---
title: "Effect of input offset voltage; PSRR"
subject: "Advanced Electronics"
chapter: 2
key: "semester_4/advanced_electronics:offset-voltage-effect-psrr"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/offset-voltage-effect-psrr
site: BCT Engineering
---

# Effect of input offset voltage; PSRR

Advanced Electronics · Chapter 2: Operational Amplifier Characterization

- Asked in: 2074 Asoj, 3+3+2 marks
- Video: [PARAMETERS OF OP-AMP | offset voltage, CMRR in op-amp, SVRR in op-amp, slew rate, thermal drift](https://www.youtube.com/watch?v=i_z6ikX80fk) — Electronics Subjectified
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/offset-voltage-effect-psrr
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-2

## Question

Show the effect of input offset voltage in Op-Amp. How can it be minimized? Define power supply rejection ratio of Op-Amp.

## Short answer

Input offset voltage Vio is amplified by (1+Rf/R1), producing a dc output error. Minimize by using offset-null terminals, low-offset op-amps, reducing dc gain, or external balancing. PSRR is ΔVio/ΔVsupply in µV/V; smaller value means better supply rejection.

## Answer

#### Effect of input offset voltage
**Input offset voltage (Vio)** is the small dc voltage that must be applied between the inputs to make the output zero. It is caused by mismatch of the input transistors (VBE and β). Its effect can be modelled as a dc source Vio in series with the non-inverting input of an ideal op-amp.

```text
            Rf
      +---/\/\/---+
      |            |
 GND -R1--+--(-)   |
          |     >--+---- Voo
 GND --[Vio]--(+)
```

With the signal input grounded, the circuit is a non-inverting amplifier for Vio:

```text
Voo = (1 + Rf/R1) · Vio
```

So the offset is amplified by the noise gain (1 + Rf/R1), the same for inverting and non-inverting configurations. Example: Vio = 6 mV, R1 = 1 kΩ, Rf = 100 kΩ gives Voo = 101 × 6 mV = 0.606 V at the output with no input. In open loop (comparator) even 1 mV can drive the output into saturation.

#### Minimizing the offset
- **Offset null terminals:** for the 741, a 10 kΩ potentiometer between pins 1 and 5 with the wiper to −VEE is adjusted for zero output.
- **External balancing (compensating) network:** a small adjustable voltage from a potentiometer across ±V, divided down, applied to the free input.
- **Use low-offset op-amps** (e.g. OP07, Vio ≈ 25 µV) or chopper-stabilized op-amps.
- **Reduce dc gain:** use the lowest gain needed; in ac amplifiers use a capacitor in series with R1 so the dc gain is 1.
- Keep the op-amp at a stable temperature, since Vio drifts (µV/°C).

#### Power supply rejection ratio (PSRR)
**PSRR** (also called supply voltage rejection ratio, SVRR) is the change in input offset voltage per unit change in supply voltage, with the other supply constant:

```text
PSRR = ΔVio / ΔV    (µV/V)
PSRR(dB) = 20 log10 (ΔV / ΔVio)
```

For the 741, PSRR = 30 µV/V typical (150 µV/V max). A smaller value in µV/V (or a larger value in dB) is better.

## Check yourself

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

1. With the signal input grounded, the output offset due to Vio is:
   A. Voo = (1 + Rf/R1) x Vio
   B. Voo = Vio
   C. Voo = (Rf/R1) x Vio with inverted sign only
   D. Voo = Vio/(1 + Rf/R1)
   Answer: A. Vio acts like a source at the + input, amplified by the noise gain (1 + Rf/R1). (Key point: Effect of Vio at output)

2. Which method reduces the effect of input offset voltage?
   A. Increase Rf
   B. Adjust a 10 kohm pot across the offset-null pins (1 and 5 on the 741)
   C. Add a larger supply voltage
   D. Use a higher-frequency signal
   Answer: B. Offset-null terminals trim the output to zero; low-offset op-amps and low dc gain also help. (Key point: Minimising offset)

3. Why does a capacitor in series with R1 help in an ac amplifier?
   A. It raises the dc gain for the offset
   B. It cancels the bias current
   C. The dc gain becomes 1, so Vio is not amplified
   D. It increases the slew rate
   Answer: C. With the capacitor blocking dc, noise gain at dc is 1, so Voo is about Vio. (Key point: AC amplifier trick)

4. PSRR of an op-amp is:
   A. The change in output per change in input signal
   B. The ratio of differential gain to common-mode gain
   C. The change in bias current per volt of input
   D. The change in input offset voltage per unit change in supply voltage
   Answer: D. PSRR = delta Vio / delta V (uV/V); smaller is better (741: 30 uV/V typical). (Key point: PSRR definition)

## Related in Advanced Electronics

- [Input offset voltage; proof of constant GBP](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainput-offset-voltage-gbp) — 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
- [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
- [Output offset voltage of 741 inverting amplifier](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aoutput-offset-741-inverting) — 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
- [Effect of load rejection](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aload-rejection-effect) — High Voltage Engineering, chapter 3
- [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×