I see, that it will have
input impendance =
Gain = 1 + R4/R3 = 3.136. So its easy to find
R3, and vice versa. But selecting both actual values R3 and R4 is the impossible task.
What is the difference between
R4=4.7k; R3=2.2k and
R4=47k; R3=22k? AFAIK, no one does ever questioned this topic in details, so I think this is a good question.
How to properly select order of magnitude for those two resistors in feedback circuit?
Upd. I know basic theory how OA works.
- OA is a comparator, sticking its output to
V-, dependent on "if
In-or less", accordingly.
- These two
R3+R4makes a voltage divider between output and a ground, for negative input terminal. And output is is working on R3+R4 circuit, establishing some level at negative.
- First plus second facts leads to behaviour, that OP amp is trying to euqalize voltage at its inputs: to make" negative terminal's voltage equal to positive's.
- And divider's ratio determines gain (how much more voltage should OA pop out of its output to equalize negative input to positive)
- Inputs does not virtually draw any current to OA, any output current goes from power rails.
What i dont understand is reasoning while choosing magnitude of R3+R4 sum. According to ohms law, total divider resistance will determine value of unwanted current(and power draw) that goes to ground. To save battery I better put there higher resistors, like 2M2 and 4M7, is that correct? Or maybe should I set this current exactly to 3.1415 mA? Or 2.71828mA? I know that humbacker coil DC resistance is 2-3 kOhms, and line level input voltage (what i want to get at output) at next stage ranges from 0.3v for consumer to 2v for profesional hardware. But nowhere current is mentioned: at next stage (the mixer) current is also not coming into op amp.