Why extreme S-parameter values

At a relatively low frequency, a bjt collector does look like a big resistor in parallel with a small capacitor. If you accept the .8 puff and 200k interpretation of the vector, the 200k sounds a bit high, but then it doesn't take much of a measurement error in a 50-ohm analyzer to make big mistakes at very high impedances: 200k is 4000 times 50 ohms! All the 200k number really means is "way, way higher than 50 ohms" which is basically true here. Just 0.1% error on the 0.9995 could account for a bunch of kohms.

The 0.8 pF does sound a tad high, given that the datasheet claims more like 0.3 total collector capacitance, but Ccb is multiplied by Miller effect, so even that's not too crazy.

As you note, things closer to 50 ohms measure more accurately.

John

[snippage]

Do yourself a favor and get a good S-parameter modeling tool that you can sweep frequency-wise. If you need something free, try Smith.exe, which I think you can still download from:

You can import the S-parameters for the infineon SiGe transistor, and run parametric analysis one frequency at a time.

If you application is below 1GHz, seriously consider another transistor, such as the BFP420. I's much easier to match down there.

Some of the effects you see are related to the S12 and S21 components. when the gain is as high as it is, the S22 is for an active device, not just a BJT junction with a parallel cap. Same for the S11, definitions aside.

Bottom line: if you see something near the unit circle, pull it in. For example, if (mag) S11 is something like 0.980 outside the frequency of interest, your input match needs to be lossy at this frequency to insure stability, because the transistor isn't going to be absorbing a lot of energy.

A good reference: Microwave Circuit Design Using Linear and Nonlinear Techniques ... Vendelin, George D. / Pavio, Anthony M. / Rohde, Ulrich L.

Frank Raffaeli

Dear Colleaques,

Checking the manufacturer-specified S-parameters of the Infineon BFP640 transistor I noticed that the values at the low-frequency end look funny. For instance, at 100MHz (Vce=2.0 Ic=2.5mA) S22 = 0.9995

Hi Frank, thanks for your comment.

That is quite true, but it still should behave like an ordinary BJT at low frequencies (unless I'm missing something). In the dc limit, you cannot even bias it using standard circuitry if it didn't behave (roughly) like an ordinary BJT. I see no reason why, looking into its collector, the impedance should suddenly look capacitive at 100MHz, even though the device is optimized for several GHz operation. Heck, it is still supposed to be modellable by the Gummel-Poon, plus layout & package parasitics.

It was exactly the stability circles I was computing when I started to feel something is wrong. Even if I was designing for, say, 2.4 GHz, the device should be stable at all frequencies including 100 MHz. Then the question would be: can I rely on the published S-parameters when computing the stability circles ? Do they really describe the plain DUT without setup parasitics? If the answer is 'yes they really do' I could move forward, but I would still like to understand what is the origin of the counterintuitive value. I must be missing something because BFP640 seems not to be the only BJT exhibiting strange-looking S-params.

This is interesting - what happened?

Regards, Mikko

OK, I'm impressed by both the VNA accuracy and by the high collector impedance, since both seem to be real. I thought that SiGe transistors had pretty darned slopey collector curves.

In typical RF fashion, the Infineon datasheet gives no DC curves. It's common for even microwave fet datasheets to not furnish enough DC data to bias the critters properly

I've played around with SiGe transistors as fast switches (I work in time domain mostly, got no use for sine waves) and found them to be surprisingly slow as switches. PHEMTs are blindingly fast and behave pretty much like a simple Gm + parasitic capacitor model would predict.

John

I don't feel my problem is the lack of tools. I'm using APLAC, and for an example, you may be interested to check the computed stability circles for the BFP640 including the parasitics of the SOT343 package (as they are not included acoording to the README in the S-param package) sweeped 0.1GHz - 20GHz at

, and

My problem was that I was not sure whether I intepret properly the S-params published by Infineon. The 0.9995

Thanks for your opinion, John. I simulated the dc characteristics from the Gummel-Poon parameters given by the manufacturer (

) and zoomed in to the saturated region just to find, indeed, a ~200kohm slope there. A surprisingly large dynamic resistance IMO!

I wonder whether a particularly narrow base region typical to SiGe transistors diminished the Early effect? According to what I weakly recollect from the top of my head, the effect should go just the other way around, though.

Regards, Mikko

Hmmm, yeah, that computes to a low-freq voltage gain, with a cc load, of 20K or something. Sounds fishy.

John

I had the same impression about the slopey curves, and I'm still suspecting there may be something wrong in these results. The device has tremendous power gain at low frequencies if the high R-dynamic is real.

Regards, Mikko