namespace backend\models;

use common\models\Admin;......

protected function getUser()
  {
      if ($this->_user === null) {
          $this->_user = Admin::findByUsername($this->username);
      }

return $this->_user;
  }

namespace frontend\models;

use common\models\User;

'components' => [
        'user' => [
            'identityClass' => 'common\models\Admin',
            'enableAutoLogin' => true,
        ],
        ...
]        

'components' => [
        'user' => [
            'identityClass' => 'common\models\User',
            'enableAutoLogin' => true,
        ],
        ...
]

namespace backend\controllers;

use Yii;
use yii\web\Controller;
use yii\filters\VerbFilter;
use yii\filters\AccessControl;
use backend\models\LoginForm;