--- title: "Current mode filters and current conveyors (CCI, CCII)" subject: "Filter Design" chapter: 9 key: "semester_5/filter_design:current-mode-current-conveyors" source: https://www.bctengineer.com/q/semester_5/filter_design/current-mode-current-conveyors site: BCT Engineering --- # Current mode filters and current conveyors (CCI, CCII) Filter Design · Chapter 9: Switched-Capacitor Filters - Asked in: 2082 Chaitra (new course), 1+3 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/filter_design/current-mode-current-conveyors - Whole chapter: https://www.bctengineer.com/md/semester_5/filter_design/chapter-9 ## Question What is the significance of using current mode filters in the electronic circuit? With basic block diagram and characteristic equations explain the concepts of first-generation current conveyor (CCI) and second-generation current conveyor (CCII). ## Short answer Current-mode filters give wider bandwidth, higher slew rate, low-voltage operation, and easy signal addition via KCL. A CCI has terminals X,Y,Z with iY=iX, vX=vY, iZ=±iX. A CCII makes Y high-impedance (iY=0), vX=vY, iZ=±iX, acting like an ideal transistor. ## Answer #### 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). ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why do current-mode filters achieve wider bandwidth than voltage op-amp filters? A. They use larger supply voltages B. Bandwidth is not limited by a fixed gain-bandwidth product, and low-impedance nodes with small voltage swings give high speed C. They always use more components D. They have higher output impedance at every node Answer: B. Signals are currents on low-impedance nodes with small swings, so bandwidth stays nearly constant with gain. (Key point: Benefit of current-mode circuits) 2. Which set describes the first-generation current conveyor (CCI)? A. iY = 0, vX = vY, iZ = ±iX B. iY = iX, vX = vY, iZ = ±iX C. iY = iX, vX = 0, iZ = iY D. vY = iX, iX = vZ, iZ = 0 Answer: B. In the CCI the current into X is also copied into Y (iY = iX), X follows Y in voltage, and iX is conveyed to Z. (Key point: CCI characteristic equations) 3. What is the key change in the CCII compared with the CCI? A. Terminal Y becomes a high-impedance input with iY = 0 B. Terminal Z becomes a voltage output C. Terminal X becomes high impedance D. The voltage vX no longer follows vY Answer: A. CCII has iY = 0, so Y draws no current, while vX = vY and iZ = ±iX still hold; this makes it far more useful. (Key point: CCII difference from CCI) 4. What distinguishes a CCII+ from a CCII-? A. The sign of vX relative to vY B. Whether Y draws current C. The sign of the Z current: iZ = +iX for CCII+ and iZ = -iX for CCII- D. The number of terminals Answer: C. The ± in iZ = ±iX selects CCII+ (iZ = +iX) or CCII- (iZ = -iX). 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