Instrumentational Amplifier

[Turbo46]

Hi<p>Im currently trying to alter the input impedance of an operational
amplifier, but cant find the right page on google. I think impedance
means, there must be an RC circuit in series with the amplifier? Does
anybody have any ideas?

[rosborne]

The impedance of an op-amp closed loop is very large. Even open loop I think you will see impedances in the range 20K and IIRC the impedance closed loop is about the open loop gain (A) times the open loop impedance or 20k * 70k or thereabouts. These values are determined by the designers of the IC and the physical geometry of the IC not by the applications engineer.
-Rick

[Ron H]

There are no ideal op amps in the real world, meaning that all of them have finite input impedance (usually parallel R-C). Do you have a particular one in mind? What problem is the input impedance causing?

[Dean Huster]

If you're talking about the input impedance of the entire circuit, it's easy to alter. If you're working with a standard inverting amplifier circuit, the input impedance is the same as the value of the input resistor. If working with a standard non-inverting amplifier circuit, simply hang a resistor from the input of the op amp to ground with a value that's equal to the input impedance you desire. All resistances used in these standard op amp circuits must have values that are at least a factor of 4 to 10 lower in value that the intrinsic input impedance of the op amp itself. If you work with FET input op amps, the value of these resistors (input resistance and feedback resistance) almost becomes a moot point.<p>Dean