Constant Negative Current Source

[BKI]

Hi,

i need a constant negative current source that generates 100mA/0mA. The circuit worked with ideal current sources.
Then i tried to use a MOSFET on the source side connected to ground, gate signal is 5V/0V - but this doesn´t work...i guess because the current is higher and unstable. Max. Vds of the Current Source Mosfet is 10V.
(see use in picture - current source is in red box)



Any simple idea how to realize this constant negative current source ?

 

[prestonee]

Try using larger device sizes, mosfet drain currents are slave to device w/l ratio and gate/drain voltages i dont see any device sizes, if these are mosfets in a package for pcb type use you should be able to pull up a data sheet to see drain current vs gate voltage. just remember Id=k*R*(Vgs-vt)^2 k =process constants(non changing) R is W over L ratio, vt is threshold voltage, Vgs is gate to source voltage of your pfet

 

[BKI]

Try using larger device sizes, mosfet drain currents are slave to device w/l ratio and gate/drain voltages i dont see any device sizes, if these are mosfets in a package for pcb type use you should be able to pull up a data sheet to see drain current vs gate voltage. just remember Id=k*R*(Vgs-vt)^2 k =process constants(non changing) R is W over L ratio, vt is threshold voltage, Vgs is gate to source voltage of your pfet

I tried it at first with a ideal current source. With increased current, the output voltage decreases, because it is pulled from the 150V supply rail. So, i really need low current of maximum 100uA which is difficult to find.

 

[nitishn5]

In which direction is the current flow? Is it from the top to the bottom?

An ideal current source will force node voltages and get the currents to flow to satisfy the Kirchoff's laws, a real one will not work like that.

 

[BKI]

In which direction is the current flow? Is it from the top to the bottom?

An ideal current source will force node voltages and get the currents to flow to satisfy the Kirchoff's laws, a real one will not work like that.

Attached is the full view of the two rails.
I need solutions for the current sources. It didn´t work with just MOSFETs. Current sources have to controlable through 0V/5V input.