URL of Featured image

esc_url( wp_get_attachment_url( get_post_thumbnail_id() ) )

A particular size:

$image_url = wp_get_attachment_image_url( get_post_thumbnail_id(), 'large' );

or,

wp_get_attachment_image_url( get_post_thumbnail_id( $prev_post->ID ), 'prev_next_thumb' )

or,

wp_get_attachment_image_src( get_post_thumbnail_id(), 'landing-page' )[0]