Detailed results are presented of two-beam optical transistor (transphasor) studies in a single InSb etalon using a CW CO laser to provide the bias beam and a Pb S1-xSExdiode laser to generate a modulated signal. The gain-bandwidth product has been measured to be ∼ 1.2 MHz for gain values ≥ 3 and is reduced at lower gain. A full expression for the absolute value of the optical gain is derived and shows good agreement with these results. An experimental method to estimate the device and material response time through the gain/ frequency dependence is presented. A medium response time of 230 ns ± 10 percent has been deduced for the conditions of these experiments.