  We have used the method of generating functional in imaginary time to derive
the current-voltage characteristics of a tunnel junction with arbitrary
tunneling conductance, connected in series with an external impedance and a
voltage source. We have shown that via the renormalized charging energy and the
renormalized environment conductance, our nonperturbative expressions of the
total action can be mapped onto the corresponding perturbative formulas. This
provides a straightforward way to go beyond the perturbation theory. For the
impedance being a pure resistance, we have calculated the conductance for
various voltages and temperatures, and the results agree very well with
experiments.
