Exercise 6.3 An opamp configuration with feedback to both inputs

Given is the circuit below; the op-amp can be assumed to be ideal.

a)

Derive an expression for the input impedance $z_{\mathit{in}}$. Please note that this is the impedance seen into the circuit from the dotted line in the circuit schematic. For this question, assume that the circuit is stable.

b)

Give a graphical representation of the input impedance as a function of frequency, assuming that $Z$ is a capacitor. If any values are missing, then assume some reasonable value and motivate your choices.

c)

Derive for which source impedances $R_G$ (values, an interval or …) the circuit operates stable for a resistive $Z$.