Comments on: How to Derive the Summing Amplifier Transfer Function http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/ Electronics Design and Modeling with Emphasis on Analog Design Wed, 02 May 2012 09:00:29 +0000 http://wordpress.org/?v=2.9 hourly 1 By: Adrian S. Nastase http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-2766 Adrian S. Nastase Sun, 28 Nov 2010 05:45:01 +0000 http://masteringelectronicsdesign.com/?p=413#comment-2766 Thank you for your comment Godwin. I am glad that this website helped you. Thank you for your comment Godwin. I am glad that this website helped you.

]]> By: Godwin http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-2183 Godwin Fri, 19 Nov 2010 06:39:57 +0000 http://masteringelectronicsdesign.com/?p=413#comment-2183 I am glad I fond this site!. Very easy to understand. All of the books only show inverting summing amplifiers but I need to design a non-inverting one. I thought it was complicated before. But when I saw your transfer function, I immediately realized that you used the Superposition Theorem to get the solution. Thanks a lot! I am glad I fond this site!. Very easy to understand. All of the books only show inverting summing amplifiers but I need to design a non-inverting one. I thought it was complicated before. But when I saw your transfer function, I immediately realized that you used the Superposition Theorem to get the solution. Thanks a lot!

]]> By: Adrian S. Nastase http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-1087 Adrian S. Nastase Sat, 03 Jul 2010 07:35:36 +0000 http://masteringelectronicsdesign.com/?p=413#comment-1087 I believe you ask about sending personal messages to the other users of this website. For this you need a forum. I do not have a forum yet. I plan to have one in the future. If any of you would like to be forum administrator(s) please let me know. In the mean time you can post responses to other people's comments. I believe you ask about sending personal messages to the other users of this website. For this you need a forum. I do not have a forum yet. I plan to have one in the future. If any of you would like to be forum administrator(s) please let me know. In the mean time you can post responses to other people’s comments.

]]> By: Kibblekly http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-1086 Kibblekly Fri, 02 Jul 2010 16:13:46 +0000 http://masteringelectronicsdesign.com/?p=413#comment-1086 How I can write PM to other users? Thanx How I can write PM to other users? Thanx

]]> By: Adrian S. Nastase http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-920 Adrian S. Nastase Mon, 10 May 2010 20:26:05 +0000 http://masteringelectronicsdesign.com/?p=413#comment-920 Thanks Aries. Glad you find this website useful. Thanks Aries. Glad you find this website useful.

]]> By: Aries http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-891 Aries Thu, 06 May 2010 16:15:30 +0000 http://masteringelectronicsdesign.com/?p=413#comment-891 Very helpful source and it help me a lot especially when some of my lecture notes did not really help me a lot in understanding fully of some particular topic related. Thank you very much for the blog. Credit for you. Thanks. Very helpful source and it help me a lot especially when some of my lecture notes did not really help me a lot in understanding fully of some particular topic related. Thank you very much for the blog. Credit for you.

Thanks.

]]> By: Adrian S. Nastase http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-777 Adrian S. Nastase Wed, 07 Apr 2010 20:45:19 +0000 http://masteringelectronicsdesign.com/?p=413#comment-777 Thank you David.  I really appreciate that. As for your question, if you connect R3 to a reference voltage, say V3, the circuit output is shifted by   –V3*R4/R3.  The new transfer function is: Vout = ((V1*R2/(R1+R2) + V2*R1/(R1+R2)) * (1+R4/R3) - V3*R4/R3 Hope this helps for now.  I will write a short article to prove this.  If you wish, please use the RSS or Facebook subscription to be notified. =============== <strong>Update</strong>: I published this article. You can find it here: <a href="http://masteringelectronicsdesign.com/a-summing-and-differential-amplifier-with-one-op-amp/" rel="nofollow">A Summing and Differential Amplifier with One Op Amp</a> Thank you David.  I really appreciate that.
As for your question, if you connect R3 to a reference voltage, say V3, the circuit output is shifted by   –V3*R4/R3.  The new transfer function is:

Vout = ((V1*R2/(R1+R2) + V2*R1/(R1+R2)) * (1+R4/R3) – V3*R4/R3

Hope this helps for now.  I will write a short article to prove this.  If you wish, please use the RSS or Facebook subscription to be notified.
===============
Update: I published this article. You can find it here:
A Summing and Differential Amplifier with One Op Amp

]]> By: david http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-775 david Wed, 07 Apr 2010 15:33:23 +0000 http://masteringelectronicsdesign.com/?p=413#comment-775 Mr. Nastase, this is a great site full of practical and useful information. Thank you for putting this up! I do have a question that I am sure you can answer. How does the transfer function of Figure 1 change if R3 is tied to a reference voltage instead of zero potential? Thank you! Mr. Nastase, this is a great site full of practical and useful information. Thank you for putting this up! I do have a question that I am sure you can answer. How does the transfer function of Figure 1 change if R3 is tied to a reference voltage instead of zero potential? Thank you!

