I have read an interesting formula that says that

\$\mbox{Gain of an amplifier} = \dfrac{\mbox{resistance connected at the output}}{\mbox{total resistance between source and gate of the amplifier}}\$

Well, this has simplified the gain calculation and helped me avoid the small signal analysis which proves to be cumbersome in quite many cases.

But I was wondering how to manage the sign in the expression, i.e. let's say for the common source amplifier the gain has a negative sign before it but the common gate amplifier has a positive sign before its gain formula.

What is the significance of the sign?

I am here referring to the MOSFET common source amplifiers and the common gate amplifiers.

  • \$\begingroup\$ Can you add a link to the document you read that states this? The sign means it either inverts the signal 180 degrees (-), or doesn't (+, 0 degrees) An amplifier is a generic term also, but I assume you mean a FET amplifier here. \$\endgroup\$
    – Oli Glaser
    Aug 25, 2011 at 7:39

1 Answer 1


I see you are talking of the resistor from source to ground, and the total resistance on output for the gain of a common source/emitter 1 transistor amplifier.

As regards the sign, I don't think the equation takes it into account as the common source/emitter amplifier is an inverting amplifier. It is just the absolute gain.

Of course you could specify the equation as $$ -\begin{equation} \ Rdrain \over\ Rsource \end{equation} $$ to make sure you always get the right polarity.

In fact (just noticed) if you check the Wiki page on Common source the (more exact) equation is as follows:

Wiki common source equation


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