Common mode impedance in operational amplifier
There is a small bias current that flows into the input, less than 1% of half of Iem at balance, for transistors with hFE &gt; 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.