Question

Hello, i cant see the logic of why in OPAMP in common mode we have virtual OPEN CIRCUIT,and in differential mode we have SHORT circuit?
Thanks.

Answer 1

In common mode of operation, we apply same signal both in phase and amplitude at the both ends of input ( at negative and positive ) terminals. The effective input voltage is the difference of voltage applied at positive terminal (V_{​​​​​​p}​​​​​) and voltage applied at negative terminal (V_{​​​​​​n}​​​). As both are equal, the effective input voltage will be zero. This leads to no current flow as both voltages are at same potential and leads to virtual OPEN CIRCUIT.

In differential mode, an exact opposite signal of the signal which is applied at positive terminal is given as input to negative terminal. Thus, here V_​​​n = - V_{​​​​​​p}​​​​​. Therefore, the effective voltage will be twice of V_{​​​​​​p}​​​​​. This is the mode where we can get maximum potential difference between positive and negative terminals thereby increasing the current flow. This leads to short circuit in differential mode.

Thank you.