Calendar Ajax is not work properly

i am  am using calender ajax on textbox but postion is not work properly when i am click on textbox then calender shown on the top of page.

my code is:

<asp:TextBox ID="txt_Cdob" runat="server" Width="120px"></asp:TextBox><cc1:CalendarExtender ID="CalendarExtender1" runat="server"
                    TargetControlID="txt_Cdob" Format="dd/MM/yyyy"
            PopupPosition="BottomRight"></cc1:CalendarExtender>