Opamp Input Impedance

One of the practical op-amp limitations is that the input impedance finite, though very high compared to discrete transistor amplifiers. For the 741 the input resistance measured to one input with the other grounded is about 2 Megohms.

For FET input devices it is typically 10^12 ohms. In practice this finite impedance is usually not as big a problem as the input bias currents since the use of negative feedback raises the effective input impedance even higher.