Filtering Query WHERE

Hello,

I want to filter my Query Select. I want to Select only id_operador < 40
This is my code:

$out['operadores'] = $db
		->select( 'operadores', 'id_operador as value, nombre_operador as label', array("id_operador" => "40"))
		->fetchAll(); 

But this, only filter the id_operador = 40. I want id_operador < 40.
How I do please??

Thank you!