ELEC4602 Chap.5 Compensation, Comparators and Sampling
Compensation, Comparators and Sampling
Week 5 links amplifier stability to decision circuits and sampled signals. Compensation deliberately reshapes dynamic behaviour so a feedback amplifier can settle rather than oscillate. Continuous and latched comparators then turn analogue differences into decisions, with offset and regeneration affecting the boundary between outcomes.
Sampling introduces a switch, storage node and clocked event, so charge injection and timing become part of accuracy. Revise these topics as one question: what state is stored or decided, what unwanted mechanism perturbs it, and what circuit or timing choice reduces the error?
What this chapter covers
- 01
Operational-amplifier compensation
- 02
Sizing and output-stage choices
- 03
Continuous comparator paths
- 04
Latched comparator regeneration
- 05
Sampling and held-state accuracy
- 06
Charge injection and offset control
Worked example · free
Diagnose a sampled decision error
- +1Observe the sampled node around the switching event to see whether a clock-correlated step appears before the comparator decides.
- +1Change sampling-switch or clock conditions and look for a corresponding change in the disturbance, which points to charge injection or feedthrough.
- +1Test the comparator with a stable direct input to characterise its decision offset independently of the sampler.
- +1Compare both error contributions and revise the sampler, offset-control method or decision timing according to the dominant evidence.
Key terms
- Compensation
- Compensation reshapes an amplifier's dynamic response to improve stability in its intended feedback condition.
- Comparator
- A comparator converts the sign of an analogue input difference into a discrete decision.
- Latched comparator
- A latched comparator uses clocked positive feedback to regenerate a small input difference into a full decision.
- Offset
- Offset is an input-referred imbalance that shifts the practical decision boundary away from the ideal one.
- Sampling
- Sampling captures a signal value at a selected time so it can be held and processed later.
- Charge injection
- Charge injection is unwanted switching charge transferred into a sampled node when a device changes state.
Compensation, Comparators and Sampling FAQ
Why does compensation trade speed for stability?
Compensation changes where the amplifier's dynamic response loses gain and accumulates phase shift. Making the closed-loop system more tolerant of feedback can reduce the speed available for a given transition. A good answer states the intended feedback condition and verifies both settling behaviour and the performance cost.
How is a latched comparator different from a continuous comparator?
A continuous comparator produces a decision from an always-active signal path, while a latched comparator uses a clocked regenerative event. Regeneration can make the decision fast and decisive, but it introduces timing, kickback and reset concerns that must be checked around the clock phases.
How can I distinguish charge injection from comparator offset?
Charge injection is tied to a switching event and appears as a disturbance on the sampled node. Comparator offset shifts the decision boundary even with a stable input. Observe timing, vary the sampling conditions and then test the comparator independently so the two mechanisms are not collapsed into one generic error.
Exam move
Study this week with timing diagrams beside circuit blocks. For compensation, sketch the feedback path and state the dynamic failure the compensation is intended to prevent. For each comparator, mark when the input is observed, when regeneration begins and when the output becomes valid. For the sampler, trace the source, switch, storage node and load across acquisition and hold.
Next, build an error table with mechanism, observable symptom, timing signature and possible control. Include amplifier instability, comparator offset, kickback, charge injection and incomplete settling. Practise diagnosis rather than definition recall: if an error follows the clock edge, if it remains with a direct input, or if it grows under a different load, explain which mechanism becomes more plausible.
End every response with a verification pair that isolates the suspected block and then retests the complete path. This mirrors good engineering practice and the rubric's emphasis on measured evidence rather than unsupported claims. Rehearse three controlled changes: move the decision instant, change the sampling transition and replace the sampled input with a stable direct value.
Predict which trace should move before running or interpreting the test. For compensation, hold the feedback configuration constant while comparing load and command size, because ringing, settling and slew limitations require different explanations. Annotate every waveform with acquisition, isolation, regeneration and observation intervals.
Then write a bounded conclusion that names the implicated block and the evidence still needed to exclude the nearest alternative mechanism.
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