I'm using a 3.7V LiPo to power a device that normally runs on 3V (and draws about 100-200mA). I've determined that my device can run properly if I can achieve an output voltage that scales the input by a constant factor of about 0.7 (I could also use a diode to simply drop the voltage rather than scale, but I'd like something less affected by current and temperature). Please note that I do not want a constant output voltage, or I’d just use a regulator. I need to track the voltage curve of the LiPo somewhat.
So the desired transfer function is:
Vout = 0.7 * Vin
I think a differential amplifier circuit of the following configuration should work (see schematic), since it gives:
Vout = Vin * (R2/R1)
(according to this article)
To achieve a scale close to 0.7 and have a low parasitic current (since the battery will always be connected), I'd like to use R1 = 1M and R2 = 680k. You can see a simulation here.
I don't know a whole lot about op-amps in practice, so I have a few questions:
When applying a load, does that inadvertently affect the output voltage, due to current draw from the output of the op-amp? In simulating, it didn't seem to have an effect, which is nice. I'm avoiding a simple voltage divider for the very reason that the load ends up affecting the desired scaling.
Is it practical to use that high of resistances? I typically see ohms in the hundreds, or maybe 10k, but I don't often see circuits using 680k or 1M. Just want to know if that's a smell of some sort. Side note: for a 1000mAh battery, it would take about 30yrs to drain, according to a quick calculation with these resistances.
Is it unwise to use an op-amp in a DC application? I typically see examples with AC circuits.
Are there any blatant bugs in my circuit design that I might be missing?
While I'm not asking for part recommendations, I am considering an LM158 or similar. If there is a good reason to avoid this for my application, letting me know would be much appreciated.
Thanks so much for your help and advice!
I'm using Rload to represent the device being powered. I don't know its exact resistance, but I estimate it to be about 20ohms on average.