Base Spreading Resistance from S Parameters

Why do you care?

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 Thanks, 
    - Win

Can't you measure it?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Start with noise figure at lowest frequencies; note that is roughly constant at low end. Presume that is roughly the same at "DC" frequencies....10KC-200MC. So at datasheet collector current(s) there is a close equivalence of NF at "DC" frequencies VS RF (1 giggle & up).

Hell one can get crazy and measure at 10Kc and specified collector current for this.

Or more crazy and measure Rbb at 10uA, 100uA, 1mA, 10mA and calculate Rbb' from the resulting curve and then DERIVE the NF! "In the daze" umpteen years ago i used to do this stuff and have forgotten a lot of the ow and why.

Make lotza measurements for a given "related" characteristics, over a number of parameters and you will see. Parameters? current, frequency, NF, Rb.

Most NPNs seem to have useful relationships that are accurate enough to be very useful.

I would not doubt that the AOE book would have some of this stuff.

PS: DAMN! I should get one.

Yes, indeed we do, a substantial amount, including test circuits, plots, and masses of measured BJT r_bb' data.

Yep.

--
 Thanks, 
    - Win

How did you measure rbb?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Assume Ebers and Moll is dead accurate, and note that the base resistance lowers the voltage across the actual base-emitter junction by the voltage drop it produces when the base current is running through the base resistance.

I once worked on a bjt hyperbolic function generator that included a compensation for base resistance - the notebook of the guy who developed the circuit was an interesting read.

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Bill Sloman, Sydney

It's explained in the text, a time-honored way. Carefully measure e_n vs current, the excess noise is due to r_bb'. If the measurement is extended to a high enough current, e.g., 5 to 50mA depending on the transistor, the e_n value asymptotes to the noise in r_bb'. Formulas in Chapter 8. I did the calculations on spreadsheet graphs.

--
 Thanks, 
    - Win

That's a poor way to do it unless r_bb' is quite high. I found transistors were down in the 1 to 2-ohm region, page 501. And some have quite high beta, e.g., 2SB737, r_bb' 1R7, beta 270.

--
 Thanks, 
    - Win

Back in the early days while we were floundering trying to obtain Spice models I devised such a DC approach... but r_bb' was on the order of 100-300 Ohms back then... emitter size 25um x 37um ;-)

(My son, Aaron, while still a student at UofA, wrote me an executable (written in Pascal), that would churn all the data and spit out all the DC model parameters :-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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"Those [of us] who dream by day are cognizant of many things which 
 escape those who dream only by night"  -Edgar Allan Poe

OK, I should have read more.

I was thinking that one could just measure the b-e diode curve and separate out the exponential and ohmic components. I'd hand-wave guess that the BFT25 hundreds of ohms.

BFT25 must be a tiny junction. Diode leakages are femtoamps and capacitances are sub-pF. Damned good diode for 35 cents... blows away a PAD1.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I need this value to do noise calculations.

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I am not equipped for this, not do I have the time. If I cannot get it from spec sheets then I will have to guess a typical value.

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A noise figure is based on the ratio of equivalent input noise power contributed by the amplifier divided by the input noise power, which is converted to dB. If I were to use Figure 9 and 10 on page 6 of the specification what input noise should I use to multiply this ratio to get the equivalent input noise power contribution from the transistor? Will the result be a voltage noise density?

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Derived from NF plotted over IC; generally NF will go up as IC (actually Ib) goes up the increase nonlinearity will give the clue.

• For most operating points,one finds that it is VERY accurate. That is where measuring actual values over a range will illuminate the internal values (Rbb', NF, GBW, etc).

I have now a partial answer to this question. The NF in those charts is based on a ratio of output noise power density to the input thermal noise power density. The input thermal noise power density is 4kT where k is Boltzmann's constant and T is temperature in Kelvin.

Now I can calculate an output noise power density given the NF in those charts of that transistor. Those charts in Fig 9, and Fig 10 of page 6 of that PDF do not give a temperature the NF was measured at, but the numeric NF on page 3 does, so I can assume those charts are for the same temperature which is 25C.

There is one question remaining before I can apply this data to the equations in this PDF:

. Into what load did the BFT25A have when the NF measurement was done? Can it be assumed to be 50 ohms, and would this be the valid resistance value to use for R_F in equation 4.6 of the lab PDF?

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In AoE III, we go into detail about the use of NF data, but I can tell you the NF values on most transistor data sheets are wildly wrong, to the point of being useless. Thankfully real transistors are generally far better.

For example, you can take some carefully-measured BJT e_n data (page 501, about 50 different types) and use the formulas to convert to NF, and compare with the datasheet values. You'll be shocked.

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 Thanks, 
    - Win

Thanks, Win. For me, Chapter 8, LOW-NOISE TECHNIQUES, and all the other chapters that reference it, are the most important parts of the book. There is information that is available nowhere else, and it took tedious and painstaking work to make the measurements and document them. I think few engineers would have the endurance and patience needed to do this work. It took a focus that is unfortunately lost today, and nobody else could afford it.

Well done. Superb work. It will never be duplicated.

Thanks.

I think most bjt nf measurements are made with tuned matching networks on the input and the output. Who knows what they were?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com