---
title: "Self bias vs fixed bias in MOSFET"
subject: "Electronics Devices and Circuit"
chapter: 3
key: "semester_3/electronics_devices_and_circuit:mosfet-self-vs-fixed-bias"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/mosfet-self-vs-fixed-bias
site: BCT Engineering
---

# Self bias vs fixed bias in MOSFET

Electronics Devices and Circuit · Chapter 3: Field-Effect Transistor

- Asked in: 2070 Ashad, 4 marks
- Video: [MOSFET Biasing: Depletion Type MOSFET Biasing (Fixed Bias, Self Bias and Voltage Divider Bias)](https://www.youtube.com/watch?v=vBvLu3euc2Q) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/mosfet-self-vs-fixed-bias
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-3

## Question

Explain why self DC biasing MOSFET circuit is better than fixed DC biasing MOSFET circuit.

## Answer

In **fixed (gate-bias) biasing** the gate voltage is set directly (for example by a fixed supply or a divider with no source resistor), so VGS is constant. In **self (source-resistor) biasing** the gate is held by a divider but a resistor RS in the source makes VGS = VG − ID RS, which feeds the drain current back to the input.

#### Why self-bias is better
1. **Negative feedback stabilises ID.** If ID rises (temperature, device change), the drop ID RS rises, VGS falls, and ID is pulled back. In fixed bias there is no such correction.
2. **Less sensitivity to device spread.** The threshold voltage Vt and constant k vary widely between devices of the same type. Fixed bias lets ID = k (VGS − Vt)² change a lot; self-bias holds it near the design value.
3. **Stable Q point and gain.** The Q point stays in saturation, so distortion and gain variation are lower. With a bypass capacitor across RS, ac gain is not reduced.
4. **Needs only one supply** in the divider form.

#### Numerical illustration
MOSFET with k = 0.5 mA/V² and Vt = 1 V, designed for ID = 0.5 mA (VGS = 2 V). Suppose Vt changes to 1.5 V.

| Bias type | ID at Vt = 1 V | ID at Vt = 1.5 V | Change |
|---|---|---|---|
| Fixed (VGS = 2 V) | 0.5 mA | 0.125 mA | −75% |
| Self (VG = 5 V, RS = 6 kΩ) | 0.5 mA | 0.43 mA | −14% |

Self-bias therefore holds the Q point far better. The cost is a lower dc voltage left for the drain-source (VDS reduces by ID RS).

## Check yourself

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

1. Why does source-resistor self-bias stabilise ID?
   A. If ID rises, ID RS rises, VGS falls, and ID is pulled back
   B. RS increases VGS when ID rises
   C. It uses two supplies
   D. It removes the threshold voltage
   Answer: A. VGS = VG - ID RS feeds ID back to the input as negative feedback. (Key point: Negative feedback)

2. Why is fixed bias poor?
   A. VGS is constant, so ID changes greatly when Vt or k varies
   B. It draws gate current
   C. It needs no supply
   D. It has no drain resistor
   Answer: A. ID = k(VGS-Vt)^2 varies strongly with device spread when VGS is fixed. (Key point: Fixed bias weakness)

3. When Vt rises from 1 V to 1.5 V, how does ID change in fixed bias?
   A. Falls 75% (0.5 to 0.125 mA)
   B. Falls 14%
   C. Stays
   D. Rises
   Answer: A. Fixed VGS = 2 V gives ID = 0.5*(0.5)^2 = 0.125 mA, a 75% fall versus 14% for self-bias. (Key point: Numerical illustration)

## Related in Electronics Devices and Circuit

- [MOSFET ID, VDS, region and small-signal circuit](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Amosfet-id-vds-region-ac-equivalent) — Electronics Devices and Circuit, chapter 3, asked 2×
- [BJT bipolar vs FET unipolar; MOSFET transconductance](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-bipolar-fet-unipolar-gm) — Electronics Devices and Circuit, chapter 3
- [Current limiting circuit in dc regulator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Acurrent-limiting-circuit-draw) — Electronics Devices and Circuit, chapter 6
- [Why divider bias is beta independent; design (24 V)](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ace-divider-design-24v-1p5ma) — Electronics Devices and Circuit, chapter 2, asked 4×
- [Design CE divider bias (12 V, 2 mA, beta 150)](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ace-divider-design-12v-2ma) — Electronics Devices and Circuit, chapter 2, asked 4×
- [Why divider bias is beta independent; design (IBQ 10 μA)](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ace-divider-design-24v-ib10ua) — Electronics Devices and Circuit, chapter 2

## Related in other subjects

- [Why Kaiser window is better than fixed windows](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-kaiser-better) — Digital Signal Analysis and Processing, chapter 5, asked 2×
- [Preventive maintenance is better than corrective](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Apreventive-better-than-corrective) — Construction Management, chapter 9, asked 4×
- [Closed loop better than open loop (cruise)](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-better-cruise-integral) — Embedded System, chapter 7
- [When wet process is better than dry](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Awet-process-better-than-dry) — Civil Engineering Materials, chapter 5