Get only the author profile picture image url inside a loop

I need to get the post author profile picture link.

I know I can use get_avatar(); but it displays whole image markup

<img src="https://.../img.png" class="avatar img-responsive" alt="image">

But need only the src of the author profile pic.

How can I got that ? Maybe we can use some php or js to take out the only src value from the total output.

My second question is, how can I add that image as post_thumbnail if no thumbnail exist for that post ?

Putting the following inside loop should fulfill your needs:

<?php
$get_author_id = get_the_author_meta('ID');
$get_author_gravatar = get_avatar_url($get_author_id, array('size' => 450));

if(has_post_thumbnail()){
    the_post_thumbnail();
} else {
    echo '<img src="'.$get_author_gravatar.'" alt="'.get_the_title().'" />';
}
?>