# Help understanding a circuit

#### johnkaz77

Joined Sep 20, 2010
33
Hi,i found this circuit that should be sinking 0 to 10uA current but i don't really understand how it works.I also want to source current,when necessary do i need a second circuit or this one does both jobs?Can someone explain?

#### Jony130

Joined Feb 17, 2009
4,974
Do you now how opamp work?
Opamp wont to keep non-inverting input voltages (Vc) equal to inverting input voltages (Vs).
Vc = Vs
So the voltage on R2 mus be equal Vc = V1 * R3/(R4+R3) = 0.02*V1.
And output current will be equal:
Iout = Vc/R2 = (0.02*V1)/R2 = 2μA*V1
So if V1 is equal 1V Iout = 2μA.
And for sourcing current you need a second circuit.

#### johnkaz77

Joined Sep 20, 2010
33
Yes,i know that,i don't understand the use of the mosfet in the circuit.
How does it "sink" current?(i'm not exactly sure on what current sink actually is).Also,the circuits you uploaded,what are they?

#### Jony130

Joined Feb 17, 2009
4,974
How does it "sink" current?(i'm not exactly sure on what current sink actually is)
Very simply, opamp opens MOSFET that the voltage on R2 is equal Vc.
And when M1 is open the current is start to flow from power supply.
From "plus" terminal of a V3--->Rload-->drain-source M1--->R2---> "minus" terminal of a power supply.

So current is "sink" or "pull" through the load by the mosfet.
http://digital.ni.com/public.nsf/allkb/A10122C63A7F5CFE86256B4C007491DD

Also,the circuits you uploaded,what are they?
First diagram with opamp show "sinking" version of a current source.
And load current is flow from:
"Plus" terminal of a V3--->R6--->source-drain M4--->load resistor--->gnd
So know the mosfet source or push current through the load.

Second diagram show BJT version of a voltage control current source.
And depending on the Vin, the current source can sink or source current to the load.