# Need help finding the transconductance author of this book is referring to

I am reading a book "Practical Electronics for Inventors" and I was trying to understand the FETs, at several places the author has mentioned about the trans conductance that he has noted with gm. I did understood what transconductance(gm)is but I took this datasheet for a reference to figure out if that is a static parameter or a parameter that depends upon various other parameters.In the datasheet there is a mention of Forward Transfer Conductance(gfs) & Output Conductance(goss). Which one is the Transconductance(gm), and what does the other conductance mean.

• Do you mean mu as in :![enter image description here](i.stack.imgur.com/0NCEc.png) and not u... Jul 21, 2017 at 15:24
• I have the book in front of me. I'm looking around page 461-464 area and can see it mentioned in a few places. What exactly are you trying to understand? Can you improve your question by narrowing it a bit?
– jonk
Jul 21, 2017 at 15:38
• Do I need to calculate it using the formula's given on the page 181, or is it a parameter that the vendor provides me directly? and what is the forward transfer conductance and the output conductance? Jul 21, 2017 at 15:41
• Maybe we have different books. My page 181 talks about "parallel impedance." My book is over 1000 pages long. It's the 3rd edition. Are you talking about $gm=\frac{\partial I_D}{\partial V_{DS}}\bigg|_{V_{DS}=V_1}$?
– jonk
Jul 21, 2017 at 15:45
• gm = Transconductance = Forward Transfer Conductance = gfs electronics.stackexchange.com/questions/302832/…
– G36
Jul 21, 2017 at 15:50

$g_m$ is a small-signal parameter that will depend on transistor biasing. Additionally, it will vary from device to device. Circuits must be designed to not depend on $g_m$ having a precise value, same as bipolar circuits must not depend heavily on $\beta$ as this will vary with biasing and temperature. $g_{fs}$ is the manufacturer measuring $g_m$ at one specific bias condition, to give a ballpark value of $g_m$. $g_{oss}$ is related to the output impedance, where $g_{oss} = 1/r_d$, again at one specific bias condition.