static dbContext with Entity Framework: Compensates or not?

Well, I use the Entity Framework and Code First to manage my data access, and I'm thinking of working with a single, static, global instance of my DataBaseContext. But I have two questions:

Here is an example of how to access the data in my bank:

CURRENT - Each method calls an instance of BaseDates

using (var db = new BaseDados())
{
    var fatura = db.Contas_Receber.Where(x => x.Chave == chaveFatura).FirstOrDefault();
}

POSTERIORLY - There will be only one instance of BaseDates , and all DAOs in my system will use this instance.

public static class DAO<T> where T : class
{
    private static SETCOM_BaseDados db;

    public static void Create(T entidade)
    {
        db.GetInstance();

        try
        {
            db.Set<T>().Add(entidade);
            ((IObjectContextAdapter)db).ObjectContext.ObjectStateManager.ChangeObjectState(entidade, System.Data.Entity.EntityState.Added);
        }
        catch (Exception ex)
        {
            throw new Exception("Erro ao inserir dados : " + ex.Message);
        }
    }
}

public static class BaseDadosExtension
{ 
    public static SETCOM_BaseDados GetInstance(this BaseDados db)
    {
        return (db == null ? new BaseDados() : db);
    }
}

Call to create a new Reward Accounts :

Contas_Receber cr = new Contas_Receber();
cr.Valor = 0;
cr.Cliente = new Cliente();

DAO<Contas_Receber>.Create(cr);