# controlling a sinking current with a positive voltage

I have found current mirrors like the basic one below often very useful.

However, recently I needed a circuit where I sink current from a ground node to the negative supply. The current level ( the impedance) needs to be controlled. The challenge is that I need to control this current with a positive control voltage. I was able to solve it with a op-amp and p-channel in typical current sink type of circuit like the conceptual circuit shown below.

The circuit was changed to accommodate the negative supply.
So the circuit below shows a positive supply variant of what I used.

However, I am curious to see if there is a discrete solution, using MOSFET and BJT's, similar to a current mirror kind of solution. To summarize:

Available: positive and negative supply (+-10V)
Control voltage is positive, output of MCU.
Restriction : not allowed to use digital pots and op-amps.
Current range: 100uA to a few mA
Accuracy not important, I am more interested in the concept.

• Does Vin have to be proportional to Iload? If not, this is basically what an NPN transistor does. In the current mirror circuit, the idea is that the left transistor converts the current to a voltage, and the right transistor converts the voltage back to a current! – user253751 Nov 22 '19 at 14:12
• hmm so how would the npn sink current from the ground node? – Navaro Nov 22 '19 at 14:14
• You can't sink current from the ground node without a negative voltage, because current flows from higher to lower voltage and the ground node is by definition 0V. Have I misunderstood something? – user253751 Nov 22 '19 at 14:14
• Yeah, either I did not formulate the question well or you just did not read it correctly. I clearly state that there is a negative supply available – Navaro Nov 22 '19 at 14:19
• Oh I see: the control voltage for the current sink is higher than the supply voltage for the load. – user253751 Nov 22 '19 at 14:23