Question

Why real op-amps have input resistance with so different values ? Is there any relation with their internal technology ?

Answer 1

Hi nick67 :)

How are you?

Will you please elaborate your question dear?

Answer 2

Hi Mustafa, I'm fine thanks and how about you ?
As you know you may have input resistance values in real op-amps that cover a range going from MOhm to GOhm. Why that ?

Answer 3

I am also fine dear..

Oh I see.. OK now I got.

You know opamp uses differential amplifiers in them by using transistors?

Answer 4

yes I know, but why the difference in Rin value ?

Answer 5

Hi nick67 are you there?

Sorry for long time taken.

Answer 6

You question is actually about why opamp has high input impedance?

Answer 7

as you know opamp is a voltage gain device.

So when you get maximum voltage on opamp it means opamp has high input impedance that's why you get voltage from source without loosing it. Right?

Answer 8

Hi Mustafa, the question is why commercial opamps may have so different input impedance values: you may find one type with less than a megaOhm and another type with more than a gigaOhm (you can check on any opamps distributor website)

Answer 9

yes dear I have understood you question.

But I think you are confuse at your question.

Look it is not meant that they have different input impedance because different manufacturer make different opamps. So there are many opamp who has more than 10^13 ohm.

Answer 10

If you ask question as why they have high input impedance then this question makes sense.

But you are asking why they have different impedances. So it depends on what manufacturer made. The high input impedance the more voltage gain.

If you are a manufacturer then I am sure you will try to make the opamp with high input impedance and low output impedance.

Is that clear now to you my dear?

Answer 11

Ok, let's take an example so that may be my question is more clear: if we look at the UA741 from TI we have a typical Ri of 2 megaOhm while if we look at the LF411 we have a typical Ri of about 1000 gigaOhm. Could you explain why that ? Is that related to the kind of technology used inside the opamps ?

Answer 12

Yes, actually opamps are made of transistors. So if transistors have high impedance then so opamps.

And now you will ask why transistors have high input impedance so it is manufactured.

Answer 13

Sorry Mustafa, really I'm trying to obtain the answer I already know :-)
Do you think all transistors have same technology ?

Answer 14

Yes they all are made from same technology, the technology that was discovered by Bradly and Schockley at Bell laboratories (generally). But transistors have different characteristics.

Answer 15

I haven't learned how op amps are made, but I know they are made of transistors. If the transistors are MOS, then their input resistance will be much larger than BJTs. Maybe it is a reason. And, even though transistors are same type(MOS or BJT), I don't think they have same resistance.

Answer 16

Hi gregfeynman, I totally agree with you: the main difference is with type of transistors.
If you are selecting your opamp and you need a big input resistance you can choose an opamp with JFET or MOSFET inside. In this case opamp inputs really are the gates of the internal unipolar transistors of the first differential stage and, as you know, they have negligible current values on the gates (for this reason they are called voltage controlled devices). If the opamp has bipolar transistors in the first stage then opamp inputs really are the bases of the internal BJTs and they need much more current (for this reason they are called current controlled devices).