# Message Boards

Posted 1 month ago
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 This may be a dumb question, but I often need to solve expressions for something other than a variable. As a trivial example, say we have a common-emitter amplifier:Vout = Vsupply - (Vin - Vbe)((hfe + 1)/hfe) * RC/REAnd I want to solve it for the DC transfer function Vout/Vin. Can Mathematica do this, at all?Now, I understand there is actually no solution here, but a small change makes it solvable (and still easy to reason around):Vout = - (Vin - Vbe)((hfe + 1)/hfe)*RC/RENow, this simple example I can solve easily. But there are complex expression created in cascades of stages and ladders that get a bit more complex in hurry... But even being to solve the simple problems would be nice as a "calculator" function. I can calculate square roots and look up logarithms in tables myself also, but I sure prefer to work with a calculator.Any ideas? Maybe I just missed something obvious?
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Posted 1 month ago
 A replacement can do something like that, unless I misunderstand you: In[7]:= Vout == Simplify[Vsupply - (Vin - Vbe) ((hfe + 1)/hfe)*RC/RE /. Vin -> Vout/DCTransferFunction] Out[7]= Vout == ((1 + hfe) RC (DCTransferFunction Vbe - Vout))/( DCTransferFunction hfe RE) + Vsupply You can leave it to Mathematica to find the replacement with Solve: Vout == Simplify[ Vsupply - (Vin - Vbe) ((hfe + 1)/hfe)*RC/RE /. Solve[Vout/Vin == DCTransferFunction, Vin][[1]]] 
 Is the intent to eliminate the variables of interest i.e. Vout/Vin->?