Reduction of phase noise in linear HBT amplifiers using low-frequency active feedback

In this paper, we investigate the effect of low-frequency active feedback in the 1/f phase modulation (PM) noise of a linear SiGe heterojunction bipolar transistor amplifier operating at 1 GHz. The voltage gain and the output resistance of the feedback amplifier were varied and their effect on the baseband collector voltage noise reduction and PM noise reduction were observed. Our results show that the measured reduction in the baseband noise when using active feedback was in close agreement with the expected reduction (within 2 dB) for all the configurations tested. While the PM noise was also reduced when active feedback was used, in some feedback configurations the PM noise reduction was not as large as the reduction observed in the baseband noise. The best amplifier PM noise obtained when using low-frequency feedback was L(f)≅-140 dBc/Hz at 100 Hz from the carrier, a reduction of 21 dB compared to the amplifier without feedback.