---
title: "Principle of biasing circuits in IC design"
subject: "Advanced Electronics"
chapter: 1
key: "semester_4/advanced_electronics:ic-biasing-principle"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/ic-biasing-principle
site: BCT Engineering
---

# Principle of biasing circuits in IC design

Advanced Electronics · Chapter 1: Operational Amplifier Circuits

- Asked in: 2069 Chaitra, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/ic-biasing-principle
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-1

## Question

Explain the principle of biasing circuit in IC design.

## Short answer

IC biasing avoids large resistors and capacitors. A single reference current, set by one resistor and a diode-connected transistor, is copied by transistor current mirrors to all stages, providing constant current sources that save area and track temperature.

## Answer

In discrete circuits transistors are biased with resistor divider networks and bypass/coupling capacitors. In ICs this is not practical, so a different principle is used:

- Resistors take large chip area (a 1 MΩ resistor needs far more area than a transistor) and have poor absolute tolerance (±20%), but **ratios** and matching of devices are very good.
- Capacitors larger than a few tens of pF cannot be made, so coupling and bypass capacitors are not possible; stages are **direct coupled**.
- Transistors are cheap, small and very well matched because they are made side by side in the same process and are at the same temperature.

So IC bias circuits use **constant current sources made from transistors (current mirrors)**. One reference current is set by a single resistor and a diode-connected transistor, and is copied to all stages by current mirrors (current steering), often with Widlar sources for small currents. The current sources also act as high-resistance active loads and tail sources, giving high gain and high CMRR, and their currents track temperature and supply changes together.

## Check yourself

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

1. Why are resistor-based bias networks avoided in ICs?
   A. They always saturate transistors
   B. Resistors are too small
   C. Resistors cannot carry dc
   D. Large resistors need much area and have poor absolute tolerance
   Answer: D. A large resistor takes far more area than a transistor and has about ±20% tolerance. (Key point: Why not resistors)

2. Why are stages direct coupled?
   A. Transistors need capacitors to match
   B. Capacitors above a few tens of pF cannot be made on chip
   C. Capacitors increase gain too much
   D. Direct coupling blocks dc
   Answer: B. On-chip capacitors are small, so coupling and bypass capacitors are not possible. (Key point: Coupling)

3. How are bias currents produced in IC stages?
   A. Zener diodes in each stage
   B. Separate resistor dividers per stage
   C. One reference current copied by current mirrors
   D. External batteries
   Answer: C. A single resistor and diode-connected transistor set IREF, which mirrors copy (current steering). (Key point: Bias method)

## Related in Advanced Electronics

- [IC biasing, Widlar advantages, differential gain](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aic-biasing-widlar-diff-gain) — Advanced Electronics, chapter 1
- [Active load preference and Widlar in IC biasing](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aactive-load-widlar-ic-biasing) — Advanced Electronics, chapter 1
- [Need for log-antilog circuits; analog multiplier](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Awhy-log-antilog-multiplier) — Advanced Electronics, chapter 6, asked 2×

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