I have this schematic of a quarter of the internals of an LM339 from this site
I want to know what the purpose of the four current sources are in this circuit, and how might they be implemented?
I know current sources can be implemented as the following;
- op-amps (not useful to this particular circuit or we would end up nesting opamps within opamps within opamps);
- a JFET "constant current diode" (also unlikely, as the LM339 is bipolar);
- a BJT current source, which I believe to be the most likely candidate in this device.
What happens to the circuit if the current sources are not present, or the supply isn't able to source enough current? I've tried simulating the circuit - warning, the simulation runs very slowly and fails when you make adjustments to the input voltages, which is either a bug in the simulator or I don't understand what I'm doing (most likely the latter)
I intend to implement this circuit using discrete components for learning purposes.