Linear, Efficient and Wideband Emitter Follower Class B Amplifier for Auxiliary Envelope Tracking Supply Modulator

In this paper, an efficient and wideband Class B amplifier is designed for the application as a supply modulator of an Envelope Tracking (ET) radio-frequency power amplifier (RFPA). The common collector configuration is employed to drive a low output impedance since the output signal of this amplifier will be used as the bias signal to the drain port of the RFPA. A quasi-complementary design, also referred to as a totem-pole with a pair of n-type transistors, is implemented to build the Class B amplifier. In this design, the transistors are properly biased to avoid crossover distortion and maintain a good level of linearity. The wideband property of the amplifier has been achieved by controlling the values of the coupling capacitors. A good efficiency is recorded by using a step-up center-tapped transformer at the emitter outputs of the transistors. Simulation results show that if the input AC signal level is chosen to be half of the maximum DC voltage level, then a transformer turn ratio of 1:6 will give an efficiency of 87%. Moreover, the bandwidth is recorded to be almost 80 MHz after using a 10 uF coupling capacitor and a 70 uF bypass capacitor. The proposed supply modulator is suitable for the base station applications of the LTE networks.