Electrical – Current source with low-side LOAD

current-sourcemosfetoperational-amplifier

Why does this work?

schematic

simulate this circuit – Schematic created using CircuitLab

But this doesn't.

schematic

simulate this circuit

In the second schematic, V2 is adjustable.

Is there a better (not very complicated) way to make an adjustable current source where the LOAD is between Source (transistor) and GND (not with the sense resistor connected to GND and the LOAD on the high-side)?

Does the op-amp have to be powered with a dual power supply?

Best Answer

Start by assuming you have an ideal opamp. The second circuit is going to try to pull the non-inverting input down to +5V. This requires that 5A flows through the 1-Ohm R2 (a 10V supply, a 5V drop across R2). However, this same current would also have to flow through the 100 Ohm load, which is not possible with the 10V supply (5A * 100 Ohm = 500V).

The circuit would work if you replaced the +5V reference with one much closer to +10V.