  We consider the infinite sequences $(A\_n)\_{n\in\NN}$ of $2\times2$ matrices
with nonnegative entries, where the $A\_n$ are taken in a finite set of
matrices. Given a vector $V=\pmatrix{v\_1\cr v\_2}$ with $v\_1,v\_2>0$, we give
a necessary and sufficient condition for $\displaystyle{A\_1... A\_nV\over||
A\_1... A\_nV||}$ to converge uniformly. In application we prove that the
Bernoulli convolutions related to the numeration in Pisot quadratic bases are
weak Gibbs.
