Expansion of the boost operator in powers of the coupling constant

Representation of the boost operator in the form of an ordered exponent [1] is analysed. A way of expanding this operator over the coupling constant powers is suggested which is of interest for the description of interaction of composite systems with large momenta. Basing on this expansion a “mixed” representation for the form factor of a composite particle is derived which exploits simultaneously the wave functions of a bound state of the usual field theory and the field theory on null-plane. The relation with the calculations in the system $P_z\to\infty$ with longitudinal momentum transfer ($q^+\ne0$) is discussed in the example of calculating the asymptotic behaviour.