loop with a square $S$. The critical current $I _{c}$ of the two junctions oscillates as a function of the external magnetic flux $\Phi = BS$ threading the loop, with the period equal the flux quantum $\Phi _{0} = 2\pi \hbar /2e \approx 20.7  \ Oe \ \mu m^{2}$ [@SQUID1977]. The voltage $V$ across the junctions at constant bias current $I$ is also periodic in $\Phi $. The classical dc SQUID is used as the detector of magnetic flux $\Phi $ or magnetic field $B = \Phi /S$ due to the periodic dependence $V(\Phi )$ at $I = const $. Therefore the maximum value $(\partial V/\partial \Phi ) _{I}$ is important parameter for the dc SQUID sensitivity [@SQUID1977]. The amplitude of the voltage oscillations $\Delta V = R _{d}\Delta I _{c}$ cannot exceed the value $R _{d}I _{c} < \Delta /e$, where $ R _{d}$ is the dynamical resistance of the Josephson junctions, $\Delta $ is the energy gap of the superconductor and $e$ is the electron charge [@Barone]. According to the relation $I _{c} = 2 I _{c,j}|\cos \pi \Phi |$ [@Barone] valid in the case of week screening $\beta = 2L I _{c,j}/\Phi _{0} \ll 1$ [@SQUID1977] the 