Stacked FET switch bias ladders
A positive-logic FET switch stack that does not require a negative bias voltage, exhibits high isolation and low insertion/mismatch loss, and may withstand high RF voltages. Embodiments include a FET stack comprising series-coupled positive-logic FETs (i.e., FETs not requiring a negative voltage supply to turn OFF), series-coupled on at least one end by an “end-cap” FET of a type that turns OFF when its V GS is zero volts. The one or more end-cap FETs provide a selectable capacitive DC blocking function or a resistive signal path. Embodiments include a stack of FETs of only the zero V GS type, or a mix of positive-logic and zero V GS type FETs with end-cap FETs of the zero V GS type. Some embodiments withstand high RF voltages by including combinations of series or parallel coupled resistor ladders for the FET gate resistors, drain-source resistors, body charge control resistors, and one or more AC coupling modules.
1. A FET switch stack, including:
(a) a plurality of series-coupled FETs having a first FET configured to be coupled to a radio frequency (RF) signal input;
(b) a first gate resistor ladder including a plurality of parallel-connected constant-valued resistors each coupled to the gate of one corresponding FET; and
(c) a second gate bias resistor ladder including a plurality of series-connected variable-valued resistors each coupled to one corresponding constant-valued resistor of the first gate resistor ladder and having resistive values that taper from higher resistive values to lower resistive values, with the higher resistive values located near the RF signal input to the first FET.
2. A FET switch stack, including:
(a) a plurality of series-coupled FETs having a first FET configured to be coupled to a radio frequency (RF) signal input;
(b) a first gate resistor ladder including a plurality of parallel-connected variable-valued resistors each coupled to the gate of one corresponding FET; and
(c) a second gate bias resistor ladder including a plurality of series-connected variable-valued resistors each coupled to one corresponding variable-valued resistor of the first gate resistor ladder and having resistive values that taper from higher resistive values to lower resistive values, with the higher resistive values located near the RF signal input to the first FET.