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.
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\$\begingroup\$ Do you mean mu as in : and not u... \$\endgroup\$– Solar MikeJul 21, 2017 at 15:24
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1\$\begingroup\$ 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? \$\endgroup\$– jonkJul 21, 2017 at 15:38
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\$\begingroup\$ 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? \$\endgroup\$– MaNyYaCkJul 21, 2017 at 15:41
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\$\begingroup\$ 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}\$? \$\endgroup\$– jonkJul 21, 2017 at 15:45
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1\$\begingroup\$ gm = Transconductance = Forward Transfer Conductance = gfs electronics.stackexchange.com/questions/302832/… \$\endgroup\$– G36Jul 21, 2017 at 15:50
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2 Answers
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\$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.
answered Jul 21, 2017 at 16:01
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The 'gm' is how tubes and bipolars and mosfets achieve gain; the output change in current (per input voltage change), multiplied by Zload, is the voltage gain.
Often, resistors in cathode or emitter or source are used, to make the 'gm' more nearly constant. Or global feedback is used, tho that is very susceptible to parasitic capacitance and in extreme phaseshifts will produce oscillations.
answered Jul 21, 2017 at 16:51