[SOLVED] transistor gain calculation

[kdg007]

I have seen a lot of calculations where they take GAIN as 10 or 20 instead of the original value in the datasheet.For example 2n2222 transistor

VCC = 5 ,Led(Vf = 3v).
For Ic = 20mA,Rc = [VCC-VF(LED)-VCE(SAT)]/Ic = [5-3-0.25]/20mA = 87 ohms.
Ib=Ic/Gain = 20mA/110 = 0.18mA
Rb = [Vcc-Vbe]/Ib = [5-0.83]/0.18mA = 23k ohms.

If my calculations are right,i took gain as 110 as per the datasheet.but some people take gain as 10 or 20 ... i want to know why is that ?????

 

[etmabreu]

I have seen a lot of calculations where they take GAIN as 10 or 20 instead of the original value in the datasheet.For example 2n2222 transistor

VCC = 5 ,Led(Vf = 3v).
For Ic = 20mA,Rc = [VCC-VF(LED)-VCE(SAT)]/Ic = [5-3-0.25]/20mA = 87 ohms.
Ib=Ic/Gain = 20mA/110 = 0.18mA
Rb = [Vcc-Vbe]/Ib = [5-0.83]/0.18mA = 23k ohms.

If my calculations are right,i took gain as 110 as per the datasheet.but some people take gain as 10 or 20 ... i want to know why is that ?????

If you assume a very very very low gain, you can be sure your transistor will saturate as wished.
If you go for the datasheet, you may get a surprise since the Ic/Ib gain is quite variable from piece to piece.

 

[chuckey]

If you take a low gain, you can be certain that the transistor saturates, if you take your 110 and Hfe is only 80, then the transistor will not saturate, it may overheat and the LED will be dim.
Frank

 

[kdg007]

so how will we know that 10 or 20 value is ok ? we can also take 40 for sure in this case right ?

 

[etmabreu]

so how will we know that 10 or 20 value is ok ? we can also take 40 for sure in this case right ?

Yes. Usually, as a rule of thumb, you think: for a low-power transistor, current gain is about 100 or 200. Making it 10 or 20 is just the 1/10 rule, which is somewhat overkill.

 

[Baas Rietrot]

I know this post is already solved...

Adding a degeneration resistor R_E might pin the gain.

Using A_v = -g_mR_C / (1 + g_mR_E)

and for g_mR_E >> 1 we can assume A_v = -R_C / R_E

With g_m out of the way the effects of β is somewhat removed and the effects of temperature on the circuit.

g_m = I_C/V_T