voltage follower current source debug help, spice file attached.

[zoulzubazz]

Attached is the LTSpice ASC file of a simple voltage follower current source but the simulations are showing wrong results. The non inverting input of the opamp is set to 2V and the inverting input should technically see the same voltage while changing the gate voltage of the MOSFET. 100 ohm resistor is used as a load. Could i get help debugging this circuit? also can the output side of three of these circuits be wired in parallel from a single power source and push different currents through different loads? thanks very much!



View attachment VF_current_source.zip

 

[Audioguru]

Please post the ASC file without zipping it.

 

[zoulzubazz]

heya i am unable to attach asc for some reason. but attached is a screen shot of the model. the simulation works fine now but am unable to get the actual circuit to work on a bread board. in the simulated model the voltage of the non-inverting input of both amps changes from 0-2V effectively changing the current through the LED from 0-2A. but when implemented using a 741 the current through a 0.1ohm is much higher than 2A, the inverting input is unable to match the non-inverting input and stays at 1.1A while non-inverting input stays at 1.1v. the MOSFET used in the actual circuit was
https://uk.farnell.com/fairchild-semiconductor/fqp50n06l/mosfet-n/dp/1611615?Ntt=1611615. the whole circuit including the opamp was fed 0-12V from a single supply.




any help would be appretiated. Thanks very much.

 

[Audioguru]

You cannot use a 46 years old 741 opamp when the input is within about 3V from a supply voltage (ground in your circuit).
Your circuit will work with an old 741 opamp if it has an additional negative supply voltage of a few volts.

Your circuit will work with an LM358 dual opamp because its inputs work fine all the way down to 0V in your circuit.

 

[zoulzubazz]

thanks a lot, will try the ckt with a rail to rail opamp. another strange thing is in voltage follower mode the 741 was closely following 2V when input was 2V, wonder what the reason for that was. thanks.

 

[Audioguru]

ICs like the 741 opamp have a range of specs. Some of them have inputs that do not work if they are closer than 3V from the positive or negative supply voltage (as listed in its datasheet) but others work fine when the input is near 2V from a supply voltage. The 741 opamp also has an output that does not get close to a supply voltage.

The old LM358 dual opamp and old LM324 quad opamp have inputs and outputs that can go to the negative rail but they cannot go near the positive rail.