While looking at this simulation, as they say no current flows at the non inverting terminal of the op-amp, but looking at the differential stage of the operational amplifier shouldn't there be some current from ground to -15V point THROUGH THE NON INVERTING INPUT?
Secondly, they define common mode impedance as the impedance between the input terminal and ground point.
Now the current producing by the action of the AC voltage source has to travel from -VEE to ground. So the impedance should be of this path, then how impedance between ground and terminal is defined as the common mode impedance?
Best Answer
There is a small bias current that flows into the input, less than 1% of half of Iem at balance, for transistors with hFE > 100.
Common mode input impedance will be very high because that bias current does not change much with small changes in input CM voltage.
In many cases you can ignore both input bias current and input CM impedance when modern op-amps are used with resistors in the few K ohm range, but it doesn’t hurt to run the numbers and establish that for a fact.