  It has long been recognized that a control current $I_a$ injected into the
section of a two-junction superconducting quantum interference device (SQUID)
is able to produce a change of its critical current $I_c$, so that a current
gain $g=|dI_c/dI_a|$ can be identified. We investigate the circumstances under
which large gains can be achieved by using vertical Josephson interferometers
which are characterized by small loop inductances. We discuss the theory of
operation of such a novel device, its performances and its advantages with
respect to planar interferometers used in the previous works. Two potential
applications are addressed.
