Amplifiers and Operational Amplifiers: O-Level Electronics module overview of voltage gain, the transistor amplifier, op-amp amplifier configurations and the comparator

What this module is about

Amplifiers and Operational Amplifiers is about making a small signal larger in a controlled way. It begins with the definition of voltage gain and how to express it in decibels, then shows how a single transistor can amplify a signal once it is correctly biased. The heart of the module is the operational amplifier: with feedback resistors it gives a precise, predictable gain through the inverting and non-inverting configurations, and without feedback its enormous gain makes it a comparator that gives a clean high or low decision. These circuits combine the transistor and sensor ideas from earlier modules into useful signal-processing blocks.

This overview ties the module together and links to every dot point, each with its own worked answers and practice questions. See the full set at /sg-o-level/electronics/syllabus/amplifiers-and-op-amps.

Voltage gain and decibels

Voltage gain and decibels defines voltage gain as the ratio of output voltage to input voltage, $A_v = V_\text{out}/V_\text{in}$, a pure number. It also shows how to express gain in decibels using $\text{gain (dB)} = 20\log_{10}(V_\text{out}/V_\text{in})$, so a gain of $10$ is $20\ \text{dB}$ and a gain of $100$ is $40\ \text{dB}$.

The transistor as an amplifier

The transistor as an amplifier explains how a small base signal controls a large collector current, so a small input voltage produces a large output voltage swing across the collector load. It stresses the need for biasing: the transistor must be set to a steady operating point so the whole signal stays in its active region and is not clipped.

Inverting and non-inverting amplifiers

Inverting and non-inverting amplifiers gives the op-amp gain equations. The inverting amplifier has gain $A_v = -R_f/R_\text{in}$, and the non-inverting amplifier has gain $A_v = 1 + R_f/R_1$. Both rely on negative feedback, which fixes the gain from the resistor values and makes it stable.

The op-amp comparator

The op-amp comparator uses the op-amp with no feedback, so its very high open-loop gain drives the output fully high or fully low depending on which input is larger. Feeding a sensor divider into one input and a reference voltage into the other gives a clean two-state output that switches at a chosen threshold, linking directly to the sensor circuits of the earlier module.

A worked op-amp gain example

How this module is examined

• Match the equation to the circuit. Decide whether the amplifier is inverting or non-inverting before choosing $A_v = -R_f/R_\text{in}$ or $A_v = 1 + R_f/R_1$.
• Carry the decibel conversion. Remember voltage gain in decibels uses the factor $20$, not $10$, in $20\log_{10}(V_\text{out}/V_\text{in})$.
• Explain feedback versus comparator. State that negative feedback sets a stable gain, while no feedback makes the op-amp swing fully high or low as a comparator.

Check your knowledge

Work through the quiz for this module to test voltage gain and decibels, transistor biasing, the op-amp gain equations and the comparator, then review the worked explanations.