# Stable current source with discrete components

The purpose of this is to compare and learn different circuit topologies, not use of the shelf solutions which obviously provide cheaper, simpler and better results. Considering this requirement (no use of ICs, not even a matched pair), the parameter ranges given below and that the load should be connected to the voltage source:

Can we get better temperature stability and input voltage regulation by an order of magnitude (e.g. < 100uA)? To focus on the main goal, let's disregard the temperature coefficient of the metal film resistors.

The load regulation of the circuit on the left is already good enough. I've used different BJTs on the right do avoid depending on any form of matching between the discrete components.

The parameters are: 10 <= Vin <= 30, 0 <= Temp. (C) <= 50, 0 <= RL <= 1200, Isource ~ 5mA

Manually adjusting the central value with a trimpot is fine, let's not add this concern either.

Note: the ranges below are simulated by fixing all but one parameter.

• Please be clearer about your question - at the moment "Can we get better temperature stability and input voltage regulation by an order of magnitude (e.g. < 100uA)?" doesn't really mean much. Also, what does this mean: "Manually adjusting the central value with a trimpot is fine"? Commented Jun 27, 2019 at 14:12
• @Andyaka by voltage regulation I mean: if the input varies from 10 to 30, other parameters fixed, is there a topology that provides less than 100uA variation at the source? By temperature stability I mean: if the temperature varies from 0 to 50C, with the other parameters fixed, is there a topology that provides less than 100uA variation at the source? Commented Jun 27, 2019 at 14:15
• Yes, I think you can improve the performance by using more complex circuits. For example: separate voltage reference (bandgap like circuit), then use an opamp like circuit (gm stage) to copy that reference voltage across a resistor. Then the circuit would be somewhat similar to what would be on an IC. To avoid having to use matched transistor pairs we can use emitter resistors to match non-matched transistors better. Commented Jun 27, 2019 at 14:20
• Realize that a suitable "opamp" (actually gm cell) can be made with as few as 4 to 6 transistors and some resistors. There's no need to make a "proper" opamp with multiple stages. see: allaboutcircuits.com/textbook/experiments/chpt-5/simple-op-amp Commented Jun 27, 2019 at 14:31
• One topology has 3 components: transistor with grounded emitter, resistor1 from VDD to collector, resistor from collector to base. Commented Jun 27, 2019 at 15:54