]]> By: Adrian S. Nastase http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-655 Adrian S. Nastase Thu, 04 Mar 2010 03:38:48 +0000 http://masteringelectronicsdesign.com/?p=413#comment-655 Gould, thank you for your input. You bring a good point here. For those readers that do not have time or patience to follow that thread, here is a short summary: the forum thread concludes that the inverting summing amplifier is preferred versus the non-inverting one due to the fact that the input currents vary as the input signals change. Indeed, if we look at Figure 1, the input current through R1 is Iin1 = (V1-V2)/(R1+R2). When the input current fluctuates independently of its own source, it creates a perceived input impedance variation that appears uncontrollable. While this is true, one has to take it with a grain of salt, like with any other circuit. I always use an Op Amp in each input, to minimize the input source impedance. If that source Op Amp has a negative feedback, its output resistance is even more reduced (see this article, <a href="http://masteringelectronicsdesign.com/the-non-inverting-amplifier-output-resistance/" rel="nofollow">The Non-Inverting Amplifier Output Resistance</a>). Also, non-inverting summing amplifiers are widely used in bipolar to unipolar converters (see this article, <a href="http://masteringelectronicsdesign.com/design-a-bipolar-to-unipolar-converter/" rel="nofollow">Design a Bipolar to Unipolar Converter to Drive an ADC</a>). In that case, the second input voltage does not change in time, so that the input impedance does not change independently of the main input source. Non-inverting summing amplifiers are also used to implement arbitrary linear functions, as described in <a href="http://masteringelectronicsdesign.com/solving-the-summing-amplifier/" rel="nofollow">Solving the Summing Amplifier</a>). As with any other electronic circuit, one has to think about the requirements of his/her project before using this circuit. I design high-end instrumentation equipment and many times I prefer the non-inverting summing amplifier versus the inverting one, because the inverting amplifier needs another amplifier at its output to invert the signal again. This adds cost, extra noise and offset in the signal chain which I prefer not to have. Moreover, when I design low voltage circuits, sometimes I do not have a negative supply, so going negative is not an option. I have to stay in the positive realm and the non-inverting summing amplifier does a good job for me. In any case, indeed, one has to use this circuit with care. Gould, thank you for your input. You bring a good point here. For those readers that do not have time or patience to follow that thread, here is a short summary: the forum thread concludes that the inverting summing amplifier is preferred versus the non-inverting one due to the fact that the input currents vary as the input signals change. Indeed, if we look at Figure 1, the input current through R1 is Iin1 = (V1-V2)/(R1+R2). When the input current fluctuates independently of its own source, it creates a perceived input impedance variation that appears uncontrollable.

While this is true, one has to take it with a grain of salt, like with any other circuit. I always use an Op Amp in each input, to minimize the input source impedance. If that source Op Amp has a negative feedback, its output resistance is even more reduced (see this article, The Non-Inverting Amplifier Output Resistance). Also, non-inverting summing amplifiers are widely used in bipolar to unipolar converters (see this article, Design a Bipolar to Unipolar Converter to Drive an ADC). In that case, the second input voltage does not change in time, so that the input impedance does not change independently of the main input source. Non-inverting summing amplifiers are also used to implement arbitrary linear functions, as described in Solving the Summing Amplifier).

As with any other electronic circuit, one has to think about the requirements of his/her project before using this circuit. I design high-end instrumentation equipment and many times I prefer the non-inverting summing amplifier versus the inverting one, because the inverting amplifier needs another amplifier at its output to invert the signal again. This adds cost, extra noise and offset in the signal chain which I prefer not to have. Moreover, when I design low voltage circuits, sometimes I do not have a negative supply, so going negative is not an option. I have to stay in the positive realm and the non-inverting summing amplifier does a good job for me.

In any case, indeed, one has to use this circuit with care.

]]> By: Gould http://MasteringElectronicsDesign.com/how-to-derive-the-summing-amplifier-transfer-function/#comment-652 Gould Wed, 03 Mar 2010 17:56:10 +0000 http://masteringelectronicsdesign.com/?p=413#comment-652 You may want to check this discussion before you use this in a circuit: http://forum.allaboutcircuits.com/showthread.php?p=170314 The input resistance to your summer will change based on the input voltages. This could be ok if you prevision for it in your design, but the non-inverting summer is not as "plug and play" as it is made out to be here. Great article otherwise. You may want to check this discussion before you use this in a circuit:

http://forum.allaboutcircuits.com/showthread.php?p=170314

The input resistance to your summer will change based on the input voltages. This could be ok if you prevision for it in your design, but the non-inverting summer is not as “plug and play” as it is made out to be here.

Great article otherwise.

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