A mobile video-conferencing device includes a camera built into the device housing and configured to capture images of a user while viewing the display, including an infrared light source and IR sensitive image sensor for capturing images of the user under low-light or uneven light conditions, or both, to permit a face detection component to detect the user’s face. The control device comprises an error amplifier having an inverting terminal and a non-inverting terminal receiving a reference voltage. The end point is determined by monitoring the DC bias voltage on the plasma generator which is used for the plasma cleaning etching in the processing chamber in an evaluation unit. Therefore, with the differential operation circuit, a necessary frequency band is not attenuated. The conical diaphragm is oriented with its apex in a downstream flow direction.