Fast optical power stabilization is demonstrated using a germanium photodiode and a silicon variable optical attenuator monolithically integrated on a compact silicon photonic platform. With an external electronic feedback circuit, the output power of the device is stabilized within an error of 2.7 dB for 22-dB input power variation. Feedback response time is less than 90 ns. The response time will be reduced to ~50 ns applying an on-chip feedback circuit.