4-8 GHz band SiGe HBT variable gain amplifiers with a feedforward configuration

Broadband monolithic SiGe HBT variable gain amplifiers with a feedforward configuration have been newly developed to improve bandwidth and dB-linearly controlled gain characteristics. For bias-current control, two types of the VGA have been made: one using a coupled-emitter resistor and the other using a HBT-based current source. Both of the VGAs achieve a dynamic gain-control range of 20 dB and a 3-dB bandwidth of 4 GHz (4-8 GHz) with the control-voltage range from 0.4 to 2.6 V. At 6 GHz, the VGAs with an emitter resistor and a current source produce a maximum gain of 9.3 dB and 8.8 dB, respectively. The total bias currents are 59 mA in the VGA with an emitter resistor and 71 mA in that with a constant current source at V/sub CTRL/ = 2.4 V.