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Op-amp gain designer

Op-amp gain designer: the circuit drawn with the example's values20kΩ180kΩ
Non-inverting ×10: R1 20 kΩ, R2 180 kΩ, flat to 100 kHz

With feedback the op-amp keeps its inputs equal: two resistors set the gain.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Given

Stage
Non-inverting
Gain wanted (V/V)
10
R1 + R2 around
100 kΩ
Standard series
E24 (5 %)
Gain-bandwidth product
1 MHz
Slew rate
0.5 V/µs
Output swing wanted
8 V

Working

  1. one plus the ratio
  2. the noise gain eats the bandwidth, for both stages
  3. the slew rate limit at full swing

Results

R120 kΩ
R2 (feedback)180 kΩ
Gain you get10 V/V (20 dB)
Bandwidth (−3 dB)100 kHz
Full-power bandwidth19.9 kHz
Input resistancevery high

The formulas

  1. G = 1 + R2 / R1 (non-inverting), G = −R2 / R1 (inverting)two resistors set the gain
  2. f−3dB = GBW / (1 + R2 / R1)the noise gain eats the bandwidth
  3. fmax = SR / (π Vpp)the slew rate limit at full swing

The usual traps

  • A gain of 100 on a 1 MHz op-amp leaves 10 kHz of bandwidth.
  • Large signals turn into triangles long before small ones roll off.

Questions

How do I choose the resistors for an op-amp gain?

G = 1 + R2 / R1 (non-inverting), G = −R2 / R1 (inverting): two resistors set the gain. f−3dB = GBW / (1 + R2 / R1): the noise gain eats the bandwidth. fmax = SR / (π Vpp): the slew rate limit at full swing.

What does it give for typical values?

Non-inverting ×10: R1 20 kΩ, R2 180 kΩ, flat to 100 kHz. R1: 20 kΩ; R2 (feedback): 180 kΩ.

What are the usual mistakes?

A gain of 100 on a 1 MHz op-amp leaves 10 kHz of bandwidth. Large signals turn into triangles long before small ones roll off.

In the app

Your circuit, drawn live

Change a value: the schematic redraws with your numbers on the parts.

The part you can buy

Results rounded to standard values (E12, E24, E96) and power ratings.

The working, shown

Every step written out. Try it first by hand, then check.

Into your notebook

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Opens in the app on 1 December, with Circuit Notebook 1.0

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