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Measuring base spreading resistance
Measuring base spreading resistance
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Old 10th March 2018, 04:01 AM   #11
Mark Johnson is offline Mark Johnson  United States
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Measuring base spreading resistance
Buy a ZTX851 (NPN transistor) for $1.13, from DigiKey. Measure its Rbb using your proposed method. See if you can confirm or refute the measured value in Horowitz and Hill's book The Art Of Electronics, 3rd edition, which says that Rbb for a ZTX851 is 1.67 ohms.

There is another transistor with an even lower Rbb in TAOE_ed3. Look at the bottom most entry in Table 8.1(a). Wow.
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Old 10th March 2018, 11:46 AM   #12
jackinnj is offline jackinnj  United States
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Measuring base spreading resistance
Quote:
Originally Posted by PRR View Post
Where??



I'm well aware of re. It dominates the part until HIGH current. I'm proposing far more than a mere 1mA, so that re drops (to 0.25r at the left side of my plot) and the near-resistance dominates over the dynamic impedance.
At higher current you begin heating the part and Vt changes accordingly. But 10mA might work! Will try it on the bench.

Mark - i have a bagful of 951/851!

Last edited by jackinnj; 10th March 2018 at 11:49 AM.
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Old 10th March 2018, 03:14 PM   #13
Elvee is offline Elvee  Belgium
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You are going to measure the parameter called RBM in spice-talk, but that is a very minor part of the problem: what you want to know is the Rbb under actual operating conditions of Vce and Ic, and it will normally be very different from RBM, because it is an extreme.
In spice, other modulating factors intervene, like RB and IRB, but even the full set only gives a very simplified idea of the reality; plus, most of the time, these parameters are absent or defaulted in many models.

The interesting part of RBB is the modulated one, and to extract it, you have to remove Re, obviously, and the elusive Rbb appears as a deviation from the regular dynamic resistance 26/Ib inherent to the BE junction.
Not an easy task, because the extraction has to take place exactly at the target working conditions.
In addition, various non-ideality factors complicate the tasks
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Old 13th March 2018, 03:54 PM   #14
jackinnj is offline jackinnj  United States
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Measuring base spreading resistance
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Originally Posted by Elvee View Post
You are going to measure the parameter called RBM in spice-talk, but that is a very minor part of the problem
and it gets "hairier" with temperature.

i think that the authors of TAoE3 backed into base resistance from the noise measurement via the apparatus shown in figure 8.92
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Old 13th March 2018, 06:26 PM   #15
Mark Johnson is offline Mark Johnson  United States
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Measuring base spreading resistance
I recommend you consider Elvee's recommendations very thoughtfully; he worked extremely hard to (unsuccessfully) measure the base spreading resistance, using his Black Knight test fixture.
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Old 2nd April 2018, 05:36 PM   #16
Maxwell3 is offline Maxwell3
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I just started to look into low noise BJT transistors. I am curious if there is any pattern of which transistors would be good. In general one needs high hfe and low rbb. But since Rbb is not usually given, I looked for transistors with low Vce(sat) and low Rce. Known low-noise transistors, such as ZTX851 are typically of this kind, medium-power switching transistors with low Vce.

I came across some relatively new transistors from Nexperia, along the lines of PBSS4021NZ. They have higher hfe and lower Vce than ZTX851 and similar. Am I on the right path? Has anyone tried one of these?
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Old 2nd April 2018, 06:44 PM   #17
scott wurcer is online now scott wurcer  United States
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Parameter extraction is a difficult process when you get down to the details. As DIY I would not go further than extracting Re from the log conformance and rbb from noise vs Ic. You can use he device to amplify its own noise so getting an answer good enough for say a MC pre-amp is pretty straight forward.

Max, you are on the right path with low Vcesat there are good physical reasons that low parasitic resistances go hand in hand with it. I doubt there are any truly ultra-low noise bi-polars with 200mV Vcesat's
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Old 3rd April 2018, 12:18 AM   #18
jackinnj is offline jackinnj  United States
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Measuring base spreading resistance
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Originally Posted by scott wurcer View Post
Max, you are on the right path with low Vcesat there are good physical reasons that low parasitic resistances go hand in hand with it. I doubt there are any truly ultra-low noise bi-polars with 200mV Vcesat's
I took the data for Vcesat from datasheets and noise from TAoEV3 Table 8.1a:
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File Type: png VceSat_Noise.png (10.2 KB, 89 views)
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Old 3rd April 2018, 12:33 AM   #19
Mark Johnson is offline Mark Johnson  United States
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Measuring base spreading resistance
Geez! It's not possible to draw a horizontal line across that graph and say: any transistor with a noise less than THIS HORIZONTAL LINE, also has a very low Vcesat.

It's also not possible to draw a vertical line across that graph and say: any transistor with a Vcesat less than THIS VERTICAL LINE, also has very low noise.

A darn shame, either of those would have been a super handy rule of thumb.
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Old 3rd April 2018, 12:45 AM   #20
jackinnj is offline jackinnj  United States
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Measuring base spreading resistance
Should have mentioned that there are 3 MAT/SSM devices from ADI which have very low Vcesat but noise stats which may be overly conservative. I used the 1kHz data where I could find it. Many of these ADI devices are EOL.
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