| 摘要 |
A cascode variable gain amplifier (VGA) includes a gain control circuit 100, a variable load circuit 110, a current steering circuit 120 and an input circuit 130. The gain control circuit 100 receives a gain adjusting voltage VAGCand generates a resistance control signal VPand a current steering control signal VCTRL. The load circuit includes variable resistors RL which may comprise FETs (fig.5), the resistance of which is adjusted by the resistance control signal VP. The current steering circuit is connected to the load circuit through plural current paths M2A and M3A, and M2B and M3B. The current ratio between the paths is adjusted by the current steering control signal VCTRL. Differential input and output terminals IN+ / IN- and OUT+ / OUT- are provided. In use, in a high gain range, the resistance adjusting signal Vp is varied but the current steering control signal VCTRLis held constant. In a low gain range, the current steering control signal VCTRLis varied but the resistance control signal VPis held constant. The highly linear VGA thereby provides a lower noise figure (NF) at low gain settings (fig.4b) than does a conventional circuit (fig. 1). |
