Apparatus to bias spool valves using supply pressure

An apparatus comprising:

• a housing of a spool valve, the housing having a supply port, a first adjacent port, a second adjacent port, a first outside port, and a second outside port, the first and second adjacent ports disposed on opposite sides of the supply port, the first adjacent port disposed between the first outside port and the supply port, the second adjacent port disposed between the second outside port and the supply port;
• a spool within the housing, the spool having a first land, a second land, and a third land, a position of the spool within the housing defining a flow path through the spool valve for a fluid received at the supply port, a variable input force acting on a first end of the spool to urge the spool in a first direction within the housing, a biasing force acting on a second end of the spool to urge the spool in a second direction within the housing opposite the first direction, when the position of the spool corresponds to a null position:
• (1) the first land blocks a first opening associated with the first outside port, (2) the second land blocks a second opening associated with the supply port, and (3) the third land blocks a third opening associated with the second outside port;
• a first fluid line to carry the fluid at a supply pressure to the supply port of the housing;
• and
• a second fluid line branching off the first fluid line to carry the fluid at the supply pressure to the second end of the spool, the biasing force based on the supply pressure of the fluid, the variable input force based on a range of proportions of the supply pressure of the fluid, the range of proportions of the supply pressure including a first proportion, a second proportion that is greater than the first proportion, and a third proportion that is greater than the second proportion.