---
title: "Feedback vs non-feedback control systems"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:feedback-vs-nonfeedback"
source: https://www.bctengineer.com/q/semester_3/control_systems/feedback-vs-nonfeedback
site: BCT Engineering
---

# Feedback vs non-feedback control systems

Control Systems · Chapter 1: Control System Background

- Asked in: 2081 Bhadra, 4 marks
- Video: [Feedback vs. Feedforward vs. Open-Loop Control | Why Each Matters in Industry](https://www.youtube.com/watch?v=O31rlFCo8a8) — Real Control Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/feedback-vs-nonfeedback
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

Differentiate between feedback and non-feedback control systems.

## Short answer

Non-feedback (open loop) acts without output measurement; transfer function G(s); accuracy depends on calibration; no disturbance rejection; usually stable. Feedback (closed loop) uses output feedback, transfer function G/(1+GH); high accuracy; disturbance reduced by (1+GH); faster but can become unstable; complex and costly.

## Answer

A **non-feedback (open-loop)** system acts only on the input; a **feedback (closed-loop)** system also measures the output and acts on the error.

| Point | Non-feedback (open loop) | Feedback (closed loop) |
|---|---|---|
| Control action | independent of output | depends on output error |
| Transfer function | $G(s)$ | $\dfrac{G(s)}{1+G(s)H(s)}$ |
| Accuracy | depends on calibration | high, self-correcting |
| Sensitivity to $G$ | $S = 1$ | $S = \dfrac{1}{1+GH}$ |
| Disturbance effect | full | reduced by $1+GH$ |
| Speed (time constant) | $\tau$ | $\tau/(1+K)$, faster |
| Stability | usually stable | can become unstable |
| Gain | higher | reduced |
| Cost, components | simple, cheap | complex, costly |
| Example | toaster, timer traffic light | AC with thermostat, motor speed control |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The transfer function of a feedback system is
   A. G/(1+GH)
   B. G
   C. GH
   D. G(1+GH)
   Answer: A. A non-feedback system has just G; feedback gives G/(1+GH). (Key point: Transfer function)

2. Sensitivity to plant parameter G is
   A. 1/(1+GH) for open loop and 1 for closed loop
   B. 1 for open loop and 1/(1+GH) for closed loop
   C. 1 for both
   D. (1+GH) for closed loop
   Answer: B. Feedback reduces sensitivity by 1+GH. (Key point: Sensitivity)

3. Time constant of a first-order plant with unity feedback compared with open loop tau is
   A. tau(1+K), slower
   B. tau, unchanged
   C. tau/(1+K), faster
   D. tau K
   Answer: C. Feedback reduces the time constant by (1+K). (Key point: Time constant)

## Related in Control Systems

- [Feedback effect in DC motor speed control](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Afeedback-dc-motor-speed) — Control Systems, chapter 1
- [Control system and why feedback makes response faster](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-feedback-faster) — Control Systems, chapter 1
- [Transfer function of op-amp with Rs ∥ C feedback](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-opamp-rs-c-r1-r2) — Control Systems, chapter 2
- [Block diagram reduction: G1–G4, H1, H2, unity feedback](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Abdr-g1-g4-h1h2-unity-2076c) — Control Systems, chapter 3, asked 3×
- [Everyday closed loop example and importance of feedback](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-example-feedback-importance) — Control Systems, chapter 1, asked 2×
- [Derivative feedback controller: block diagram and effect](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aderivative-feedback-bd-effect) — Control Systems, chapter 7, asked 2×

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