  We study the classical and quantum $G$ extended superconformal algebras from
the hamiltonian reduction of affine Lie superalgebras, with even subalgebras
$G\oplus sl(2)$. At the classical level we obtain generic formulas for the
Poisson bracket structure of the algebra. At the quantum level we get free
field (Feigin-Fuchs) representations of the algebra by using the BRST formalism
and the free field realization of the affine Lie superalgebra. In particular we
get the free field representation of the $sl(2)\oplus sp(2N)$ extended
superconformal algebra from the Lie superalgebra $osp(4|2N)$. We also discuss
the screening operators of the algebra and the structure of singular vectors in
the free field representation.
