1. Why does op amp generally work in reverse proportion?
The significant difference between the reserve input method and the phase input method is: Reserve input method, due to the termination in phase with a balanced resistor to ground, and there is no current in this resistance , so the phase terminal is approximately equal to the ground potential, known as "virtual ground". While the reserve terminal and the phase terminal are very close to each other, so there is "virtual ground" on the reserve terminal. The benefit of a virtual ground is that there is no common-mode input signal, and even if the opamp's CMRR is not high, there is no common-mode output guaranteed. There is no "virtual ground" in the phase input connection . When using single-ended input signal, it will have a common-mode input signal. Even with high common-mode rejection ratio op amp, there will still be common-mode output. Therefore, reverse input connection is the general use .
2, Even if some op amps do not enter any voltage, there will be output, and the output is not small, so VCC / 2 is often considered as a reference voltage . There is output when op amp is of any input .This situation is due to the design of op amp itself caused by asymmetric structure, That is, we have often said that the input offset voltage Vos, which is a very important operational amplifier performance parameters. Op amp commonly used VCC / 2 as a reference voltage because the op amp is in a single power supply, at this time the actual reference op amp is VCC / 2. So a VCC / 2 DC bias is often set in the op amp positive terminal. When the positive and negative dual power supply ,it is often refered . There should be paid attention to many things when considering the choice of op amp
, in the not very strict conditions, op amp's operating voltage, output current, power consumption, gain bandwidth product, price and so on should be considered. Of course, when operational amplifiers are used under special conditions, different factors need to be considered.