# Help understanding a circuit

Discussion in 'Homework Help' started by johnkaz77, Sep 23, 2010.

1. ### johnkaz77 Thread Starter Member

Sep 20, 2010
33
0
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?

File size:
44 KB
Views:
34
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
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.

File size:
15.9 KB
Views:
31
File size:
29 KB
Views:
27
3. ### johnkaz77 Thread Starter Member

Sep 20, 2010
33
0
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?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
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

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.

5. ### johnkaz77 Thread Starter Member

Sep 20, 2010
33
0
Thank you,you've been really helpfull!I have one last question,how can i combine these two (current source/current sink) in order to have one output?(depending on a digital signal to choose either source or sink?)