For the circuit,
(a) Derive an expression for $Vout/Vin$ if $Ri = ?$, $R0 = 0$ and $A$ is finite.
(b) What value of open loop gain A is required for the closed loop gain to be within 1 percent of its ideal value ?
The + input is at zero potential. $V+=0$. The circuit amplifies until $V- =0$ (the – input is virtual ground).The – input does not draw current (ideal amplifier). It means both the resistances 1 K and 100 K have the same current.
$I*100 K= Vout$
$I*1 K = Vin$
$A(closed) =Vout/Vin = 100/1 =100 (1)$
The relation between open and closed gain is
A(closed) = A(open) / (1+ A(open)*Feedback)]