EC.21)For a stable operational amplifier-feedback network, the ideal closed loop gain can be calculated if which of the following conditions is true:(a)A negligible differential voltage applied at input terminals, produces a significant output voltage
(b)The current required at either of the op-amp terminals is negligible
(c)both (a) and (b) are true
(d) only (a) is true
Answer is (c)
The above two conditions are used to describe the amplifier working characteristics.
Consider for the first condition ,
that is the opamp basically amplifies the input voltage
For the non-inverting amplifier
$V_a\bumpeq \frac{Z_1}{Z_1+Z_2} V_o$
considering ideal conditions $V_a = V_i, \therefore \frac{V_o}{V_i} = \frac{Z_1 + Z_2}{Z_1}$
For the second condition , consider inverting amplifier - By Kirchoff's current law $I_a +I_b \bumpeq 0$
$I_a \bumpeq \frac{V_i }{Z_1} ; I_b \bumpeq \frac{V_o}{Z_2} $
Combining above, $\frac{V_o}{V_i} = -\frac{Z_2}{Z_1} $
