.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: 9999;
    outline: 0!important;
    transition: .4s;
}

.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;
}