help with finding the beta

Discussion in 'Homework Help' started by herbgriffin, Feb 24, 2011.

1. herbgriffin Thread Starter New Member

Feb 14, 2011
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how to find beta given minimum=40 , maximum = 120...
Ic = 150 mA , Vce=10V ...how can i find the beta , and if the specific beta is found will the Ic and Vce values change.?

2. Georacer Moderator

Nov 25, 2009
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Can you be a bit more specific about your exercise? It would be ideal if you posted it.

Theoretically, you can find the value of beta, with Ic and Vce known, from a corresponding graph like the following, but which is specific to each model of BJT.
You find the corresponding value of Ib, and the use the equation Ic=b*Ib.

3. Audioguru Expert

Dec 20, 2007
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1,185
You said that the minimum beta is 40 and the maximum beta is 120.
But you want to know the beta at 150mA and 10V.
The datasheet has a graph of "typical" beta but you can't buy a transistor with "typical" beta because some are low and some are high as in the first sentence.

A transistor circuit can be designed so that a transistor with a low beta or a transistor with a high beta makes little difference. But some people design a circuit that works only with one beta number then many circuits won't work.

4. Ron H AAC Fanatic!

Apr 14, 2005
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It is actually specific to each individual transistor, not just each model. Perhaps this is what you meant, as I'm pretty sure you know this.

5. Georacer Moderator

Nov 25, 2009
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Well, you have to start somewhere. If you want to make use of the above chart, you have to accept it as a good description of your current transistor model.

How do you suggest of making use of such a diagram? Building a new, correct diagram for each transistor by taking measurements?

6. Audioguru Expert

Dec 20, 2007
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You should never use a simplified model of a transistor to learn how to bias one. Then the hFE is a certain number and not a range of numbers and the Vce is also a certain number not affected by temperature.

A student would simply bias a transistor with a single resistor from the positive supply and not use an emitter resistor. Then the transistor is a thermometer instead of an amplifier.

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7. Georacer Moderator

Nov 25, 2009
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Hey, I am a student too, even though many people in the forum seem to forget it, so don't think too highly of me.

I know that simple biasing techniques are only for educational purposes. My question was if the Ic-Vce chart has any useful information for implementation purposes.

8. Ron H AAC Fanatic!

Apr 14, 2005
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674
If it's typical data, then no. Minimum beta is about the only useful beta data (I'm a poet!) point for reliable linear circuit design, and even that is almost useless for designing saturated switches. Most transistors will prescribe that saturation voltages are specified for Ic/Ib=10, and occasionally, 20.
The V-I curves are helpful in getting an understanding of how transistors work, but not for circuit design.

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