Passive
Elements
Signal
Sources
Element Adjoint
I
OUT
V
IN
I
IN
N
V
OUT
+
−
N
I
OUT
I
IN
Reciprocal Networks
N
V
OUT
+
−
N
A
I
OUT
I
IN
Interreciprocal Networks
I
IN
V
IN
I
OUT
V
OUT
=
V
IN
V
IN
R
V
OUT
C/2
R
+1
C
V
IN
R
I
OUT
C/2
R
CCII
−
C
E
B
C
I
IN
V
OUT
T(s) = = =
I
OUT
V
IN
I
IN
s
2
+ 2/RC[2Q(1
−
K) + 1]s + 4KQ
2
/R
2
C
2
4KQ
2
/R
2
C
2
OPA861
www.ti.com
SBOS338G –AUGUST 2005–REVISED MAY 2013
ACTIVE FILTERS USING THE OPA861 IN of the operational amplifier becomes a negative
CURRENT CONVEYOR STRUCTURE second type of Current Conveyor (CCII), as shown in
Figure 43. Both arrangements have identical transfer
One further example of the versatility of the Diamond
functions and the same level of sensitivity to
Transistor and Buffer is the construction of high-
deviations. The most recent implementation of active
frequency (> 10MHz) active filters. Here, the Current
filters in a Current-Conveyor structure produced a
Conveyor structure, shown in Figure 43, is used with
second-order Bi-Quad filter. The value of the
the Diamond Transistor as a Current Conveyor.
resistance in the emitter of the Diamond Transistor
controls the filter characteristic. For more information,
refer to application note SBOS047, New Ultra High-
Speed Circuit Techniques with Analog ICs.
Figure 43. Current Conveyor
The method of converting RC circuit loops with
operational amplifiers in Current Conveyor structures
is based upon the adjoint network concept. A network
is reversible or reciprocal when the transfer function
Figure 44. Networks
does not change even when the input and output
have been exchanged. Most networks, of course, are
nonreciprocal. The networks of Figure 44, perform
interreciprocally when the input and output are
exchanged, while the original network, N, is
exchanged for a new network N
A
. In this case, the
transfer function remains the same, and N
A
is the
adjoing network. It is easy to construct an adjoint
network for any given circuit, and these networks are
the base for circuits in Current-Conveyor structure.
Figure 45. Individual Elements in the Current
Individual elements can be interchanged according to
Conveyor
the list in Figure 45. Voltage sources at the input
become short circuits, and the current flowing there
becomes the output variable. In contrast, the voltage
output becomes the input, which is excitated by a
current source. The following equation describes the
interreciprocal features of the circuit: V
OUT
/V
IN
=
I
OUT
/I
IN
. Resistances and capacitances remain
unchanged. In the final step, the operational amplifier
with infinite input impedance and 0Ω output
impedance is transformed into a current amplifier with
0Ω input impedance and infinite output impedance. A
Diamond Transistor with the base at ground comes
quite close to an ideal current amplifier. The well-
Figure 46. Universal Active Filter
known Sallen-Key low-pass filter with positive
feedback, is an example of conversion into Current-
Conveyor structure, see Figure 46. The positive gain
Transfer Function Filter Characteristics
The transfer function of the universal active filter of Five filter types can be made with this structure:
Figure 46 is shown in Equation 7.
• For a low-pass filter, set R
2
= R
3
= ∞,
• For a high-pass filter, set R
1
= R
2
= ∞,
• For a bandpass filter, set R
1
= R
3
= ∞,
• For a band rejection filter, set R
2
= ∞; R
1
= R
3
,
(7)
• For an all-pass filter, set R
1
= R
1S
; R
2
= R
2S
; and
R
3
= R
3S
.
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