# Revelations on circuit analysis during a 2 hour flight...

#### Distort10n

Joined Dec 25, 2006
429
I have already completed a KCL analysis on a differential amplifier a while ago, and I posted it to my website.
Recently, I came across a common-mode calculator for Instrumentation amplifiers. This calculator is used to determine the voltages of the internal nodes to ensure that they will not violate the input-common mode ranges of the remaining internal amplifiers. The voltage expressions seemed complicated, but after looking at them it seemed that each voltage source on the input was multiplied by the appropriate signal gain equation, and then they were all combined together (see first attachment).

So, I decided to test that against a typical KCL analysis and it works! If each voltage source is treated separately like the others are not involved, then is this the superposition theorem at work? I will have to dig out my text book to review the text book definition of superposition.

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#### hgmjr

Joined Jan 28, 2005
9,027
Is the voltage applied to this opamp a single floating voltage source of magnitude (V1-V2) or are V1 and V2 two seperate voltage sources with one end of each voltage source connected to ground as is commonly the case?

Can you provide a link to the information on the instrument amplifier calculator? I would like to read up on that jewel.

hgmjr

#### Distort10n

Joined Dec 25, 2006
429
I will have to remember to get the file tomorrow. It is an excel calculator.

#### hgmjr

Joined Jan 28, 2005
9,027
I'm looking forward to seeing it. I am interested in learning whether the two voltages V1 & V2 represent a floating voltage source or two separate ground referenced voltage sources.

hgmjr