Comments on: Solving the Differential Amplifier – Part 2

By: Adrian S. Nastase

Adrian S. Nastase — Wed, 07 Feb 2018 03:58:29 +0000

In reply to Jackson. I assume that the input voltage is Vin = 4.017 mV, not mOhm. For an academic exercise I would use the differential amplifier in this article, where R2/R1 = R4/R3 = gain. In this case the gain is the ratio between Vout and Vin which is 249. So, I would choose R1 = R3 = 100 ohm and R2 = R4 = 24.9 kohm, 1% tolerance. For a practical application I would use an instrumentation amplifier, since the input signal is small and could be buried in noise. Or at least a low noise op amp.

By: Jackson

Jackson — Wed, 03 Jan 2018 16:13:58 +0000

Hi Adrian can you help me to solve an academic exercise which is very important for me. It involves thermocouple with differential amplifier. The question states that Vin=4.017 mOhm and Vout=1V, please calculate resistance for R1, R2, R3 and R4. Please help me !

By: Adrian S. Nastase

Adrian S. Nastase — Tue, 02 Nov 2010 06:19:25 +0000

In reply to Seasons. Well, it has a subtitle, "Design a Differential Amplifier with the Coefficients Identification Method". However, it is part of a series of 3 articles, so I think the main title is appropriate.

By: Seasons

Seasons — Sat, 30 Oct 2010 07:55:55 +0000

Best you could make changes to the post name title MasteringElectronicsDesign.com : Solving the Differential Amplifier â€“ Part 2 to something more specific for your blog post you create. I enjoyed the blog post nevertheless.

By: senti32

senti32 — Sat, 02 May 2009 15:12:11 +0000

no, i power the opamp 0 to 5V. the source is 0.5V peak to peak and i need the output to be 1.5V peak to peak.

By: Adrian S. Nastase

Adrian S. Nastase — Sat, 02 May 2009 14:52:32 +0000

In this case you need to bias the input so that the output is somewhere in the middle of 0 to 5V. You did not mention what is the DC level at the OpAmp’s output, but let’s assume it has to be exactly in the middle, at 2.5V.

Connect the source at V1, connect R3 to ground and connect R2 to a DC level of 2.5V. If you decrease the input signal down to zero, you will see that the OpAmp output is 2.5V. As you increase the AC level, the output signal rides on top of 2.5V.

Choose R1, R2, R3, R4 so that R4/R1 = R2/R1 = 3, your gain.

By: senti32

senti32 — Sat, 02 May 2009 07:28:45 +0000

thank you very much for explaining the differential amplifier. i have a question: i need to connect between v1 and v2 a AC source. is there a easy way to find the resistors for a given output range?

By: Adrian S. Nastase

Adrian S. Nastase — Sat, 02 May 2009 05:56:17 +0000

If your OpAmp has a bipolar source (+/- 12 V as an example) you can connect the AC source directly to V1 V2 inputs. Make R4/R3 = R2/R1 and equal with the gain you need. Chances are your source does not float and has a ground connection. Connect its output at V1 and connect R3 to ground. This is a classic way to design a non-inverting amplifier with the gain less than 1. Otherwise, if your gain is greater than 1, you can eliminate R1 and R2, connect the output of your source to the non-inverting input of U1 and connect R3 to ground. The gain will be 1+R4/R3.