  We present a design and protocol to achieve an essential feature of an
optical transistor, namely the amplification of input signal with the use of
discrete solitons in waveguide arrays. We consider the scattering of a discrete
soliton by a reflectionless potential in the presence of a control soliton. We
show that at the sharp transition region between full reflectance and full
transmittance, the intensity of the reflected or transmitted soliton is highly
sensitive to the intensity of the control soliton. This suggests a setup of
signal amplifier. For realistic purposes, we modulate the parameters of the
reflectionless potential well to achieve a performance of amplifier with a
controllable amplification. To facilitate the experimental realization, we
calculate the amplification factor in terms of the parameters of the potential
well and the input power of the control soliton. The suggested signal amplifier
device will be an important component in the all-optical data processing.
