Extend linearity range of this opamp circuit

Question

Sorry if my question is too naive, I'm new to this. I am trying to design a controlled current source for currents up to about 1A (diagram below). My requirement is that the current depend only on the value of a single voltage source and a single resistance (which I can precisely choose).

schematic

^{simulate this circuit – Schematic created using CircuitLab}

The problem is that the output voltage of the op-amp = gate voltage of MOSFET is becoming equal to the rail voltage, causing saturation at about 300mA itself (i.e. before reaching 1A). Is there a way I can improve the above circuit to prevent the op-amp from saturating and getting a 1A current?

EDIT: I've changed the design following τεκ's answer and swapped the FET and Load. This should ensure that FET's source voltage doesn't go above 5V, but the gate voltage still goes to 14V and saturates the op-amp. Thanks in advance.

@τεκ I haven't chosen an op-amp yet, I'm simulating the circuit on LTSpice. I'll later choose a precision op-amp. — VP06, Mar 07 '18 at 15:48
Thanks, I just realized R(load) matters a lot in my design above, but it shouldn't in @τεκ's answer (swap FET and load), however that still doesn't help. I'm going to edit my question for that. — VP06, Mar 07 '18 at 15:58
If you short out the MOSFET (connect source and drain), what is the load current? — τεκ, Mar 07 '18 at 16:01
@QMrules that's your problem - the load value is too big as per my answer. — Andy aka, Mar 07 '18 at 16:09

Andy aka · Accepted Answer · 2018-03-07T15:08:09.387

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With a 5 ohm sense resistor and a 5 volt control signal you will get 1 amp through the load if the load's value of resistance is low enough. For instance, if the load is 10 ohms then you need 15 volts at the source of the MOSFET to drive 1 amp through 10 ohms plus 5 ohms.

Plus you need another 2 to 10 volts more on the gate to get enough conduction (generalism alert) through the MOSFET. So, given your values, even if the load was zero ohms, you will need a supply voltage that is at least 7 volts and more likely 10 to 15 volts. This assumes the op-amp output is rail-to-rail.

I will note that you have shown V2 as 1 volt and this could only ever push 200 mA through 5 ohms.I assume this is an error on your part as is not putting a ground symbol at the bottom of R1.

edited Mar 07 '18 at 15:08

answered Mar 07 '18 at 14:56

Andy aka

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Thanks, I understood why it's happening.. @τεκ's answer puts the load above the FET, so the source voltage is now at most 5V. This is still not solving the issue, and the gate voltage shoots up rapidly from 4V onwards and op-amp saturates.. I'm unable to see why.. – VP06 Mar 07 '18 at 15:55
Op-amp power supply voltage? Op-amp? Load resistance? Possibly 45 ohms? – Andy aka Mar 07 '18 at 16:04
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You can't push 1 amp through a 50 ohm load if the supply is only 15 volts. – Andy aka Mar 07 '18 at 16:08
I've chosen 15V supply voltage for op-amp and LM741 for simulation. Load resistance would be 50ohm at max. – VP06 Mar 07 '18 at 16:18
Thanks a lot! Now I understand why it's not working. – VP06 Mar 07 '18 at 16:24
[Reasons not to use a 741](https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp/304522#304522) – Andy aka Mar 07 '18 at 16:43

τεκ · Answer 2 · 2018-03-07T16:37:34.347

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Swap the load and MOSFET.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

edited Mar 07 '18 at 16:37

answered Mar 07 '18 at 15:01

τεκ

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It's not working.. The gate voltage again shoots up to 14V, and saturates the op-amp.. – VP06 Mar 07 '18 at 15:51
I think you might be missing a ground on the bottom of R1. – Spehro Pefhany Mar 07 '18 at 15:59
Sorry, that was *my* fault. I've edited the schematic in the question. – VP06 Mar 07 '18 at 16:04

Extend linearity range of this opamp circuit

2 Answers2