operational-amplifier
I'm wondering if we use batteries instead of stable power supply for the opamp power rails what would be the consequences.
If we use an opamp for precision application would using batteries a bad idea since the battery voltage drifts might effect the opamp output directly?
As shown you have a perpetuum mobile, none of which work. You are using the opamp as the energy source for itself and that dog won't hunt.
You can do what you want if you use external switches driven by the opamp - 2 x bipolar transistors or MOSFETS. The opamp then supplies only the drive energy to turn the "switches" on and off.
An opamp version of the circuit below would do what you want. The opamp (here = 555) can be powered from the output - initially its Vdd is two diode drops below Vcc_in but this rises as the circuit bootstraps its own supply.
Circuit from here
An alternative is to build a simple boost converter - oscillator (opamp or whatever) inductor, jellybean BJT transistor, a few resistors, diode, caps.Zener clamp output to regulate. Choose inductor size, frequency to suit desired power level.
Various examples here
including circuit below from here - good design notes on that page.
No. There has to be some reference. IF you find a standalone current sink part, it has the reference built in.
The LM10 is a good choice for your circuit: it has a low-voltage reference built-in, and it will work over a wide supply range.
A modern alternative to LM10 is TL103 or TSM102.
Best Answer
Assuming you're using the opamp in a feedback configuration circuit, which you probably do because you need some precision.
Also assuming that the voltages at inputs and output of the opamp do not exceed what the opamp can handle at the given battery supply voltage. I'm thinking (common mode) input voltage range and output voltage range with a certain load connected.
Then the battery voltage drift should be irrelevant.
In a properly designed circuit the gain (and offset) of the opamp will then determine the output voltage, not the supply voltage.
So yes, you can add a regulator but I don't think you have any reason to do so. It will only use up some of the battery voltage (and power) while perhaps not improving anything.
What you might need is a "low battery" detection and indicator (for example, a flashing LED) to indicate that the battery voltage has become too low and accuracy might be compromised.