  We report results of investigation of amplitude calibration for very long
baseline interferometry (VLBI) observations with Korean VLBI Network (KVN).
Amplitude correction factors are estimated based on comparison of KVN
observations at 22~GHz correlated by Daejeon hardware correlator and DiFX
software correlator in Korea Astronomy and Space Science Institute (KASI) with
Very Long Baseline Array (VLBA) observations at 22~GHz by DiFX software
correlator in National Radio Astronomy Observatory (NRAO). We used the
observations for compact radio sources, 3C 454.3, NRAO 512, OJ 287, BL Lac, 3C
279, 1633+382, and 1510-089, which are almost unresolved for baselines in a
range of 350-477 km. Visibility data of the sources obtained with similar
baselines at KVN and VLBA are selected, fringe-fitted, calibrated, and compared
for their amplitudes. We found that visibility amplitudes of KVN observations
should be corrected by factors of 1.10 and 1.35 when correlated by DiFX and
Daejeon correlators, respectively. These correction factors are attributed to
the combination of two steps of 2-bit quantization in KVN observing systems and
characteristics of Daejeon correlator.
