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In a textbook, it says that the ideal op-amp should exhibit following electrical characteristics and one of them is - **

Infinite input resistance (R) so that almost any signal source can drive it and there is no loading on the preceding stage

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Can someone explain the statement especially why high input resistance is a good thing?

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  • \$\begingroup\$ What would it mean if the input to the op-amp had low input resistance? \$\endgroup\$ – Harry Svensson Nov 25 '17 at 23:34
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Extremely high input resistance mean that changes in the input voltage do not result in significantly changing input current. If your source has an output resistance greater than zero, a given voltage change will be attenuated at the input like any resistive voltage divider.

It's also desirable to have the input bias current very low.

The two requirements are independent- an input with a significant but very constant bias current can still have a high input resistance.

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Consider the conceptual circuit shown below of a thevenin equivalent source driving the equivalent circuit of an amplifier. Rin is the input resistance, A is the voltage gain and Rout is the output resistance.

schematic

simulate this circuit – Schematic created using CircuitLab

Note that the source impedance R2 forms a voltage divider with the input resistance. This means that the higher Rin, is the more of the source voltage will be dropped across it and therefore amplified. The smaller Rin is the more voltage would be dropped across the source impedance, we call this effect loading.

A high input impedance means it will also draw a negligible current, which is one of the golden rules for Op-Amp analysis as mentioned in the answer below.

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High input resistance makes possible use of high output impedance signal sources. For example, feedback circuits with high resistor and low capacitor values. Also, some physical signal sources have intrinsically high impedance, which makes high input impedance of amplifier very desirable.

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Wikipedia's Operational amplifier to the rescue. (Did you check?)

These ideals can be summarized by the two "golden rules":

  1. In a closed loop the output attempts to do whatever is necessary to make the voltage difference between the inputs zero.
  2. The inputs draw no current.

The first rule only applies in the usual case where the op-amp is used in a closed-loop design (negative feedback, where there is a signal path of some sort feeding back from the output to the inverting input). These rules are commonly used as a good first approximation for analyzing or designing op-amp circuits.

None of these ideals can be perfectly realized. A real op-amp may be modeled with non-infinite or non-zero parameters using equivalent resistors and capacitors in the op-amp model. The designer can then include these effects into the overall performance of the final circuit. Some parameters may turn out to have negligible effect on the final design while others represent actual limitations of the final performance that must be evaluated.

The infinite input impedance means, amongst other things, that the op-amp will not significantly load the signal being monitored.

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  • \$\begingroup\$ What you report from Wiki does not call for infinite input impedance which infact does not seem to be mentioned. Once item 1 (voltage difference between inputs zero) is fulfilled any impedance higher than zero ohms is enough for point 2 too. It just comes from Ohm's law since I=V/R=0/R=0 for any R not zero \$\endgroup\$ – carloc Nov 26 '17 at 8:03

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