Finite-gain amplifiers with composite operational amplifiers: input impedance and stability limitations

An investigation is conducted of the input impedance of non-inverting amplifiers realized by the four composite OAs (operational amplifiers) namely, C20A-1, C20A-2, C20A-3, and C20A-4. Also, the author reexamines the necessary and sufficient conditions of stability. In this regard he adopts the OA two-pole model. It is shown that for the C20A-1 and C20A-4 realizations the input impedance becomes finite and dependent on GB (gain-bandwidth product). Thus, at relatively high frequency this impedance must be taken into consideration. The condition of stability for the four amplifier realizations (inverting and non-inverting) is derived