The T-Coil Circuit Demystified

The T-coil circuit is finding new life on-chip in wideband circuits. Its design remains shrouded in myth, with complicated design equations whose derivations can be very involved. There is almost no intuition to guide its user. We show that the symmetrical T-coil is actually an equivalent of the classic lattice network. Using the Brune equivalent of the lattice and the bisection theorem, the apparent complexity dissolves and insights follow. All transfer functions can be derived in one or two steps, and it becomes clear why driving a capacitor load through the symmetrical T-coil gives twice the bandwidth of inductive peaking. Some insights extend to the asymmetrical T-coil circuit.