Current mode filters and current conveyors (CCI, CCII)

Significance of current mode filters

In current-mode circuits the signal is carried by currents instead of voltages. Benefits:

• Wider bandwidth, not limited by a fixed gain-bandwidth product as in voltage op-amps; bandwidth stays nearly constant with gain.
• Higher slew rate and speed, since internal nodes have low impedance and small voltage swings.
• Work well at low supply voltage (modern CMOS), with good dynamic range.
• Addition of signals is simply joining wires (KCL), so circuits need fewer components; easy electronic tuning.

First-generation current conveyor (CCI)

A three-terminal block with ports X, Y and Z (Smith and Sedra, 1968).

        iY                iZ
  vY ---->[Y        Z]---->--- vZ
           |  CCI    |
  vX ---->[X         ]
        iX

Characteristic equations:

$$i_Y = i_X, \qquad v_X = v_Y, \qquad i_Z = \pm i_X$$ $$\begin{bmatrix} i_Y \\ v_X \\ i_Z \end{bmatrix} = \begin{bmatrix} 0 & 1 & 0 \\ 1 & 0 & 0 \\ 0 & \pm 1 & 0 \end{bmatrix}\begin{bmatrix} v_Y \\ i_X \\ v_Z \end{bmatrix}$$

The voltage at X follows Y; the current fed into X is copied into Y and conveyed to the high-impedance output Z.

Second-generation current conveyor (CCII)

Y is made a high-impedance input (draws no current), which makes the device far more useful:

$$i_Y = 0, \qquad v_X = v_Y, \qquad i_Z = \pm i_X$$ $$\begin{bmatrix} i_Y \\ v_X \\ i_Z \end{bmatrix} = \begin{bmatrix} 0 & 0 & 0 \\ 1 & 0 & 0 \\ 0 & \pm 1 & 0 \end{bmatrix}\begin{bmatrix} v_Y \\ i_X \\ v_Z \end{bmatrix}$$

• CCII+: $i_Z = +i_X$; CCII−: $i_Z = -i_X$.
• Y acts as a voltage input, X as a low-impedance voltage-follower output/current input, Z as a high-impedance current output. A CCII is like an ideal transistor (Y = gate/base, X = source/emitter, Z = drain/collector) and is used for current-mode integrators, biquads and inductor simulation.