Engineering Finding out the input impedance in a negative feedback op-amp

[alan123hk]

Just for the sake of accuracy - you are not correct.
The input resistance of a non-inv. opamp configuration will be, of course, increase due to negative feedback. In your reasoning you are using "infinity" which is not realistic.
It is another question if the formal calculation of the enlargement of the input impedance is important - if compared with real conditions (see my separate answer, post#28). But it is simply false to state that "negative feedback does not effect its input impedance".

Of course, infinite impedance does not actually exist, I just tried to use a mathematical model, so I don't think this is incorrect, but it can be said that it does not accurately describe the actual situation. For example, we cannot say that the calculation results obtained by using an ideal voltage source, an ideal current source, or an ideal transformer are incorrect because they are unrealistic. If they are incorrect, then it seems that we should not apply them extensively in textbooks. I just want to use it to describe a situation, that is, a thing is already big, and if we add it, the actual effect will not change.

As for the benefits of increasing the input impedance, this is of course subjective, which will vary according to different people's opinions and application conditions.

 

[alan123hk]

The following nanopower operational amplifier is an interesting example.

https://www.ti.com/lit/ds/symlink/tlv8802.pdf?ts=1633508956509&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLMP91002

Its specifications do not specify input resistance, so users can only assume that its input resistance is close to infinity during the design process.

Of course, it also has input bias current and input capacitance. The bias current is extremely low, about 100fA. The input differential mode capacitance and common mode capacitance are 7pf and 3pf respectively, and these capacitive reactances are almost negligible in the case of extremely low frequency operation.

 

[DaveE]

Its specifications do not specify input resistance, so users can only assume that its input resistance is close to infinity during the design process.

No! The designer should assume that the input impedance hasn't been characterized. They should assume that:

1) It isn't a required screening in manufacturing.

or

2) The marketing people didn't think it was worth the effort to do the characterization and to publish it in the "typical" section.

There is a difference between unknown, unpublished, zero, and infinite. There are many, many times in my career that I had to deal with uncharacterized or poorly characterized performance specs that mattered to me but not so much to the mass market. This is why the BOM at EVERY reputable manufacturer has approved suppliers for each individual part. Just because the parts have the same (or very similar) part numbers or data sheets, doesn't mean they are the same.

As an aside, one of the things they never teach EEs in school is how data sheets are made, what they mean, and how to use them. They are primarily a marketing document.