Which statement describes a practical nonideal property of real op-amps?

• A. Infinite bandwidth
• B. Zero input bias current
• C. Finite gain-bandwidth product
• D. Zero output impedance

Answer: (C)

Real op-amps are not ideal; a practical nonideal property is a finite gain-bandwidth product. The open-loop gain of an op-amp is high but limited, and as signal frequency rises, that gain falls. For many devices, the gain-bandwidth product stays roughly constant, so the product of gain and frequency is about fixed. This means you can’t get both very high gain and wide bandwidth at the same time—the higher the closed-loop gain you use, the smaller the bandwidth you’ll have. For example, if the gain-bandwidth product is about a few megahertz and you set a closed-loop gain of ten, the usable bandwidth is on the order of hundreds of kilohertz. This limitation is a hallmark of real op-amps.

Infinite bandwidth would require the amplifier to maintain gain at all frequencies, which real devices can’t do due to internal capacitances and other parasitics. Real op-amps also have nonzero input bias currents and a nonzero output impedance, which are other practical nonidealities.