Operation and characteristics of an ideal operational amplifier: power…

The idea

An ideal operational amplifier is a differential voltage amplifier with two inputs, an output, and power supply connections. The inverting input is marked with a minus sign and the non-inverting input with a plus sign. The output voltage is proportional to the difference between the input voltages, multiplied by the open-loop gain. For an ideal op-amp the open-loop gain is infinite, so even an infinitesimal difference between the inputs would drive the output to saturation; the input resistance is also infinite, so no current flows into either input. Power supply connections provide the positive and negative rails that limit the output voltage. These ideal properties simplify circuit analysis, especially when negative feedback is applied.

What earns the marks

• Identify the inverting input, non-inverting input, output, and the positive and negative power supply connections on an op-amp symbol.
• State that the output voltage is proportional to the potential difference between the two inputs, with the proportionality constant being the open-loop gain.
• Explain that infinite open-loop gain means any finite output is produced by a vanishingly small input potential difference, so the inputs are at almost the same potential when negative feedback is used.
• Explain that infinite input resistance means no current flows into either input terminal, so the op-amp draws no input current.
• Describe the role of the power supply connections in setting the maximum positive and negative output voltages available from the op-amp.

Common mistakes and how to put them right

• Assuming current flows into the op-amp inputs.

  Put it right: With infinite input resistance the ideal input current is zero, so the correct statement is that no current enters the inputs.

• Confusing the inverting and non-inverting inputs.

  Put it right: The inverting input is marked with a minus sign and reverses the sign of the output relative to that input.

• Believing the output can exceed the power supply rails.

  Put it right: The output saturates at values set by the positive and negative supply voltages.

Exam tips

• Label the op-amp symbol clearly with the inverting input, non-inverting input, output, and supply rails when drawing or annotating a diagram.
• Use the ideal assumptions of infinite open-loop gain and infinite input resistance when analysing circuits with negative feedback.
• Remember that the power supply connections determine the saturation limits, so quote them when discussing output voltage limits.