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transistor problem

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Fahime

New Member
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.

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)

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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