What are the properties of an ideal op-amp, and how does it work as a comparator and Schmitt trigger?
Operational amplifier properties and the comparator: the ideal op-amp, open-loop gain, the comparator with and without hysteresis, and the Schmitt trigger.
A focused answer to WJEC A-Level Electronics operational amplifier properties and the comparator, covering the ideal op-amp model, open-loop gain, the inverting and non-inverting comparator, hysteresis, and the Schmitt trigger.
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What this dot point is asking
The operational amplifier is the central analogue building block on the course, and the comparator is its simplest use. WJEC expects you to state the properties of an ideal op-amp, explain open-loop behaviour, describe the comparator with a reference voltage, and explain hysteresis and the Schmitt trigger. Comparator output questions and the noise-immunity argument for a Schmitt trigger are dependable exam earners.
The answer
Properties of the ideal op-amp
Real op-amps fall short (finite gain, slew-rate limits) but the ideal model is accurate enough for the design calculations on this course.
Open-loop behaviour and the comparator
One input is usually held at a fixed reference (from a potential divider or Zener), and the signal is applied to the other. The comparator then signals whenever the input crosses the reference.
The problem with a plain comparator
A plain comparator has a single switching point. If the input is noisy and changes slowly, it crosses the reference repeatedly, and the output chatters between high and low. This false multiple switching is unacceptable in, say, a count or a control output.
The Schmitt trigger and hysteresis
Examples in context
- Example 1. A light-activated switch
- An LDR divider feeds a comparator whose reference sets the light level at which a lamp turns on. Because the op-amp's open-loop gain is so high, the output snaps cleanly between off and on as dusk falls, with no dim intermediate state.
- Example 2. A thermostat with a Schmitt trigger
- A temperature sensor feeding a Schmitt trigger switches a heater on at a low threshold and off at a higher one. The hysteresis stops the heater rapidly cycling on and off around a single temperature, extending the relay's life and giving steadier heating.
- Example 3. Cleaning up a noisy logic input
- A slow, noisy signal arriving on a long cable is passed through a Schmitt-trigger input before the logic. The two thresholds reject the noise and produce a single clean edge, which is why many logic chips have Schmitt-trigger inputs on their clock and reset lines.
Try this
Q1. State two properties of an ideal op-amp and what each means for the input current and the output voltage. [2 marks]
- Cue. Infinite input resistance means it draws no input current; zero output resistance means the output voltage does not sag under load.
Q2. A comparator has its non-inverting input at a reference. The inverting input rises to . State the output (high or low) and why. [2 marks]
- Cue. Output goes low, because the inverting input is now higher than the non-inverting input, so .
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20194 marksAn op-amp comparator has its inverting input held at a reference of and a sensor voltage applied to its non-inverting input. The supply rails are and . State the output for a sensor voltage of and for , and explain why the output saturates.Show worked answer β
A comparator without feedback runs in open loop, where the enormous gain drives the output fully to one rail or the other.
For a sensor voltage of : the non-inverting input () is below the inverting reference (), so the output goes low, near .
For : the non-inverting input is above the reference, so the output goes high, near .
It saturates because the open-loop gain is so large (typically over ) that even a tiny voltage difference between the inputs is amplified beyond the supply rails, so the output can only sit at one rail or the other.
Markers reward the low output below the reference, the high output above it, and the open-loop-gain explanation of saturation.
WJEC Eduqas 20214 marksExplain why a plain comparator may switch repeatedly on a slowly changing noisy input, and how a Schmitt trigger overcomes this.Show worked answer β
A plain comparator switches whenever the input crosses the single reference voltage. If the input is noisy and changing slowly, it crosses the reference many times in quick succession, so the output chatters between high and low.
A Schmitt trigger uses positive feedback to create two different switching thresholds, an upper and a lower, a gap called hysteresis. Once the output switches, the threshold moves away, so noise smaller than the hysteresis gap cannot switch it back.
The input must rise past the upper threshold to switch one way and fall below the lower threshold to switch back, so a single clean transition replaces the chatter.
Markers reward the multiple-crossings cause, the two thresholds (hysteresis) from positive feedback, and the clean switching result.
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Sources & how we know this
- WJEC Eduqas GCE A-level Electronics specification β WJEC Eduqas (2017)