When the non-inverting input terminal of an op-amp is equal to that of the inverting input terminal (ideally), this represents the concept of virtual short or virtual ground.
Virtual Short Property:
In an ideal op-amp with negative feedback, the two input terminals appear to be at the same potential even though they are not directly connected. This is called a virtual short.
Key Characteristics:
1. Vid = 0 (Ideally, the voltage difference between inputs is zero)
2. No current flows between the input terminals
3. The inputs are not physically connected but act as if they are
The correct answer is typically: (b) Vid = 0
This property is fundamental to op-amp circuit design. When Vid = 0, the non-inverting and inverting inputs are at the same potential, allowing us to analyze circuits using this virtual short assumption. This makes circuit analysis much simpler and is a key principle in negative feedback amplifier design.