# Reading transistor datasheet - Reg

#### Electronic_Maniac

Joined Oct 26, 2017
253
Hi all,

I have understood the theoretical portion of the transistor. Now, I am trying to select a part and figure the actual current calculations in my circuit.

I have 30mA of collector (load current). This is the transistor I am choosing for my design : https://www.onsemi.com/pub/Collateral/MMBTA05LT1-D.PDF

MMBTA06

So, Once I have figured out the collector current through my transistor, I need to calculate my required base current.
But while calculating the required base current, how will I choose the Hfe value? In the datasheet it is given as 100 (For Ic=100mA and Vce=1.0V)

But my actual Ic is just 30mA. How will I fix my gain now?

Thanks.

P.S. It would be better if you give me some direction of steps/approach once I complete this gain calculation.

#### ericgibbs

Joined Jan 29, 2010
13,654
Morning EM,
Is the transistor being used in its active region or as a switch.?

E

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#### Electronic_Maniac

Joined Oct 26, 2017
253
Morning EM,
Is the transistor being used in its active region or as a switch.?

E
Morning @ericgibbs .

Sorry one small question before that. What will be the difference in which data I have to look for in the transistor datasheet if I operate in active or switch? Can you tell me one circuit example which can help me understand in situation I have to operate under active or as a switch.

And regarding the graphs you have attached, that was my doubt exactly. In the Ic Vs Hfe, the Vce is fixed as 1.0V. How will I take this value based on some fixed Vce. My circuit will have different Vce right? So, how can I take this data from this graph which has some fixed Vce ?

And regarding the Ic Vs Vce graph, How will I choose the values from that because the gain is fixed at 10 right?

Can you please tell me the order of choosing the parameters and values.

Sorry for long paras and questions. Unable to understand.

#### ericgibbs

Joined Jan 29, 2010
13,654
hi,
Those two graphs show the Active case Gain approx 200 , saturated case Gain assumed 10.
In saturated case, with Ic =30mA, make the Ib =3mA

E

#### AlbertHall

Joined Jun 4, 2014
11,506
You can't be too precise in any calculation using the gain figures as any figure given can only be 'typical', 'max', or 'min'. You won't know exactly what gain any particular transistor will have. You need to design around that.

#### LvW

Joined Jun 13, 2013
1,300
You can't be too precise in any calculation using the gain figures as any figure given can only be 'typical', 'max', or 'min'. You won't know exactly what gain any particular transistor will have. You need to design around that.
Yes...I agree - and, more than that:
Normally, for linear amplification purposes, we add DC negative feedback to the transistor circuit (in most cases: Emitter resistor RE).
This method has tremendous benefits: It makes the whole circuit much less sensitive to the transistor parameters (current gain, Vce), which very often are known with great uncertainty only (tolerances of 200...300 %).