---
title: "Active load preference and Widlar in IC biasing"
subject: "Advanced Electronics"
chapter: 1
key: "semester_4/advanced_electronics:active-load-widlar-ic-biasing"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/active-load-widlar-ic-biasing
site: BCT Engineering
---

# Active load preference and Widlar in IC biasing

Advanced Electronics · Chapter 1: Operational Amplifier Circuits

- Asked in: 2069 Asar, 2+2 marks
- Video: [ECE 3110 - Lecture 6 - Part 1 - Active Load Circuits](https://www.youtube.com/watch?v=bPnUpM2-mZk) — EEStream
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/active-load-widlar-ic-biasing
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-1

## Question

Why active load is preferred than passive load? What is the advantage of using Widlar current source in biasing circuits for IC design?

## Short answer

Active loads offer high small-signal resistance with small dc drop, double differential gain, and save chip area. The Widlar source generates small currents using small resistors, gives high output resistance Ro≈ro(1+gmRE), and reduces sensitivity to supply and β variations.

## Answer

#### Why active load is preferred over passive load
- **Much higher gain:** gain = gm × load resistance. A transistor current source gives a small-signal resistance ro (50 kΩ to MΩ) while dropping only a few tenths of a volt dc. A resistor of the same value would need a very large dc drop and supply voltage.
- **Gain is doubled in a differential pair:** the current-mirror load converts the differential output to single-ended without losing half the signal (Ad = gm(ro2‖ro4) instead of gm·RC/2).
- **Less chip area:** a transistor occupies much less area than a large resistor.
- **Large output swing** and better use of a low supply, since the dc drop is small.
- **Better CMRR** and good temperature tracking (matched devices).

#### Advantage of Widlar current source in IC biasing
- Gives very small bias currents (µA) with small resistors: IO·RE = VT ln(IREF/IO), so a 10 µA source needs only about 12 kΩ instead of about 1.4 MΩ, saving chip area.
- Higher output resistance, Ro ≈ ro(1 + gm RE), so the current is more constant.
- The output current is less sensitive to supply voltage and β changes because of the emitter degeneration.
- Used for the low input-stage current of the 741, giving low input bias current and high input resistance.

## Check yourself

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

1. Why does an active load give much higher gain than a resistor?
   A. It increases gm
   B. It removes the input stage
   C. It has lower resistance
   D. It gives a large small-signal resistance ro with only a small dc drop
   Answer: D. Gain = gm × load; a transistor gives large ro without needing large dc voltage. (Key point: Active load gain)

2. What is the advantage of Widlar source in IC biasing?
   A. Larger currents than IREF
   B. Needs large capacitors
   C. Small µA currents with small resistors
   D. No need for transistors
   Answer: C. IO·RE = VT ln(IREF/IO): a 10 µA source needs about 12 kΩ instead of about 1.4 MΩ. (Key point: Widlar advantage)

3. Where is the Widlar source used in the 741?
   A. Feedback network
   B. Output stage
   C. Low input-stage bias current
   D. Power supply
   Answer: C. It gives the low input-stage current, so low input bias current and high input resistance. (Key point: 741)

## 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
- [Principle of biasing circuits in IC design](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aic-biasing-principle) — Advanced Electronics, chapter 1
- [Current mirror requirements, Widlar resistance, best source](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Acurrent-mirror-best-source) — Advanced Electronics, chapter 1
- [Benefits of Widlar source and output resistance](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Awidlar-benefits-output-resistance) — Advanced Electronics, chapter 1, asked 2×
- [Current mirror conditions and Widlar output resistance](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Awidlar-essential-conditions-output-resistance) — Advanced Electronics, chapter 1, asked 3×
- [Current mirror advantages and Widlar output resistance](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Acurrent-mirror-advantages-widlar) — Advanced Electronics, chapter 1

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