Differential amplifiers comprise a differential input stage, first, second, third, and fourth fixed current sources, a first resistive element, and a current-folding element. The present invention provides for a process for converting hydrocarbonaceous oils comprising contacting the hydrocarbonaceous oils with the catalyst under suitable hydrocarbon conversion conditions. Customer data of the user is held securely in a database of the payment gateway and is not transmitted over the Internet or held in the mobile terminal. Each bank of reference signals has a corresponding bank of switches, with each switch associated with a particular reference signal in the bank.