Most of the radio frequency (RF) variable gain amplifiers (VGA) are fabricated in CMOS process and GaAs process.The gain variability is usually realized when the biased voltage/current of transistors is changed or an attenuator in VGAs is built.A RF VGA variable gain amplifier based on high performance RF SiGe heterojunction bipolar transistor(HBT) is presented.The designed VGA consists of a feedback path with a PIN diode.The PIN diode forward resistance is controlled by its forward biased voltage

and hence the dB-linear variable gain is realized when the PIN diode forward biased voltage is altered.After the high-frequency small-signal equivalent circuit of the VGA with the PIN diode feedback is built

the gain controlling mechanism is analyzed.A complete design and implementation of a 1.8GHz VGA with the proposed VGA topology was demonstrated.The test results show that the dynamic gain scope reaches 15dB at the frequency of 1.8GHz when the controlling voltage changes from 0.6V to 3.0V.The noise figure is lower than 5.5dB and the minimum noise figure achieves 2.6dB.The input and output matching keeps well and the linearity is also acceptable in the whole voltage variable range.