# Voltage controlled current source

Discussion in 'Homework Help' started by PsySc0rpi0n, Oct 31, 2015.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Hi...

I'm studying a circuit that is a current source controlled by a voltage source using OpAmps.

This circuit is the following:

The question I have is:
What are the limitations of such a circuit if we use an UA741 OpAmp???

I know at least 2 limitations regarding this specific OpAmp, which are:
1 - The output voltage is never the OpAmp supply voltage. It always drops about 2V. So if the OpAmp supply has +12V and -12V, probably, at the best scenario, saturated OpAmp, it will output around ≈10 V and ≈-10 V.

2 - The Slew Rate is somehow bad... I'm not sure the value, and it can vary within the same reference part and so on, but is slow, maybe somehting like 0.5V/μs.

But I'm not very sure if this is what teacher wants us to answer. I mean, depending ion the application the circuit is intended to be, it may be a limited choice or not!
So, what else can be said about the circuits limitations?

2. ### dl324 Distinguished Member

Mar 30, 2015
3,378
651
There are other things to consider, but I can't mention them for fear of giving you the answers. You should ask your teacher to clarify expectations.

But one thing you can do is read the datasheet to find output swing and slew rate specs so you don't have to make assumptions.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Limitations of such a circuit could be related to:
offsets, linearity, drift, noise, thermal effects, ....
some of which can be laid at the doorstep of non-ideal behavior of the '741...

4. ### dl324 Distinguished Member

Mar 30, 2015
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651
This is the homework forum. Identifying the non-ideal attributes affecting the circuit might be the question.

5. ### crutschow Expert

Mar 14, 2008
13,501
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Perhaps the question should be expanded as to how the op amp's non-ideal behavior affects the output current.

6. ### JoeJester AAC Fanatic!

Apr 26, 2005
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In a way, that is the question. I agree with there should be more clarity from the instructor on what they want and you posed the question better than the TS's inquiry and possibly better than the instructor as the TS is one of the better known students here.

7. ### WBahn Moderator

Mar 31, 2012
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One question to ask yourself is whether the choice of the '741 is meant to be in relation to an ideal opamp, or in relation to a "better" opamp.

As already suggested, I'd look at some of the key specs on the '741 and describe what limitation those put on the circuit and compare that to what the limits would be with an ideal opamp and perhaps what they might be with a "better" opamp (of your choosing).

8. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Ok, as this is in an academic context, probably some of the items suggested here won't be of much interest as offsets, linearity, drift, noise, thermal effects.

Maybe that would be the best approach... What would be those key specs?

I will talk about current limitation, maybe slew rate that is of "poor quality" if we need to apply this circuit to a fast switching state application.

9. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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And I also have another question/problem.

One other question teacher is asking is what would we change on the circuit so that the current would increase with increasing the input voltage.

The previous one had an inverted proportionality relationship between load current and input voltage.
Now we need to change the circuit to have a direct proportionality between load current and input voltage. So, my first thought was to replace the P channel mosfet by an N channel mosfet... And in fact, this works but I find the Vgs and Vds curves weird... I think these curves on the P channel mosfet were ok and I think I understood them, but I can't quite understand what is going on with these curves for the N channel mosfet! I was expecting some kind of mirrored curves for the N channel mosfet compared to the P channel mosfet!

This is the P channel plots

and here is the N channel mosfet plots

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,991
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How can '741 output current limitation have any effect on Iout??
Also have you check '741 input common-mode range , Aol , Ib, etc.?

11. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Hi Jony130... I don't even know those terms... Input common-mode range, Aol (gain on open loop???), Ib???

Feb 17, 2009
3,991
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13. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Ok, let me look again to the N mosfet version...

About the current, it will be limited because the more we decrease vin the more current we have, but we will only be able to decrease current down to 0V. Then, there is no more possible current. If the load requires more current than what the circuit can deliver, then our current source is limited and short for the load demands!

14. ### Jony130 AAC Fanatic!

Feb 17, 2009
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I really don't understand what you are trying to say, but what all of this has to do with opamp limitations ??

15. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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It's about the circuit's limitations using the UA741 specifically. I don't know what else to say about limitations!

16. ### MrAl Distinguished Member

Jun 17, 2014
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515
Hi,

You can not use the N Mosfet like that because the integral body diode will conduct all the time, meaning you will not have control over the output current. The Spice model you are using is too incomplete to show the correct simulation results.
See if you can find another way to connect the N channel Mosfet so that it works as desired.

A more subtle issue is using the Vcc line as a reference. We dont usually like to do that unless the accuracy is not expected to be that great for the circuit. See if you can figure out why.

When considering the output current limitation (if any) of the 741 (or other op amp) think about what kind of static input impedance the gate of the Mosfet presents to the output of the op amp, then figure out the DC current requirement out of the op amp.

17. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
Ok, the previous problem it's solved, I guess. I just placed the N-MOSFET on the low side at it looks like it's working as supposed. I have another circuit where I need help to interpret it! I'm going to create a new thread!