The variations of the phase and amplitude of klystron
output due to the change in klystron cathode voltage is investigated
in this note. The mechanism and the effectiveness
of the feedback control to suppress the variations are
given. To understand the limitation of the feedback, both
proportional controller and proportional-integral controller
used in feedback loop are simulated and analyzed respectively
for superconducting cavity and normal conducting
cavity. The tolerances of the droop and ripple in cathode
voltage are shown according to the data and results obtained.
All the simulations and calculations are performed
with MATLAB. The data and results are listed in detail so
as to enable comparison with further studies and measurements
at ESS.