Base emitter junction capacitance when (1-VBE/VJE) is negative ?? - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: October 09, 2023, 10:59:26 am
content: The only equation I've seen or used for Cje is Cje = Cjeo/(1-VBE/VJE)^MSo what are you really supposed to do when the bottom is a negative number to the power of something like 0.3279 ? I've never seen a warning to take the absolute value, or flipping the sign, or any mention of having to find complex roots, and then having a complex number for Cje.So what are you supposed to do ?

author: moffy
author_rank: Super Contributor
timestamp: October 09, 2023, 11:46:34 am
content: I'd be more worried when Vbe = Vje and the bottom goes to 0! Obviously not allowed, it's an approximation.

author: MathWizard
author_rank: Super Contributor
timestamp: October 09, 2023, 11:52:11 am
content: Yeah so what are we supposed to do ?

author: mawyatt
author_rank: Super Contributor
timestamp: October 09, 2023, 12:07:35 pm
content: Believe this (Vbe~Vje) is when the PN junction starts to become heavily forward biased, generally this equation is estimating the reversed biased PN junction, where "VBE" is negative, but "VBE" is replaced with the junction potential for a general solution. See pages 11 and 12 from Cal Berkeley lecture notes.Various flavors of SPICE utilized fudge factors when transitioning from reverse PN bias to heavily forward biased PN junctions, these fudge factors helped smooth out the abrupt capacitance change.Besthttps://inst.eecs.berkeley.edu/~ee105/fa14/lectures/Lecture07-P-N%20Junction%20Diode.pdfEdit: BTW you can see this behavior with a LCR meter and biasing the PN junction in both forward and reverse while measuring the equivalent capacitance. Here's and example of a diode (T4, 1N4148 like) and the equivalent series C measured at 100KHz with a small signal (50mv) and DC bias changed. Note the abrupt Cs change at +0.35 volts forward bias. The same characteristic happens with SBD but at lower forward bias (~0.05vdc).Vdc in volts Cs in PfReverse Biased0 11.1-0.2 9.41-0.4 8.31-0.6 7.52-1 6.47-1.5 5.64-2 5.06Forward Biased0.2 14.610.3 33.430.35 357.30.4 6950

author: MathWizard
author_rank: Super Contributor
timestamp: October 09, 2023, 01:49:11 pm
content: Ok I'll have a look. For my example with a BC547B VBE=0.6545V and Vje=0.65V. The depletion capacitance is ~33pF, and if I used the absolute value in the above eqn' I'd get a Cpi of ~100pF, so I'm just checking the AC model vs the BJT sim.

author: MathWizard
author_rank: Super Contributor
timestamp: November 26, 2023, 12:24:24 am
content: Here's what I was looking for, I've been using the absolute value when the bottom goes negative, and it was never equal to zero in my design's, so I never had to worry about that case.https://www.multisim.com/help/components/bjts/bjt-model/But I wonder if for the Base-Collector eqn's when it reference's Vbe</=FC*Vjc and Vbe>FC*Vjc if they mean Vbc ? or is it really Vbe ? I think the default value of FC is 0.5.That absolute value approx is not great, for a BC547B w/ 2mA and 0.657=Vbe, the Cje would be about 50% of the above model.

author: MathWizard
author_rank: Super Contributor
timestamp: November 27, 2023, 04:30:41 pm
content: Well something's still not right. Take for example a BC547B common collector buffer with IC=1.548mA, VBE=0.6471V.FC=0.9579, so VBE>FC*Vje=0.9579*0.65=0.6226VAnd the using the lower eqn would give Cje=622pF, and then with Cdiff=24.5pF, Cpi~647pFBut comparing this AC model with just using the Cjeo/(1-VBE/Vje)^Mje ~ 80pf, then Cpi =104pF , vs 647p. So if you're trying to cancel that out at some frequency, that sounds like a big difference to me.The latter version, matches the BJT sim almost perfectly in the time domain. On a FFT log scale, they still match better than I figured, but its off between 1-100MHz, and the equation that should be wrong, matches way better.So what's going on? Again, how do you people find Cpi ?

author: mawyatt
author_rank: Super Contributor
timestamp: November 27, 2023, 04:57:15 pm
content: The transitional capacitance as "seen" from the base of a bipolar transistor is a complex subject as one crosses from reverse to forward bias, enormous amounts of charge gets stored and augmented as the BC junction becomes forward biased (device saturation). This is why such complex base drive circuits are encountered in actual practice.Not knowing how good the bipolar model is, one issue is that bipolar transistors have excess base delay which often isn't modeled correctly, or at all. This is one the many issues with various models from who knows where. We used an extension of GP when designing in an early complementary bipolar process days (later the process was called CBIC V2), to help with these GP limitations.Much later there were better bipolar models than GP, or Extended GP; ViBIC, Mextram and HiCUM for example. These offer better fidelity than GP in various ways, but require much more effort and expense in generating the model, so not often available.Remember the bipolar AC model is just a linearized model about the operating point, whereas the Time Domain model should better reflect the dynamic charge transfer mechanisms (~capacitance) within the BE and BC junctions due to the highly non-linear behavior of such.Best,

author: MathWizard
author_rank: Super Contributor
timestamp: November 27, 2023, 05:30:36 pm
content: Well I've barely tried to measure the actual BJT behaviour, but in this AM TX I'm making, I better try harder to measure them and see.My overall circuit sim, is missing too many parasitic's and real world values to match all the way through. And trying to calculate all the Zin's and Zout's between stages, some stuff seems to work, but compared to the sim and my eqn's, IDK whats right and what's wrong.I haven't gotten much into the semi-conductor physics yet, a little bit though.

author: MathWizard
author_rank: Super Contributor
timestamp: November 27, 2023, 10:08:51 pm
content: Measuring the input and output of the buffer is not going well, with cheap 10x probes, long GND lead's, and aligator clip coax coming from the AWG.I wrote down a bunch of numbers, but I had the AWG cable going over my scopes touchscreen. And moving the cables upsets the measurements anyways. And looking at the numbers, the gain goes up to 1.92V/V at 85MHz, so reflections and what ever else are making this pointless.But at least I can see what works, when I get an acceptable AM TX output. Right now I'm getting 1.5Vpp RF, mixes with 1.4Vpp AF, to make about 3Vpp out of the buffer of the diff-amp modulator. But now I'm testing the power buffer before the antenna.But the corner frequency seems to be around 25MHz, but this buffer also wants to oscillate at 57MHz when I had just the probe and AWG connected but output turned off. SO IDK. I need some RF cables/accessories and a NanoVNA