---
title: "Short note: Current steering circuits"
subject: "Advanced Electronics"
chapter: 1
key: "semester_4/advanced_electronics:short-note-current-steering"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-current-steering
site: BCT Engineering
---

# Short note: Current steering circuits

Advanced Electronics · Chapter 1: Operational Amplifier Circuits

- Asked in: 2074 Chaitra, 3 marks; 2073 Chaitra, 5 marks (asked 2 times)
- Video: [MOS Current Steering Circuits](https://www.youtube.com/watch?v=uNlcNhTGwF8) — Infinity Solution's Concept Builder
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-current-steering
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-1

## Question

Write a short note on current steering circuits.

## Short answer

A current steering circuit generates one reference current and distributes it to many IC stages via current mirrors. NPN mirrors sink current, PNP mirrors source current, and output currents scale with emitter area. This uses only one resistor, ensures matching, and biases op‑amp stages with high‑output‑resistance sources.

## Answer

A **current steering circuit** generates one reference current on the chip and then copies ("steers") it to many points of the IC using current mirrors, so all stages are biased by constant current sources.

```text
                 +VCC
        +---------+---------+
        |         |         |
    Q3 (diode)   Q4        Q5     PNP mirror:
        |         | I3      | I4  sources current
        |         v         v
        R       to load   to load
        |
        |       from load from load
        |         | I1      | I2
    Q1 (diode)   Q6        Q7     NPN mirror:
        |         |         |     sinks current
        +---------+---------+
                 -VEE
  (base of Q1 drives Q6, Q7; base of Q3 drives Q4, Q5)
```

**Working:**
1. Diode-connected transistors Q1 and Q3 in series with resistor R set IREF = (VCC + VEE − VBE1 − VEB3)/R (or (VCC − VBE)/R for a single mirror).
2. The VBE of Q1 is applied to the bases of several NPN transistors (Q6, Q7, …). Each copies IREF, so they **sink** current from the circuits connected to them.
3. The reference current also flows through a diode-connected PNP (Q3), whose VEB drives PNP transistors (Q4, Q5, …) that **source** current into loads.
4. Different output currents are obtained by scaling the emitter areas: I = IREF × (area of output transistor / area of reference transistor), e.g. a 2× area transistor gives 2IREF.

**Advantages:** only one resistor and one reference are needed; all bias currents track each other with temperature and supply; transistors are cheaper than resistors in IC area; high output resistance of current sources gives high gain and good CMRR.

**Use:** biasing of op-amp stages (e.g. the 741 uses one reference current for its input, gain and output stages).

## Check yourself

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

1. What is the idea of current steering?
   A. Each stage gets its own large resistor network
   B. Using an op-amp to regulate each stage
   C. Using capacitors to bypass bias
   D. One reference current is generated and copied by mirrors to many points
   Answer: D. A single reference current is steered to many stages by current mirrors. (Key point: Idea)

2. How are loads supplied with current in a steering circuit?
   A. PNP mirrors sink and NPN mirrors source
   B. Only NPN mirrors are used and they source
   C. Only resistors source current
   D. NPN mirrors sink current and PNP mirrors source current
   Answer: D. NPN transistors copying Q1's VBE sink current; PNP transistors copying Q3's VEB source current. (Key point: Source and sink)

3. How are different output currents obtained from one reference?
   A. By scaling emitter areas: I = IREF × (area ratio)
   B. By changing the supply voltage
   C. By changing β
   D. By adding capacitors
   Answer: A. A 2× area output transistor gives 2 IREF. (Key point: Different currents)

## Related in Advanced Electronics

- [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
- [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

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