We have considered an InGaAs/InAlAs/InP/InGaAs ballistic tunnel transit-time oscillator under dc and small high frequency ac electric field. Using the quantum transmitting boundary method coupled with the Poisson equation, we have studied electron tunnelling through the barrier assuming that the electrons have a nonparabolic dispersion, and have got the steady state field-current characteristics (E-j) of the oscillator. As a result of the small signal analysis based on these characteristics, we find the negative resistance windows, which are the work regions of the oscillator. The windows get much deeper with the increase of the dc electric field. Their frequencies are in the terahertz (THz) range, which may be used to develop efficient and powerful sources of this range.