Phase noise reduction in microwave bipolar transistor amplifiers through active feedback

In this paper, we document the effects of low-frequency active feedback on the 1/f phase modulation (PM) noise of linear and compressed SiGe heterojunction bipolar transistor (HBT) and Si bipolar junction transistor (BJT) amplifiers operating at 1GHz. Low noise, high frequency transistors manufactured by NEC were used in a common-emitter (CE) configuration and powered with DC batteries to ensure low supply noise. The feedback path consisted of a low noise operational amplifier, the LT1028, connected as a voltage referenced difference amplifier. In general, the measured reduction in the baseband collector voltage noise with active feedback agreed with the theoretical values. While the PM noise of the amplifiers was reduced when using active feedback, the reduction was lower than expected, therefore other factors must contribute to the amplifiers´ PM noise. The lowest PM noise achieved was L(10 Hz) ≈ -144 dBc/Hz for the Si BJT amplifier operating in 4dB compression and L(10 Hz) ≈ -139 dBc/Hz for the SiGe HBT amplifier operating in 4dB compression. These represent reductions of approximately 12 dB and 9 dB from the respective open loop configurations.