.btn_up {
     position: fixed;
     bottom: -50px;
     right: 100px;
     width: 50px;
     height: 50px;
     border-radius: 50%;
     border: 2px solid #ccc;
     background: #333;
     opacity: .5;
     z-index: 100;
     outline: 0!important;
     transition: .4s;
    -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
 }

.btn_up:after {
    content: '';
    width: 10px;
    height: 10px;
    border-top: 2px solid #ccc;
    border-left: 2px solid #ccc;
    transform: rotate(45deg);
    position: absolute;
    top: 4px;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto;
}

.btn_up.active {
    bottom: 15px;
    right: 15px;
}