Hi All,
Could you please explain me how to measure the Early Voltage of a BJT experimentally in lab (Undergraduate Electronic Devices Lab)?
I am also interested to know the different parameters of a BJT (in addition to Early Voltage) that can be measured experimentally. The typical input and output characteristic curves are common experiments, but what additional measurements / observations can be added to these experiments with BJT?
Regards,
Raghu
The Forward Early voltage vAF is determined by extrapolating the curve of the IC versus VCE in the active region of operation where IC>= ICSAT till intercepts the the VCE axis in a voltage equals the forward Early voltage.
The equation of this straight line can be expressed as:
IC= ICSAT + VCE / rCE, where rCE IS the inverse of the slope of the I-V characteristics.
It is the incremental collector to the emitter resistance in the active region.
By definition, at IC=0, VCE= VAF, It follows that:
VAF= ICSAT X rCE.
WHICH IS THE PROOF OF THE ABOVE RELATION BY MUSTAFA FAYEZ..
Hello Raghu, you can calculate the early voltage from certain measurements as follows: you can draw the curve between Collector current and
you can draw the curve between Collector current and Collectro-Emitter voltage. The slope of this curve is the output resistance which equals (Va+Vce)/Ic, you can then caculate Va since u get Ic and Vce from the curve.
Check Sedra
http://www.amazon.com/Microelectronic-Circuits-Electrical-Computer-Engineering/dp/0199339139/ref=sr_1_1/181-4374515-8442007?ie=UTF8&qid=1450862858&sr=8-1&keywords=sedra
The Forward Early voltage vAF is determined by extrapolating the curve of the IC versus VCE in the active region of operation where IC>= ICSAT till intercepts the the VCE axis in a voltage equals the forward Early voltage.
The equation of this straight line can be expressed as:
IC= ICSAT + VCE / rCE, where rCE IS the inverse of the slope of the I-V characteristics.
It is the incremental collector to the emitter resistance in the active region.
By definition, at IC=0, VCE= VAF, It follows that:
VAF= ICSAT X rCE.
WHICH IS THE PROOF OF THE ABOVE RELATION BY MUSTAFA FAYEZ..
Thank you Mustafa and Abdelhalim.
From your replies, I get that ICSAT is the intercept on y-axis (Ic axis). It will be varying for different values of base current in the output characteristics. If so, how is the Early Voltage maintained constant? (Assuming the slope of the curve is almost constant)
The slope of the curve is not constant. Experimentally the slope of IC versus VCE in the active region increases linearly with the ICSAT. This is proved also by direct measure of the small signal parameters of the transistor including hoe which is 1/ rCE. To this fact please see the link:http://www.zen22142.zen.co.uk/Theory/tr_model.htm
So, VA is constant with ICSAT and the and the parameter which changes is the rCE. such that rCE =VA/ ICSAT.
There is also a theoretical proof of this relation
In the active region we can express the collector current by:
IC= qDdelta n/ wb with q the electronic charge, d the diffusion coefficient of the minority carriers in the base delta n is the excess minority electrons at the edge of the base region adjacent to the emitter. and Wb is is the neutral base width.
If VCE increases Wb decreases and the slope of the excess minority carrier increases causing the early effect by increasing the collector current above its maturation value
Accordingly one can express IC as
IC= ICSAT( 1 + VCE/VA), DIFFERENTIATING THIS RELATION WE GET tCE= VA/ ICSAT.
wish you success. If you wish we can discuss the other spice parameters of the BJT.
- Badges
- Science topic