  The current and future cosmic microwave background (CMB) experiments fielding
kilo-pixel arrays of transition-edge sensor (TES) bolometers require accurate
and robust gain calibration methods. We simplify and refactor the standard TES
model to directly relate the detector responsivity calibration and optical time
constant to the measured TES current $I$ and the applied bias current
$I_{\mathrm{b}}$. The calibration method developed for the Cosmology Large
Angular Scale Surveyor (CLASS) TES bolometer arrays relies on current versus
voltage ($I$-$V$) measurements acquired daily prior to CMB observations. By
binning Q-band (40GHz) $I$-$V$ measurements by optical loading, we find that
the gain calibration median standard error within a bin is 0.3%. We test the
accuracy of this "$I$-$V$ bin" detector calibration method by using the Moon as
a photometric standard. The ratio of measured Moon amplitudes between detector
pairs sharing the same feedhorn indicates a TES calibration error of 0.5%. We
also find that for the CLASS Q-band TES array, calibrating the response of
individual detectors based solely on the applied TES bias current accurately
corrects TES gain variations across time but introduces a bias in the TES
calibration from data counts to power units. Since the TES current bias value
is set and recorded before every observation, this calibration method can
always be applied to raw TES data and is not subject to $I$-$V$ data quality or
processing errors.
