Everyone learns the op-amp. Almost nobody learns the current conveyor, even though Sedra and Smith proposed it back in 1968 as a more general analog primitive. If you've ever needed a wideband transimpedance stage, a precision current mirror with voltage input, or a filter that runs cleanly at hundreds of MHz, the current conveyor is often a better answer than an op-amp.

A second-generation current conveyor (CCII) has three terminals: Y (high-impedance voltage input), X (low-impedance voltage output that tracks Y), and Z (high-impedance current output that mirrors the current flowing into X). The defining equations are beautifully simple:

• V_X = V_Y (X follows Y like a buffer)
• I_Y = 0 (Y draws no current)
• I_Z = ±I_X (Z sources/sinks whatever current X is carrying)

The sign choice gives you CCII+ or CCII−. That's the entire abstraction.

Why bother? Op-amps have a fixed gain-bandwidth product — push voltage gain up and bandwidth collapses. Current conveyors don't. Their gain is set externally by an impedance ratio, and the internal signal path is current-mode, so parasitic node capacitances don't roll off the transfer function the same way. A CCII-based transimpedance stage can hit hundreds of MHz where an op-amp version dies at 10 MHz.

Concrete example — voltage amplifier. Tie Y to your input, put R₁ from X to ground, and R₂ from Z to ground with the output taken at Z. Since V_X = V_in, the X-terminal current is I_X = V_in/R₁. That current is conveyed to Z, developing V_out = I_Z·R₂ = V_in·(R₂/R₁). Same gain formula as a non-inverting op-amp — but with no gain-bandwidth trade-off.

Rule of thumb: if your desired gain × required bandwidth exceeds ~50% of your op-amp's GBW, reach for a current conveyor (AD844 is the classic, and it also exposes the Z node explicitly — many "CFA" op-amps are secretly CCII+ blocks with a buffer). Typical AD844 achieves 60 MHz at a gain of 10, versus a general-purpose op-amp with 10 MHz GBW that would give you only 1 MHz.

Watch out for: X-terminal input impedance isn't truly zero (typically 50 Ω on the AD844), so at low R₁ values it eats into your gain accuracy. Also, Z-terminal parasitic capacitance sets your ultimate high-frequency pole — keep the Z node's load light and its wiring short.