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ACTIVE FILTERS USING THE OPA861 IN

OUT

C/2

OUT

C/2

CCII

−

OUT

T(s) =

OUT

2 + 2/RC[2Q(1

− K) + 1]s + 4KQ2/R2C2

4KQ

2/R2C2

OPA861

SBOS338 – AUGUST 2005

output becomes the input, which is excitated by a

CURRENT CONVEYOR STRUCTURE

current source. The following equation describes the

interreciprocal features of the circuit: VOUT/VIN =

One further example of the versatility of the Diamond

IOUT/IIN. Resistances and capacitances remain un-

Transistor

and

Buffer

the

construction

changed. In the final step, the operational amplifier

high-frequency (> 10MHz) active filters. Here, the

with infinite input impedance and 0

Ω output im-

Current Conveyor structure, shown in Figure 40, is

pedance is transformed into a current amplifier with

used with the Diamond Transistor as a Current

Ω input impedance and infinite output impedance. A

Conveyor.

Diamond Transistor with the base at ground comes

quite

ideal

current

amplifier.

The

The method of converting RC circuit loops with

well-known Sallen-Key low-pass filter with positive

operational amplifiers in Current Conveyor structures

feedback, is an example of conversion into Cur-

is based upon the adjoint network concept. A network

rent-Conveyor structure, see Figure 43. The positive

is reversible or reciprocal when the transfer function

gain of the operational amplifier becomes a negative

does not change even when the input and output

second type of Current Conveyor (CCII), as shown in

have been exchanged. Most networks, of course, are

Figure 40. Both arrangements have identical transfer

nonreciprocal. The networks of Figure 41, perform

functions and the same level of sensitivity to devi-

interreciprocally when the input and output are

ations. The most recent implementation of active

exchanged,

while

the

original

network,

filters in a Current-Conveyor structure produced a

exchanged for a new network NA. In this case, the

second-order Bi-Quad filter. The value of the resist-

transfer function remains the same, and NA is the

ance in the emitter of the Diamond Transistor controls

adjoing network. It is easy to construct an adjoint

the filter characteristic. For more information, refer to

network for any given circuit, and these networks are

application note SBOS047, New Ultra High-Speed

the base for circuits in Current-Conveyor structure.

Circuit Techniques with Analog ICs.

Individual elements can be interchanged according to

the list in Figure 42. Voltage sources at the input

become short circuits, and the current flowing there

becomes the output variable. In contrast, the voltage

Figure 40. Current Conveyor

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