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walid

Beta of the BJT

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All of us know that beta of any BJT is not constant value and it is change depending on temperature and biasing conditions...etc.
L know three methods to primarily determine its value:
1- data sheets (not always available)
2- measuring it using hfe function of a digital multimeter
3- put the BJT in a simple circuit and measure Ic & Ib then calculate it.
Now the question is:
Which method of the three can I rely on? Or you may have a different method more effective.

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Hi Walid,
If you are designing a BJT circuit then a datasheet must be available or use another transistor that has one. Then design the circuit so that it works well with any beta that is specified in the wide range on the datasheet. ;D

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hi walid!
beta of  a  transistor is determined as:
setting up a known value of Vce for the transistor.in sinking mode apply the collector current of say 5mA.sense the base current and divide the values.u will get different betas for different VCEs........thats sure.
;D

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Just a little something I found out about beta. At low currents the beta is low. As you increase the collector current the beta get's higher. Increase it more and the beta goes lower. So there is place on the current curve where the beta is at a high point. I don't know what this means but I found it out with little work. And as you know the beta is determined by two things, the VCE and the amount of collector current you have. Keep in mind too that the lower the beta, the greater the difference between the emitter current and the collector current. And as always, linearity is based on the fact that the current is multiplied by the same amount from base current to base current.

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Guest Alun

I always use lots of negitive feedback in your design to reduce the gain a lot so the beta of the transistor has little effect on the operation of the circuit.

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And as always, linearity is based on the fact that the current is multiplied by the same amount from base current to base current.

Linearity of a transistor is based on a lot more than only its beta.

Take a transistor with a 10k collector resistor and a 20V supply. Bias it so that the collector voltage is halfway, at 10V. Its Q-point is at 1mA.
Now feed a sine-wave voltage directly to the base of the transistor until it nearly saturates on the positive peaks of the sine-wave. You will see that when the sine-wave swings in a negative-going direction that the collector voltage swings much less.
It takes a positive-going voltage of only about 20mV for the transistor to saturate, but more than 50mV in the negative-going direction for it to be near cutoff. The collector voltage swings about 5V in each direction. That is extremely non-linear! :o

A transistor's transconductance non-linearity is caused by the logarithmic relashionship of the collector current to a linear change in its base voltage.

post-1706-14279142352743_thumb.png

post-1706-14279142352954_thumb.png

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DATASHEETS LIST BETA IN THE SATURATION REGION.........
it is a convention for all manufacturers.

Hi Prateek,
I am sorry but I disaggree with you.
When a transistor is saturated then it takes much more than the higher-voltage beta for its saturation voltage to be low.
The 2N3904 has a guaranteed minimum beta of 100 at 10mA of collector current and a Vce of 1.0V, but when guaranteeing its max saturation voltage the manufacturer uses a base current of 1/10th the collector current whiich is 10 times more than its listed beta.

post-1706-14279142355163_thumb.png

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Hi AudioGuru,
If I select the above transistor and if the Ic -Vce 'test conditions' don't exist in my circuit i.e I have my own Ic Vce depending on my curcuit conditions then what beta should I consider?

Hi Shekhar,
In the above transistor, beta won't be any lower than it is with a Vce of only 1V, unless the transistor is used as a saturating switch when it needs much more base current to saturate well.
Design the circuit so it works with the range of beta of the transistor by using a voltage divider for its base voltage and/or negative feedback from its collector and/or negative feedback from an emitter resistor. ;D

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hi audioguru!
in saturation region i guess a transistor has a beta which is lower than its value in active region. for hard saturation beta in saturation ,beta in saturation is generally 10 times less than in active region.do u agree with this?
base current is higher than usual once the transistor is in saturation.......


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Hi Prateek,
Some experts say that a transistor's base-collector junction must be forward-biased for a transistor to be fully saturated. The forward-biased junction needs extra current in addition to beta for its base-emitter junction.

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hi audioguru!
i carry out the Q point and hence the beta analysis for a transistor only in CE region.
cause i use it mostly as my amplifier.
for such a case i found the results which i just told.i too believe that a transistor is saturated when VCE is 0 i.e we move towards forward bias from reverse side.
Ic is maximum.

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