# Does this voltage to current amplifier setup work?

I'm trying to drive a current-controlled load circuit with a higher current than in the initial circuit. As far as I'm aware for the following voltage to current amplifier the load doesn't affect the current through R2. Does this mean the load can be a BJT controlled circuit with its own voltage supply without affecting the current through R2?

simulate this circuit – Schematic created using CircuitLab

• Besides the fixes provided in the answer, u have to make sure that the BJT Q1 operates in "Linear" condition which means Vce > 0.5v , otherwise it wouldn't work as expected. Commented Jun 26, 2020 at 0:31

It would work if you fixed it: -

• The red box is connecting the power supply to ground correctly
• The purple box is fixing the supply polarity
• The blue box is a warning to take care you have the right power rails on the op-amp

Does this mean the load can be a BJT controlled circuit with its own voltage supply without affecting the current through R2?

Not with this circuit but if you want a ground referenced load then using a PNP transistor and R2 connected to the positive supply can achieve constant current through the load - see circuit 4 in this answer to see what I mean. Also this answer and this answer and this answer for the same example and some theory.

• Thanks a lot! Where is the extra current through the transistor going if not through R2 then? Commented Jun 25, 2020 at 19:36
• Current travels from the positive terminal of V1 through the load resistor (R1) and passes from collector to emitter and through to R2 (the sense resistor) and then back to the supply negative of V1. Commented Jun 25, 2020 at 20:58
• The TL0xx series of Jfet op-amps often need a bipolar supply to be stable. One way to cheat is to connect a 10K resistor from op-amp output to its Vcc pin. This creates internal offsets that make the op-amp stable.
– user105652
Commented Jun 25, 2020 at 22:57