# Common-Source Voltage Gain Estimate

1. May 30, 2013

### hisotaso

1. The problem statement, all variables and given/known data
Working through some homework problems we are given the desired gain and input resistance, and are asked to choose what type of amplifier (mosfet or bjt) and what configuration would be ideal. The solutions are given. In the solutions, and in the design example in the text, the author uses the common-source voltage gain estimate I have listed in the "relevant equations" section.

In the design example, the author says "referring back to table xx.xx we see our design estimate for voltage gain of a common-source amplifier is..." and uses the expression I have given. Also, in the given solutions (from the author), in each example where a common-source is considered, that expression is used as an estimate. Now, that expression is not in that table, nor is it anywhere in the chapter where the common-source amplifier is discussed and it's various values derived.

My question is if any of you have seen this expression, is it a valid expression, and does it make any difference if there is a source resistor or not?

(I should add that this would be just one more of many many errors/oversights I have found so far)

2. Relevant equations

$\frac{VDD}{VGS-VTN}$

3. The attempt at a solution

In the analogous case for the common-emitter amplifier, the estimation of 10*Vcc is only valid if there is no emitter resistor. My feeling is that the expression given above is only valid when there is no source resistor.

2. May 31, 2013

### hisotaso

I found an answer to the question, it is a valid approximation.