¿ªÔÆÌåÓý

  1. LTspice
  2. Topics

Estimating Base spreading resistance for a bipolar transistor via LTspice

 

Hello friends,
?
Is there a way of estimating the base spreading resistance of a bipolar transistor by using LTspice? Perhaps via getting the transistor's S-parameters?
Many thanks in advance!
 

For a BJT model, the parameters Rb, Rbm and Irb determine the base spreading resistance. The resistance is Rb at low currents and falls to the value of Rbm as current increases past the value of Irb.
?
The base spreading resistance is not present in LTspice unless it has been specified in the model. Many models use Rb=10 as a default value. There are many thousands of garbage models in existence which bear only a passing resemblance to the BJTs they are named after (likely the result of a failed attempt to batch process a dataset into SPICE models).
?
Rb can be estimated by measuring the base voltage noise of a transistor at the collector current you plan to use it at. This does not always give the same value of Rb as the high frequency estimation method where you determine Rb based on the rise in transconductance as frequency approaches Ft (this is possible with s-parameters).
?
There are still other methods of extracting Rb that are given in multiple long PDFs about the SGP model, although I can't find any of them at the moment.
?
Here is a page with a calculator for using the noise method:
http://www.dicks-website.eu/low_noise_amp_part4/part4.html
 

I meant to say the fall in transconductance as frequency increases.
 

¿ªÔÆÌåÓý

Do you want to experiment with this in relation with noise voltage?
You could probably build a theoretical model circuit of your transistor and adjust an explicit Rbb' until you have coincidence between the simulation and the real-life behavior.

Le 22/02/2025 ¨¤ 18:31, Kamran Ahmed via groups.io a ¨¦crit?:

Hello friends,
?
Is there a way of estimating the base spreading resistance of a bipolar transistor by using LTspice? Perhaps via getting the transistor's S-parameters?
Many thanks in advance!
 

Thank you keantoken. I am interested in the Rbb at high current values. Would the s-parameters help for this estimation? If if does, then please provide some guidelines on how to do so. I am already familiar with deriving s-parameters for a bjt in LTspice.
 

Hi Jerry, I am investigating a NMOSFET based class E oscillator. The BJT will be used as a driver for the mosfet and part of the feedback loop will include the rbb and the cap_be of the BJT in series with the input capacitance of the mosfet. Hence I am after the high current version of the rbb.
 

In that case I would suggest capturing waveforms on a transistor driving a dummy load, and matching them by modifying Rb for a model that is known to already have correct high-frequency behavior. One step further would be to get the low-current Rb with the noise method and then adjusting Rbm and Irb to match the waveform.
 

"In that case I would suggest capturing waveforms on a transistor driving a dummy load, and matching them by modifying Rb for a model that is known to already have correct high-frequency behavior. One step further would be to get the low-current Rb with the noise method and then adjusting Rbm and Irb to match the waveform."
?
A practical & useful suggestion. Thank you!
 

I am wondering whether the noise at higher current values won't be affected by the warming of the device.? I know this to be a limiting affect for JFETs.
 

¿ªÔÆÌåÓý

When temperature increases, the current resulting from the reverse-biased gate-drain junction increases exponentially, so yes, noise increases with temperature.

Le 25/02/2025 ¨¤ 16:36, Jack Walton via groups.io a ¨¦crit?:

I am wondering whether the noise at higher current values won't be affected by the warming of the device.? I know this to be a limiting affect for JFETs.

1 - 10 of 10