# Input Resistor Voltages of Non-Inverting Summing Op Amp

Discussion in 'Homework Help' started by gurg2k1, May 10, 2013.

1. ### gurg2k1 Thread Starter New Member

Feb 26, 2013
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Hello, I am having trouble calculating the input resistor voltages for a non-inverting voltage adder Op Amp circuit. I have attached a schematic below, but I would like to make a note about calculating based on a 2V DC input. I understand that the input resistors (R2 and R3) see 2V and 1V from the voltage divider of R4 and R5, the input voltage equals 1.5V, and the output equals 3V. However, I am having trouble with calculating the voltage drop across R2 and R3 mathematically.

Originally I thought that R2 and R3 would each drop their supply voltage of 2V and 1V, but during the lab I measured 500mV across R2 and -500mV across R3 (with respect to the input source). This is definitely not what I was expecting, and now I am wondering where I went wrong in my calculations. My textbook is not much help, as it only shows how to calculate gain and output voltage.

Any help would be greatly appreciated!

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2. ### WBahn Moderator

Mar 31, 2012
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First, you need to clean up some of your termilogy because the way you are saying things is quite confusing (and this may simply be a english-as-second-language issue, in which we will work through it).

When you say that a resistor "sees" a voltage, that implies you are talking about the voltage across the resistor since that is the only voltage that the resistor cares about. You seem to be using it to refer to the voltage, relative to ground, on one side of the resistor. But which side? See why it's confusing.

The you say that you thought that some resistors would each drop their supply voltage. What does that mean? Resistors don't have supplies.

If you measure a voltage across something, there there is no "with respect to" something else involved. If you say that you measured a voltage on a particular node, then THAT has to be with respect to something else. But if you measure the votlage ACROSS something, then the measurement is the voltage on one node with respect to the other node, and you need to specify which of those two nodes it is with respect to.

So, with that in mind, calculate (or properly state) the following:

With an input voltage of 2V DC (the "input voltage" is the voltage applied to Node V1 wrt ground), what do you get for the voltage at V2 and what do you get for the voltage at the non-inverting input to the opamp?

For your lab measurements, when you say you measured 500mV and -500mV, state what this measurement was, specifically which node was the negative load of the meter connected to and which node was the positive meter connected to? I'm guessing that for the measurement across R2 you had the positive proble at V1 and the negative probe at the non-inverting input of the amp and for the measurement across R3 you had the positive probe at V2 but the negative problem still at the non-inverting input of the amp. If so, then you measured exactly what I would have expected (except that I would not have expected them to be exactly 500mV in magnitude).

Also consider this, in your first paragraph you said that you understand that R2 and R3 see 2V and 1V from the voltage divider and that the voltage at the input (of the opamp) is 1.5V. So, with just that information, what is the voltage across R2 and R3?

Apr 26, 2005
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