Bias_CircuitsSheet 1 of 5 Microwave Bipolar/FET Bias circuits RF/Microwave transistors/FET’s require some form of circuit to set the correct bias conditions for a particular RF performance. There are two main types used an active current mirror and a passive self-bias circuit. The main purpose of the bias circuit is to maintain the drain/collector current regardless of any drift in the DC current gain of the GAsFET/Bipolar. In the case of a GAsFET, over time the transconductance falls, with a resulting drop in the bias current. The Sparameters of a device are fixed and don’t age so long as the correct bias current is maintained. A drop in bias current, over life, will cause the RF device gain and output power to fall. A reduction in the gain of the device in the oscillator may cause oscillation to sto……