Question 1

What is a T-coil circuit and how does it extend bandwidth?

Accepted Answer

A T-coil or bridged T-coil is a circuit with two mutually coupled inductors and a bridge capacitor that extends signal bandwidth beyond simple inductive peaking. At low frequencies the signal passes through the inductors while at high frequencies it bypasses them through the bridge capacitor. This provides two complementary signal paths that maintain nearly flat frequency response over a wider bandwidth.

Question 2

How do parasitic resistances affect the T-coil transfer function?

Accepted Answer

Parasitic resistances Ra and Rb in the inductor windings reduce the DC gain because they add to the voltage divider between R1 and R2. The DC gain drops from R2 over R1 plus R2 to R2 over R1 plus Ra plus Rb plus R2. They also slightly reduce the bandwidth and shift the frequency of minimum attenuation. Toggle the Compare No Parasitics option to see the difference.

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T-Coil Circuit (Bridged T-Coil Transfer Function)