In the Figure, the u-i relationship of element X is[mathjaxinline]i = { m{A}}{{ m{e}}^{u/B}}[/mathjaxinline] , wherein A and B are all constants. Analyze the relationship between the output voltage [mathjaxinline]u_o[/mathjaxinline] and the input voltage [mathjaxinline]u_i[/mathjaxinline] , and select the operation function of the circuit.