Calculating input impedance of ICIS opamp circuit
I believe you have accidentally assumed that \$V_- = 0V\$ when you said


  Since almost all the input current goes through \$R_2\$, voltage at the meeting point of the 3 resistors is given by \$−i_{in}R_2\$.


While it actually should be the voltage drop:

$$V_- - V_x = i_{in}R_2$$

You can still keep the equation

$$V_{out} = -A_{VOL}V_-$$

But the KCL at the crossing point (\$V_x\$) becomes:

$$i_{in} + \frac{V_{out} - V_x}{R_L} = \frac{V_x}{R_1}$$

This results in the expression

$$Z_{in} = \frac{V_-}{i_{in}} = \frac{(R_1 + R_2)R_L + R_1R_2}{R_L + (A_{VOL} + 1)R_1}$$

You can verify this formula by simulating the schematic below. I tried some combinations and I'm fairly certain that the input impedance is correct.

I haven't been able to get to the equation in your book though. I may need more information about their assumptions.







simulate this circuit &ndash; Schematic created using CircuitLab