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current gain @ small signal and high signal

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faustoleali1974

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BJT current gain @ small signal and high signal

Dear Sirs, I'm looking for an explanation of why, usually in some exercises of circuit amplification, the bjt is given with two different current gains, one @ small signal and one @ large signal (polarization) -> why is B-small-signal (hfe) different (almost always is LARGER) from B-large-signal (Hfe) ?

Thank you in advance

Fausto
 
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Hello,

If you look to the datasheet of (for example) a BC847/547, then you will see that the small signal hfe depends on collector bias current.
hfe@ 1uA > 100 (guessed).
hfe@ 1mA = 190
hfe@ 10mA = 185
hfe@ 100 mA = 80.

If your momentary current (that is AC peak + bais) peaks significantly above 10 mA, Ic is no longer 180*Ib.

In many large signal application, the peak collector current has excursions that are in the region where hfe is low and therefore the "effective" hfe for large signal applications is less then for small signal applications where the bias current is in the region of high hfe.

Also when using transistors at low current (say below 10 uA), the small signal current gain can be significantly below the value for moderate current.

Nowadays there are small signal transistors (not darlingtons) that have exceptional high hfe and low saturation voltage, even at current > 1A.
 
WimRFP said:
Hello,

If you look to the datasheet of (for example) a BC847/547, then you will see that the small signal hfe depends on collector bias current.
hfe@ 1uA > 100 (guessed).
hfe@ 1mA = 190
hfe@ 10mA = 185
hfe@ 100 mA = 80.

If your momentary current (that is AC peak + bais) peaks significantly above 10 mA, Ic is no longer 180*Ib.

In many large signal application, the peak collector current has excursions that are in the region where hfe is low and therefore the "effective" hfe for large signal applications is less then for small signal applications where the bias current is in the region of high hfe.

Also when using transistors at low current (say below 10 uA), the small signal current gain can be significantly below the value for moderate current.

Nowadays there are small signal transistors (not darlingtons) that have exceptional high hfe and low saturation voltage, even at current > 1A.
Click to expand...

Thank you sir now it seems a little bit clear to me, BJT are so complicated to me !!
 

Hi Fausto,

In a few words:

hFE=IC/IB : DC current gain

hfe=dIC/dIB : AC, or incremental, or small signal current gain

Regards

Z
 

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