Bias Circuit for RF Power Amplifiers

Abstract

Bias circuits often affect the instantaneous bandwidth (the bandwidth of the signal that is taken through the PA at a given time) over which a pre-distorter can linearize the transmitter’s output signal. The resulting nonlinearity is difficult to remove completely even by most advanced pre- distortion techniques. In order to keep up with the increasing data rates in telecom industry, there is a desire to increase the mentioned bandwidth. This thesis describes clearly the effects of bias circuit which causes the baseband currents to generate significant variations in the baseband voltages at the transistor terminals. This paper proposes internal decoupling (large capacitance inside the drain package of the transistor) to lessen baseband resonance problem by moving the resonance to much lower frequency with a weaken amplitude. It also proposes the use of snubber circuit to attenuate the weak resonance. Two models of packaged RF power transistor were available. These were identical except that one had internal drain baseband decoupling and the other not. Both were internally pre-matched Si LDMOS transistors designed mainly for class AB operation in the 1.8 GHz band. The transistor with internal decoupling seems to alleviate the baseband impedance variation problem thus making it easier to pre-distort. Simulation result gave a baseband impedance of 0.3Ω at 11 MHz and 11Ω at 194 MHz for the transistor with internal decoupling and for the standard transistor respectively. The measurement results gave 2Ω(manufacturers measured with un-optimized board and it was 16Ω) at 242 MHz and 4Ω at 164 MHz again for the transistor with internal decoupling and for the standard transistor respectively. For RF performance, both transistors were designed for best efficiency and gave 58% for the transistor with internal decoupling and 55% for the standard transistor. Both transistors gave output power more than 100W.