The operational amplifier should be treated as an important system…

The idea

An operational amplifier, or op-amp, is a high-gain differential voltage amplifier supplied as an integrated circuit. Treating it as a system building block means you analyse circuits by connecting standard blocks rather than by examining internal transistors. The ideal model assumes infinite open-loop gain, infinite input impedance, zero output impedance and infinite bandwidth. In a circuit with negative feedback, these assumptions give two working rules: the potential difference between the inverting and non-inverting inputs is effectively zero, and negligible current enters either input. For example, in an inverting amplifier, the input resistor and feedback resistor form a potential divider because the inverting input sits at virtual earth. This lets you predict gain from resistor values and design sensor, comparator and summing circuits reliably.

What earns the marks

• States that an operational amplifier is a differential amplifier with very high open-loop voltage gain, usually supplied as an integrated circuit.
• Explains that treating it as a building block means using standard configurations such as inverting, non-inverting, comparator and summing circuits.
• Applies the ideal op-amp assumptions: infinite open-loop gain, infinite input impedance, zero output impedance and infinite bandwidth.
• Uses the virtual earth or virtual short idea for a negative-feedback circuit, so the inverting input is at approximately the same potential as the non-inverting input.
• States that negligible current flows into either input, so current through the input resistor equals current through the feedback resistor.
• Derives or uses gain relationships from resistor ratios, for example the inverting amplifier gain is the negative ratio of feedback resistance to input resistance.
• Explains that negative feedback stabilises gain and reduces distortion, making the block predictable in a larger system.
• Describes how op-amp blocks can be cascaded or combined, for example a summing amplifier feeding a filter or comparator.

Common mistakes and how to put them right

• Treating the op-amp as a device that creates energy rather than as an amplifier that needs a power supply.

  Put it right by stating that the output cannot exceed the supply rails and that the op-amp shapes a signal using external components.

• Assuming the inputs are always at the same potential in every circuit.

  Put it right by specifying that the virtual short applies only when negative feedback is present and the output is not saturated.

• Using the open-loop gain directly in a negative-feedback design.

  Put it right by using the external resistor ratio, because negative feedback sets the closed-loop gain.

• Thinking that current flows into the input terminals in an ideal op-amp.

  Put it right by applying the ideal assumption of infinite input impedance, so input current is negligible.

Exam tips

• Sketch the op-amp block with its two inputs and one output, then label the feedback path before doing any algebra.
• When deriving a gain, write the virtual-earth condition and the current-balance equation as separate lines so the examiner can follow your reasoning.
• Check whether the circuit has negative feedback; if it does not, do not use the virtual short rule.
• Give final gains as dimensionless ratios or in decibels only if the question asks for decibels, and keep resistor values in the same unit throughout.