# Extend linearity range of this opamp circuit

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). 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.

• What opamp are you using? – τεκ Mar 7 '18 at 15:34
• @τεκ I haven't chosen an op-amp yet, I'm simulating the circuit on LTSpice. I'll later choose a precision op-amp. – QMrules Mar 7 '18 at 15:48
• Big, big question - what is the resistance of the load? – WhatRoughBeast Mar 7 '18 at 15:55
• 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. – QMrules Mar 7 '18 at 15:58
• @QMrules that's your problem - the load value is too big as per my answer. – Andy aka Mar 7 '18 at 16:09

## 2 Answers

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.

• 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.. – QMrules Mar 7 '18 at 15:55
• Op-amp power supply voltage? Op-amp? Load resistance? Possibly 45 ohms? – Andy aka Mar 7 '18 at 16:04
• You can't push 1 amp through a 50 ohm load if the supply is only 15 volts. – Andy aka Mar 7 '18 at 16:08
• I've chosen 15V supply voltage for op-amp and LM741 for simulation. Load resistance would be 50ohm at max. – QMrules Mar 7 '18 at 16:18
• Thanks a lot! Now I understand why it's not working. – QMrules Mar 7 '18 at 16:24

Swap the load and MOSFET. simulate this circuit – Schematic created using CircuitLab

• It's not working.. The gate voltage again shoots up to 14V, and saturates the op-amp.. – QMrules Mar 7 '18 at 15:51
• I think you might be missing a ground on the bottom of R1. – Spehro Pefhany Mar 7 '18 at 15:59
• Sorry, that was my fault. I've edited the schematic in the question. – QMrules Mar 7 '18 at 16:04