This article presents a study of the Class G amplifiers used to improve the efficiency versus back-off characteristic of power amplifiers. With this technique, the amplifier's drain bias is switched between two or three values depending on the detected envelope magnitude in order to improve the efficiency when the signal level is lower and the power amplifier needs less biasing voltage. Several options will be presented, comparing the performance in terms of efficiency and linearity for each case. Using a 3GPP signal, a great improvement of the PAE vs. backoff is obtained, with a value of 40.3 % at 20 dBm of output power instead of 27 %, value obtained with a fixed biasing voltage of 3.5 V
