Operational Amplifiers and Gain

The chip that makes signals bigger

An operational amplifier, or op-amp, is a tiny building block with one main job: it makes an electrical signal bigger. How much bigger is set by its voltage gain, which is simply the output voltage divided by the input voltage. A gain of 10 means the output signal is ten times the size of the input. On its own an op-amp has an enormous gain, so it is tamed by feeding some of the output back to the input, called feedback, which sets a sensible, chosen gain using two resistors. Two things matter as well as the number. Push the gain too high and the range of frequencies the amplifier handles cleanly, its bandwidth, shrinks. And if the amplified signal tries to swing beyond the supply voltage, its peaks are flattened off, which is called clipping. This module covers the gain rules, the two main circuits, the summing amplifier and these two limits.

Words for the amplifier

Learn what each of these means before you use it.

Inverting against non-inverting

There are two main op-amp amplifier circuits, and they differ in their gain rule and in what they do to the signal.

How to work out a gain

A gain question is nearly always the same shape, so learn the routine. First decide which circuit you have, inverting or non-inverting, because that decides the rule. Next read the two resistor values from the circuit. Then apply the rule: for the inverting circuit, divide the feedback resistor by the input resistor; for the non-inverting circuit, do the same division and add one. To go the other way and choose resistors for a target gain, pick a sensible feedback resistor first, then work out the input resistor the rule requires. Always check the answer is believable: a gain below one from a non-inverting circuit, or a gain with no units, means something has gone wrong. Name the circuit, read the resistors, apply the rule.

Match the circuit to its behaviour

Which gain?

A non-inverting amplifier has a feedback resistor of 90 kilohms and a second resistor of 10 kilohms. What is its voltage gain?

• 10
• 9
• about 0.11
• 100

Order the amplifier system

Put the parts of a typical amplifier system in the order a signal passes through them.

• The source produces a small signal
• The preamplifier boosts that weak signal first
• The mixer combines the signals together
• The power amplifier drives a large output
• The loudspeaker turns it into sound

Spot the true op-amp facts

Tap the TWO statements that are true.

• Voltage gain is the output voltage divided by the input voltage
• An inverting amplifier turns the signal upside down
• Raising the gain always widens the bandwidth
• Clipping makes a signal larger than the supply voltage with no distortion

Work out the gain

An inverting amplifier has a feedback resistor of 100 kilohms and an input resistor of 20 kilohms. Work out the size of its voltage gain, ignoring the minus sign. Divide the feedback resistor by the input resistor.

Complete the op-amp facts

The output voltage divided by the input voltage gives the voltage _____. Feeding part of the output back to the input to set a controlled gain is called _____. An amplifier that keeps the signal the same way up is the _____ type. An amplifier that adds several input signals into one output is the _____ amplifier.

When the signal clips

Imagine an op-amp running from a supply that can push its output no higher than ten volts and no lower than minus ten volts. Feed in a small, smooth wave and set a modest gain, and the output is a bigger copy of the same smooth shape, sitting comfortably inside those limits. Now turn the gain up too far. The amplifier tries to produce a wave whose peaks would reach fifteen volts, but it simply cannot go past ten. So the tops and bottoms of the wave are sliced flat at the supply limits, and the smooth curve becomes a shape with flat edges. That flattening is clipping, and it is heard as harsh distortion. The lesson for design is that gain is not free: a gain that looks fine on paper can wreck the signal if it drives the output into the supply rails.

Build the gain rule

Choose the words that complete the rule for an inverting amplifier.

Choose the circuit

Read each case and choose the best answer, then think about why.

• A design needs a voltage gain of exactly 1 and must keep the signal the same way up. Which choice makes sense?
• An engineer keeps raising the gain and notices the amplifier now handles a much narrower range of frequencies. What has happened?
• The output of an amplifier shows a wave with flattened tops and bottoms. What is the most likely cause?

Gain and bandwidth

Select the TWO statements that are true about an operational amplifier.

• An inverting amplifier gain is the feedback resistor divided by the input resistor
• Increasing the gain tends to reduce the bandwidth
• Clipping happens because the gain is set too low
• A non-inverting amplifier can have a gain below one

Explain operational amplifiers

A friend is revising Electronics and does not understand what an op-amp does or how its gain is set. Explain it using the ideas from this module.

• Explain what an operational amplifier does and what voltage gain means
• Explain the difference between the inverting and non-inverting circuits
• Give the gain rule for one of the two circuits, in words
• Explain what a summing amplifier is for
• Finish by explaining what clipping is and why too much gain causes it