---
title: "Positive/negative bias current effect and reduction"
subject: "Advanced Electronics"
chapter: 2
key: "semester_4/advanced_electronics:positive-negative-bias-current"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/positive-negative-bias-current
site: BCT Engineering
---

# Positive/negative bias current effect and reduction

Advanced Electronics · Chapter 2: Operational Amplifier Characterization

- Asked in: 2068 Chaitra, 7 marks
- Video: [Op-Amp: Input Bias Current and Input Offset Current Explained](https://www.youtube.com/watch?v=c3X7_ohCTSw) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/positive-negative-bias-current
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-2

## Question

Explain the effect of positive and negative input bias current on output voltage of an op-amp and suggest methods of reduction.

## Short answer

Positive and negative bias currents produce opposite polarity output errors: Vo=IB−·Rf − IB+·Rcomp(1+Rf/R1). Choosing Rcomp=R1||Rf cancels the main error, leaving Rf·Ios. Additional reduction: FET-input op-amps, smaller resistors, offset-null, and ac coupling.

## Answer

**Input bias currents** are the dc base (or gate) currents of the input transistors. The current into the non-inverting input is called the positive input bias current IB⁺ (or IB2), and the current into the inverting input the negative input bias current IB⁻ (or IB1). (The currents flow into the op-amp for NPN input transistors and out of it for PNP inputs; only the sign of the error changes.)

```text
                 Rf
          +----/\/\/----+
          |             |
 Vi --R1--+--> IB1 -(-) |
                     >--+---- Vo
     +-----> IB2 ---(+)
     |
   Rcomp
     |
    GND
```

#### Effect of IB⁻ (inverting input)
With Vi = 0 and Rcomp = 0, the inverting input is a virtual ground, so no current flows in R1 and IB⁻ must flow from the output through Rf:

```text
Vo1 = +IB⁻ · Rf
```

#### Effect of IB⁺ (non-inverting input)
IB⁺ flows through Rcomp and makes V+ = −IB⁺·Rcomp. The circuit amplifies this as a non-inverting amplifier:

```text
Vo2 = -IB⁺ · Rcomp · (1 + Rf/R1)
```

#### Total effect
By superposition:

```text
Vo = IB⁻·Rf - IB⁺·Rcomp(1 + Rf/R1)
```

The two currents produce outputs of **opposite polarity**, so they can be made to cancel.

#### Methods of reduction
1. **Compensating resistor:** choose Rcomp(1 + Rf/R1) = Rf, i.e. **Rcomp = R1‖Rf**. Then

```text
Vo = Rf(IB⁻ - IB⁺) = Rf·Ios
```

   which is much smaller since Ios << IB.
2. Use FET-input or super-β op-amps with very small bias currents (pA).
3. Use smaller resistor values (or a T-feedback network for high gain), since the error is proportional to Rf.
4. Null the remaining error with the offset-null pot (741: 10 kΩ between pins 1 and 5).
5. In ac amplifiers, use coupling capacitors so the dc gain is unity.

## Check yourself

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

1. With Vi = 0, Rcomp = 0, the effect of the inverting-input bias current is:
   A. Vo1 = +IB- x Rf
   B. Vo1 = -IB- x Rf
   C. Vo1 = 0
   D. Vo1 = IB- x R1
   Answer: A. IB- flows from the output through Rf. (Key point: Effect of IB-)

2. The effect of IB+ is:
   A. Vo2 = +IB+ x Rf
   B. Vo2 = -IB+ x Rcomp(1 + Rf/R1)
   C. Vo2 = IB+ x R1
   D. Vo2 = 0
   Answer: B. V+ = -IB+ Rcomp is amplified as in a non-inverting amplifier. (Key point: Effect of IB+)

3. Why can the two effects be cancelled?
   A. They have the same polarity
   B. They are ac
   C. They produce outputs of opposite polarity
   D. They have different frequencies
   Answer: C. Vo = IB-.Rf - IB+.Rcomp(1 + Rf/R1). (Key point: Opposite polarity)

4. Choosing Rcomp = R1||Rf leaves:
   A. Vo = Rf x IB
   B. Vo = 0 exactly
   C. Vo = R1 x IB
   D. Vo = Rf x Ios
   Answer: D. The residual is set by the small offset current. (Key point: Reduction)

## Related in Advanced Electronics

- [Bias current effect; closed-loop output impedance](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Abias-current-output-impedance) — Advanced Electronics, chapter 2
- [Output voltage of two op-amp IA](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atwo-opamp-ia-output) — Advanced Electronics, chapter 4
- [IA features; 3 op-amp IA output voltage](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aia-features-3opamp-output) — Advanced Electronics, chapter 4
- [Effect of input bias currents and mitigation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Abias-current-effect-mitigation) — Advanced Electronics, chapter 2
- [3 op-amp IA output and common-mode rejection](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Athree-opamp-ia-cm-rejection) — Advanced Electronics, chapter 4
- [Input impedance of inverting op-amp](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainverting-input-impedance) — Advanced Electronics, chapter 2

## Related in other subjects

- [Voltage gain of non-inverting op-amp](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-noninverting-gain) — Basic Electronics Engineering, chapter 4, asked 2×
- [Non-inverting op-amp voltage gain](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Anon-inverting-amplifier-gain) — Fundamental of Electrical and Electronics Engineering, chapter 6
- [Op-amp properties and differentiator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aopamp-props-differentiator) — Basic Electronics Engineering, chapter 4, asked 2×
- [Transfer function of op-amp phase-shift circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-opamp-allpass-r1-c-r2) — Control Systems, chapter 2, asked 2×