  An expression for the microwave power generated by a spin-torque oscillator
in the presence of thermal noise is derived. This expression, when used in a
subcritical regime, demonstrates that generated power is determined by the
noise level and provides a simple way to experimentally determine the threshold
current for microwave generation. The same expression gives a good quantitative
description of the experimentally measured dependence of the generated power on
the bias current in a moderate super-critical range of bias current variation.
