A simple level converter. It assumes the +4V supply can sink current, so why not just have 2 resistors between +4 and +10 and use the opamp as a buffer?
I know what you mean. This was just someone's idea of an example but he should have thought of a better one. I wondered if there was any justification for using 4 resistors.
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The op-amp version has a -2 voltage gain for variations in the +4 line to the output. If you want that, you can't get it with passive components.
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If you use a pair of resistors to get 7V out of the 10V and the 4V, then when the 4V changes by dV, the output will change by significantly less than dV and in the same direction.
If you use the setup you show and the 4V changes by dV, then the output will change by -2*dV. If what you need is to pick out and amplify some signal that's riding on the 4V, then the pictured circuit would be the way to go.
You don't say what the circuit is from or what you want to use it for, so it's impossible to answer your "why not". My suggestion was just that -- one possible reason "why not", which you then have take and exercise your own brain cells to see if it makes sense or not.
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My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
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