We investigate transport in nanotransistors in the Landauer–Büttiker formalism. A systematic linearization of the general expression for the current response yields the quantum version of the small signal equivalent circuit. This equivalent circuit can be compared with classical schemes so that explicit quantum mechanical expressions for the circuit elements can be extracted. Reducing our analysis to an effective Y-parameter description of the equivalent circuit we find the multi-terminal Büttiker formula except for one extra term. We show that this extra term is essential for the operation of transistors. An application of our theory to a simple transistor model yields a description of mismatch oscillations in the source-drain current experimentally observed in nano-transistors.