In an open loop op-amp configuration, the voltage drop across the source resistance of a differential amplifier is determined by the input bias current and the source resistance value.
When an op-amp is connected in an open loop configuration, the input impedance is very high (typically in the range of mega-ohms). The main current that flows through the source resistance is the input bias current.
Key points:
1. The input bias current is typically in the range of nanoamperes to picoamperes
2. The voltage drop across the source resistance = Input Bias Current x Source Resistance
3. V = IB x Rs
For a differential amplifier in open loop mode, the source resistance should be low to minimize offset voltage errors. Therefore, the voltage drop across the source resistance is kept minimal but is non-zero, making the answer usually in the range of a small value.
Based on op-amp theory, the correct answer is typically: (a) Zero or close to zero, since the input impedance of an op-amp is extremely high and negligible current flows through the source resistance in the ideal case.