Why do we want infinitely high input impedance in an ideal opamp?

Why do we want infinitely high input impedance in an ideal opamp?

• A. The higher the impedance, the lower the current drawn from the source
• B. The higher the impedance, the higher the current drawn from the source
• C. Impedance has no effect on current drawn
• D. Infinite impedance causes instability

Answer: (A)

The main idea is that a source should be loaded as little as possible by the amplifier. An ideal op-amp is modeled as having infinite input impedance, so no current flows into its input. That means the signal source delivers almost no current to the amplifier, preserving the original signal voltage and avoiding loading effects that would alter the signal you’re trying to amplify. In other words, as input impedance grows, the current drawn from the source drops toward zero, and with infinite impedance it effectively becomes zero. Real devices come close to this idea, with very large input impedance and only tiny input bias currents, so the loading is negligible. If the input impedance were finite, the source would have to supply current into the input, which would interact with the source impedance and could attenuate or distort the signal. The notion that input impedance has no effect on current, or that infinite impedance would cause instability, doesn’t align with how loading and signal transfer work in amplifier circuits.

The main idea is that a source should be loaded as little as possible by the amplifier. An ideal op-amp is modeled as having infinite input impedance, so no current flows into its input. That means the signal source delivers almost no current to the amplifier, preserving the original signal voltage and avoiding loading effects that would alter the signal you’re trying to amplify. In other words, as input impedance grows, the current drawn from the source drops toward zero, and with infinite impedance it effectively becomes zero.

Real devices come close to this idea, with very large input impedance and only tiny input bias currents, so the loading is negligible. If the input impedance were finite, the source would have to supply current into the input, which would interact with the source impedance and could attenuate or distort the signal. The notion that input impedance has no effect on current, or that infinite impedance would cause instability, doesn’t align with how loading and signal transfer work in amplifier circuits.