Self-consistent calculations of the parameters of self-sustained MIP and DCP in argon and in model gases in the diffusion-controlled regime have been carried out. The calculated parameters of argon microwave produced plasmas (MIP) and direct current plasmas (DCP) are in satisfactory agreement with experimental data. The use of the proposed invariant group of parameters, namely specific absorbed power P_d, plasma gas pressure p, discharge tube radius R and tube wall temperature T_W, for the comparisons of different types of electrical discharges have been grounded. The results of calculations in argon using different models of discharge and the results of calculations in model gas plasmas are given.