the relation between virtual short and virtual ground in an op-amp
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Answer:
Applying the concept of a virtual short can greatly simplify the analysis of an op-amp feedback amplifier. Since the non-inverting terminal is grounded (v+ = 0), the virtual short means that the non-inverting terminal is likewise at zero potential (v- = 0)! We refer to this condition as a virtual ground.
Explanation:
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Answer:
What is the virtual short of an op-amp?
Virtual short is a circuit node with a steady voltage that is not directly connected to a voltage reference such as ground or a DC voltage source.
The concept of virtual short is commonly used for op-amps. Figure 1 shows a negative feedback amplifier using an op-amp.
Suppose that it is the ideal op-amp, which has:
infinite open-loop gain (AV)
infinite input impedance
zero output impedance
Because the input impedance is infinite, all of the current (i1)
flowing through R1 flows through R2.
i1 ＝ (Vi – V−) / R1 = (V− - Vo) / R2 (1)
The output voltage of the op-amp is given by the equation:
Vo = AV(V+ - V−) (2)
From Equation 1 and Equation 2, V+ is calculated as follows:
V+ ＝ {1 + (R1 + R2) / (AV * R1)} * V− - R2 * Vi / (AV * R1)
Because AV is infinite, we obtain V＋ = V−.
Hence, the voltage at the negative inverting input (V−) is equal to
the voltage at the grounded noninverting input (V+).
Roughly speaking, in the event of there being a large difference in voltage between the two inputs, the output voltage becomes abnormally large because an op-amp has a very large open-loop gain (AV).
Even real op-amps have an open-loop gain (AV) around 100 dB (105). This means that, in order to obtain a typical output voltage of several volts, the two inputs must have almost the same voltage (with a difference less than several 10–5 volts).