Using $wpdb

I’m using a $wpdb and it is not recognize, am I missing something out?

my page calls an ajax to a certain php file which is I name get_place.php

this is my ajax, its working

$.ajax({
cache: false,
url: baseurl + ‘/php/get_place.php’,
type: ‘post’,
data: ‘latlang=’ + place.geometry.location + ‘&q=’ + 1
});

and this is the get_place.php file

if (isset($_POST['latlang'])){
	$latlang = $_POST['latlang'];
}

if (isset($_POST['q'])){
	$q = $_POST['q'];
}

if($q==1){
        $wpdb -> query("INSERT IGNORE INTO <code>tbl_branch</code> SET <code>br_latlang</code> = '".$latlang."';");
}
else{
	$result = $wpdb -> get_results( "SELECT REPLACE(<code>br_title</code>, ' ', '&nbsp;'), <code>br_content</code> FROM <code>tbl_branch</code> WHERE <code>br_latlang</code> = '".$latlang."';" );
	$row = mysql_fetch_array($result);
	echo "<div id=\"infobox_content\">
		<div id=\"siteNotice\"></div>
		<h3 id=\"firstHeading\" class=\"firstHeading\">".$row[0]."</h3>
		<div id=\"bodyContent\">
		".$row[1]."
		</div>
		</div>";
}

it gives me an error that $wpdb is not recognized

Notice: Undefined variable: wpdb in C:\xampp\htdocs\public\lianas\wp-content\themes\lianas\php\get_place.php on line 15

Fatal error: Call to a member function get_results() on a non-object in C:\xampp\htdocs\public\lianas\wp-content\themes\lianas\php\get_place.php on line 15

any ideas?