BJT modes, inconsistent information - Page 1
Forum section: electronics / beginners

author: petert
author_rank: Regular Contributor
timestamp: July 27, 2020, 07:07:05 am
content: Hello,I was revisiting the various modes of NPN transistors, and got differing information from various sources. I'll quote two of them:Falstad has this source code to determine which mode a BJT is in:(This generates purely informative strings for the GUI, it is not used for the actual simulation, so I wonder if it is correct.)Code:pnp = (pnpflag) ? -1 : 1; // pnp == 1 for a npn transistorif (vbc*pnp > .2)arr = vbe*pnp > .2 ? "saturation" : "reverse active";elsearr = vbe*pnp > .2 ? "fwd active" : "cutoff";So, it assumes that Vbc is forward biased when it is larger than 0.2V, but it assumes the same for Vbe. I would expect Vbe to be forward biased only at 0.6-0.7V.At what voltage is Vbc really forward biased and how does it relate to Vce(sat)?Saturation mode in SparkFun's tutorialis described as such:QuoteFalstad seems to either ignore Vce(sat) or (falsely?) use it as threshold voltage.Could somebody write a simple if then else statement that actually states the necessary conditions for saturation, active mode, and cut-off mode summarized?

author: petert
author_rank: Regular Contributor
timestamp: July 28, 2020, 10:42:17 am
content: Ok, since it seems the question is not clear enough, let me ask it differently.Slide 25 here:https://www.slideshare.net/NapexTerra/lecture-8-bjt1says that:QuoteAnd alsoQuoteYet in saturation mode it holds that, VCE = VCE(sat) = 0.2 V.But the "Collector Characteristic Curves" on slide 21 clearly show that VCE can reach much higher than VCE(sat).And since VCE decreases as IC (and IB) increases, what exactly do they mean with VCE reaching its maximum?Confusingly VCE(sat) is given as typically 0.2V on slide 25, yet as 0.7V on slide 21 (where it delimits the saturation region in the graph).Graph in slide 26 names this voltage that delimits saturation from active region VCE(sat).I saw similar confusing information in other material, but this summarizes nicely all in one place.Can someone clear this up please?

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2020, 01:01:02 pm
content: It should be once VCEreaches its minimum value, the transistor is said to be in saturation.VBEis notmally higher than the usual 0.7V, when the transistor is in saturation.

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2020, 04:39:18 pm
content: Quote from: petert on July 27, 2020, 07:07:05 amFor saturation that is true but the collector current definitely exists at lower Vbe voltages. See below.QuoteVbc could be zero volts and still the collector current follows the base-emitter voltage like it should. Translinear circuits usually operate with Vbc=0. This falls apart at high currents where the base spreading resistance contributes a significant part of Vbe.

author: petert
author_rank: Regular Contributor
timestamp: July 28, 2020, 04:41:12 pm
content: Thank you.So in saturation mode VCE should not exceed VCE(sat)? Will VCE(sat) vary depending on some factors (such as Ib or Ic), or does it stay constant?The slides suggest it would be constant, but the DC load line graph in slide 26 makes me wonder.Is there a voltage drop across CE, similar to VCE(sat) when the NPN-transistor is in active mode?Collected links just for future reference (the questions remain):Info on how to create a stable bias independent of beta, and other useful details:https://home.zhaw.ch/~kunr/Elektronik/Presentations/Bipolar_Lect7.pdfandhttps://home.zhaw.ch/~kunr/Elektronik/Presentations/Bipolar_Lect8.pdfSecond answer on electronics-stackexchange: "How do I saturate an NPN transistor?"BJT Small Signal AnalysisVarious quick notes:https://www.nhn.ou.edu/~bumm/ELAB/Lect_Notes/BJT_FET_transitors_v1_1_1.htmlBJT Tutorial for active and saturation mode with many examples:https://www.bu.edu.eg/portal/uploads/Engineering,%20Shoubra/Electrical%20Engineering/833/crs-14354/Files/Tutorial%20on%20BJT%20-%20Part%20(1).pdfhttps://www.bu.edu.eg/portal/uploads/Engineering,%20Shoubra/Electrical%20Engineering/833/crs-14354/Files/Tutorial%20on%20BJT%20-%20Part%20(2).pdfElektronik Kompendium: Übersteuerung und Sättigung (Transistor)Transistorkunde (Physik Grundpraktikum - Uni Saarland)

author: Benta
author_rank: Super Contributor
timestamp: July 28, 2020, 06:49:55 pm
content: It's a typo in the foil pack. The text on slide 25 should read:"Once VRC(or ICRC) reaches its maximum value, the transistor is said to be in saturation."