transistor problem

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Fahime

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Hi,
You know that a BJT transistor has different HFE. For example between 70-700 in its active region. You can also measure hfe by a multimeter.
Now my question is that:
when I'm designing a bias for a transistor to operate in its active region, which BETA(HFE) should I insert in the calculations?
The input voltage is a positive square wave.( between 0 and Vp)

Please help me. I'm confused!
 
I would pick either the arithmetic mean or the geometric mean of the extreme values. I would also caution you that designing a circuit which depends on the beta of a transistor is extremely risky.
 
I design transistor circuits using the minimum beta for a transistor. Then I am certain that every good transistor will work in the circuit.
Circuits that are designed using the median beta will have many of them not work properly.
 
I agree with Audioguru,
I too have designed lots of transistor circuits and design using the minimum
beta.
 
k7elp60 said:
I agree with Audioguru,
I too have designed lots of transistor circuits and design using the minimum
beta.
Click to expand...

Yes, it's standard procedure, often you can buy different versions of the same transistor (usually with a different letter at the end), in different gain groupings - so choosing a particular transistor is often part of the process as well.
 
Fahime said:
Hi,
The input voltage is a positive square wave.( between 0 and Vp)
Click to expand...

If you're trying to amplify a low level square wave you may consider a saturated bjt? Why are you biasing the transistor in the active region? Which are the expected input/output levels and waveforms?
 
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