# Help With Circuit

Discussion in 'General Electronics Discussion' started by Rajinder, Nov 13, 2016.

1. ### Rajinder

470
9
Jan 30, 2016
Hi all,
i have attached the following circuit. it is basically a opamp driving a n channel fet used as a constant current source.
the drain of the fet is connected to 0V, the source is at -5v. the gate is driven from the opamp output via 100R.

my understanding that the opamp will drive the fet into conduction, this switches the current through the fet which develops a voltage drop across the 1M resistor. This keeps both of the inputs of the opamp terminals at the same voltage (minus the opamp input offset voltage). hence the current output is given by I = Vin / 1M. the ciruit reaches equilibrium when enough opamp output voltage is generated to control the fet to allow exactly the current to flow through the 1M to create a voltage drop equal to vin.

Am i correct in saying that if Vin is -0.5V, the opamp output is driven positive ie. towards 0V?

My other question is what equations satisfy the fet control i.e. is it in linear mode? saturation mode? could someone help with this part of the circuit. could someone explain with equations what is happeining with the fet.

Raj

File size:
151.1 KB
Views:
110
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Your circuit effectively has positive feedback rather than negative feedback. It will act more like a Schmitt trigger than a current sink.

To work, the resistor needs to be between the source and ground and the non inverting input connected to the source of the mosfet.

3. ### Rajinder

470
9
Jan 30, 2016
Hi Steve,
Thanks, if i use a n channel fet, then i can have the drain to 0V with resistor to non-inverting input and then source to output with the other end to -v supply.
Do you remember that you helped me once before?
I am still trying to understand the circuit you helped me with.
i have attached the circuit but you also pointed out that i needed a n channel not a p channel fet.
You also mentioned that the current output was (Vin * R2) / (R1 * R3).
I thought it would be simply -Vin / R3
I have a question, if i do not have a -Ve supply to the end of the source what would happen, would it not function as a constant current source?
best regards,
raj

File size:
18.9 KB
Views:
59
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Sorry, I was using my phone at the time and misread the 0V rail for 9V -- I assumed it was the positive rail.

OK, so this is a current source which requires a negative voltage input.

I'm going to have to go back and look at that circuit again. It looks like I've drawn the mosfet with the drain and source reversed, which then means that (of course) a P channel mosfet is required.

The source must be closest to the most +ve rail (in this case 0V) and the drain toward the negative rail. The op amp output goes negative with respect to 0V to turn the mosfet on.

For the circuit you've drawn the output current is Vin/R3 (note that Vin must be negative because the current is -ve with respect to the 0V rail.

The other half of the circuit has a gain given as -(R2/R1), so for a given (positive) input voltage Vin, the output is Vin * ( -R2/R1 ) / ( R3 ) = (Vin * R2) / (R1 * R3)

Last edited: Nov 15, 2016
5. ### Rajinder

470
9
Jan 30, 2016
Hi Steve,
Thanks for the help. I did point this out in my previous post when you drew the circuit i.e. that the get you have is a p channel.
I have replaced this with a n channel. Drain to 0V with sense resistor and source going to load with the other end of load to -ve. Is that correct I.e. n channel drain at 0v with source at -ve? It seems to work OK.
Best regards,
Raj

6. ### Audioguru

3,042
678
Sep 24, 2016
Your schematic is completely wrong if an N-channel Mosfet is used as a follower. Even the negative feedback to the opamp from the drain of the Mosfet becomes positive feedback instead of negative feedback.

7. ### Rajinder

470
9
Jan 30, 2016
Hi,
Thanks for your reply. The schematic is incorrect. I have a n channel with drain to 0v, in between drain and 0v is the sense resistor. Then this is connected to the opamp. If the drain is at 0v then I need the source to be at a lower potential I.e. -5v. Is this not correct?

8. ### Audioguru

3,042
678
Sep 24, 2016
A current source uses negative feedback. Your circuit has positive feedback.

9. ### Audioguru

3,042
678
Sep 24, 2016
I see that you do not show an N-channel Mosfet in your schematic, It is an upside down P-channel.
Schematics should always show a part number.

File size:
39.5 KB
Views:
55
10. ### Rajinder

470
9
Jan 30, 2016
Hi Sorry, it is a n channel mosfet that i have. incorrectly it is a p channel in the diagram.

11. ### Rajinder

470
9
Jan 30, 2016
i will show the correct schematic, give me 2 minutes

12. ### Rajinder

470
9
Jan 30, 2016
The circuit has a -0.5v at vin, the source is fed back to the inverting input. fet is n channel d to 0v, inverting input connected to -5V via 10K to allow inverting input to be driven to same potential as vin i.e. -0,5v. the output is taken from the 1M, so -0.5/1M is the current o/p.

Note: vin is fed from the inverting opamp
I get both inputs of the opamp to read -0.5V i.e. constant voltage,
the resistor is constant i.e. 1M
therfore I = v / r = -0.5v / 1M

This is supposed to be a -ve constant current source.

I wasnt sure how the opamp regulates the fet

File size:
81.3 KB
Views:
60
13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Why do you say upside down?

14. ### Audioguru

3,042
678
Sep 24, 2016
Your new circuit is completely wrong.
1) A current source does not have a resistor in series with its output. Yours has 1 Megohm.
2) The (-) input of the opamp cannot be -0.5V unless the load voltage is +450.5V.
3) You have the output resistor connected to the source of the Mosfet but instead the drain must be the output.
4) Your circuit is missing a current sensing resistor connected from the source and (-) input of the opamp to ground.

Basically if the input is a positive voltage then the Mosfet must be N-channel and the load connects to the positive supply. If the input is a negative voltage then the Mosfet must be P-channel and the load connects to the negative supply.

15. ### Audioguru

3,042
678
Sep 24, 2016
Because the drain and source are swapped. My "corrected" copy fixed it.

(*steve*) likes this.
16. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
That's a lowecase m. Perhaps it is a 1 milliohm current sense resistor. However, that's a pretty low value resistor.

17. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Sorry, I was looking at your corrected version.

I think I noted that the mosfet was shown backwards in the original thread.

And here is yet another reason not to start new threads... I'll try to find the original thread with the full discussion.

25,448
2,809
Jan 21, 2010
19. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Thanks for the correction, by the way.

20. ### Rajinder

470
9
Jan 30, 2016
Hi guys,
Sorry for the confusion. Steve do you have the new circuit with correction. Thanks for the help with this.