So I was reading the datasheet of TI's NE5532 and I noticed something that I don't quite understand:
Chapter 7.5 (page 5) says that the NE5532 has an input resistance (I guess input impedance?) of 30kOhm to 300kOhm. I don't really understand the internals of opamps but my initial thought was that this input resistance seems rather low for an opamp (most ones I've seen are 1MOhm - 1GOhm). From what I know opamps usually have a relatively high input impedance so that they do not affect the input signal by very much.
As far as I understand 188.8.131.52 (page 9) then mentions that this particular circuit used 36kOhm (+-2%) resistors (R1-R4).
I have to questions which relate to that:
- Doesn't the low input resistance of the NE5532 totally screw up the "voltage dividers" that are used to create the inverter and the bias voltage with these particular resistor values? The opamp's input resistance isn't even 10 times as large as the input impedance.
- What exactly does input resistance mean? I understand how resistor networks and Kirchhoff's law work, but if I look at the opamp's schematic in 8.2 I don't really understand anything: What is this (possibly virtual) input resistor connected to? How does it affect my circuit and how can circuits like above work properly with such low input resistance? If the opamp would have a an input resistance, which is a magnitude higher than the impedance of the input signal, I wouldn't worry, but like that I totally don't know how things work.