how to use featured image as background image in wordpress code example

Example: wordpress featured image as a background image

<?php
  	if ( has_post_thumbnail() ) {
    	$image_b1 = get_the_post_thumbnail_url();
  	}
	else {
  		$image_b1 = get_stylesheet_directory_uri().'/images/noimage.png';
	}
?>
  
 <div style="background-image: url(<?php echo $image_b1; ?>);">