2

In some texts I encounter they mention transconductance when it comes to FETs not BJTs. I have seen myself many texts and skimmed through them, the word transconductance does not appear until FETs. Before they teach BJTs and they use hfe and other parameters but not gm.

For BJTs they do not necessarily menton transconductance, instead they use hfe i.e forward current transfer ratio.

Having said that, I also saw some tutorials which use the transconductance when they teach BJTs.

Could there be a reason to employ transconductance concept more for FETs than in BJTs?

user1245
  • 3,955
  • 4
  • 37
  • 87
  • 1
    What is the gate current in a FET? – Sredni Vashtar Mar 25 '18 at 18:08
  • Good point, besides capacitive effects the gate current is zero. I can see why gm used in FETs. But what could be the reason not to use gm concept when teaching BJTs. What do you think? – user1245 Mar 25 '18 at 18:26
  • In the past, the hybrid parameters were the preferred tool to represent small signal models for transistors. Probably because they were pedagogically the easiest option after a linear circuit theory course, I don't know. They are good for plug and chug analysis, of the kind used in technical institutes. Even Millman, a staple in engineering electronic textbooks used h-params in his books. That was in the late 60s, early 70s. The gm route is better from a design point of view and it has slowly crept into lower level education as well. Maybe the widespread use of MOSFET helped in affirming it. – Sredni Vashtar Mar 25 '18 at 18:38
  • Could you also mention the mathematical relation between hfe and gm in BJT? Maybe that would yield something. – user1245 Mar 25 '18 at 18:40
  • I think you'll find transconductance was a term used to describe the properties of valves (tubes for our American cousins) – JIm Dearden Mar 25 '18 at 18:41
  • \$g_m=\dfrac{I_e}{v_{be}}\$ but gm varies with DC input *Vbe* – Tony Stewart EE75 Mar 25 '18 at 18:43
  • So can we say the only reason is that gm came to be used because the FETs do not have gate current. But later this gm started to appear in BJTs. Weird.. – user1245 Mar 25 '18 at 18:43
  • @TonyStewart.EEsince'75 Can you write an equation which relates gm and hfe in a BJT? hfe = Ic/Ib right? Im trying to understand do we really need this transconductance concept for BJTs. Because this gm does not appear in old texts in BJT chapters. It only appears in FET chapters. – user1245 Mar 25 '18 at 18:45
  • If you know \$r_\pi\$ what do you expect it will be? – Tony Stewart EE75 Mar 25 '18 at 19:32
  • @atomant I wouldn't say that's the only reason. I think we should look at it the other way around: hybrid parameters were used because they provided an easy model to BJTs small signal analysis. In his 1969 book, Millman uses gm for BJTs, but when studying low frequency amplifiers and feedback amplifiers he used h-params. Perhaps later on - I do not know when the shift occurred in mainstream education - the widespread use of MOSFETs showed the advantages of using a transconductance model and that has been made the 'default' model for small signal analysis in BJT circuits too. – Sredni Vashtar Mar 25 '18 at 20:43
  • ...I can see three advantages: works at higher frequencies, it's more apt to design (you can easily estimate gm, rpi, rbb', ro for a given polarization), and it allows you to draw parallels with mosfet amplifiers. But I am pretty confident (albeit not sure) early BJT designers used gm, not h-param. (and that's the reason I am limiting myself to comments, I do not have solid data for a full-fledged answer.) – Sredni Vashtar Mar 25 '18 at 20:43
  • \$g_m=h_{FE}/r_{\pi}\$ but with Re added, it is simpler to use \$h_{FE}\$ – Tony Stewart EE75 Mar 25 '18 at 21:53

1 Answers1

0

Universally, you need the gm (transconductance) to compute the stage amplification. For tubes, bipolars, FETs.

analogsystemsrf
  • 33,703
  • 2
  • 18
  • 46