We consider the sensitivity of optical receiver used in digital fiber-optics communication link. This sensitivity is understood as the smallest optical power which must be fed to the photodiode to assure BER = 10-9. It is assumed, that the only sources of noise are the photodiode and the first stage of receiver preamplifier. Approximate expressions describing advantages of using APD over PIN diode was derived. The optical transimpedance receiver for 34 Mb/sec system was used as calculation example.