Reading Transistor Datasheet: Effect of Measurement Conditions

[kunilkuda]

Hi,

I'm pretty new to the forum. So I hope I posted on the right channel.

I want to ask about the measurement condition that is put next to transistor parameter. I have BC846A from NXP. The datasheet can be downloaded from **broken link removed**.

The datasheet said that hFE group A is typically 180, under measurement condition VCE=5V;Ic=10uA. And hFE is 110-450 under measurement condition VCE=5V;Ic=2mA. The datasheet also has hFE vs Ic graph, which is taken under condition that VCE=5V.

My question is if I'm using BC846A for audio signal, and its VCE is less than 5V, can I expect to have hFE as specified in the datasheet ?

Thanks for your help.

Regards,
-daniel

 

[kak111]

If we look at graph

Fig.2 DC current gain as a function of collector current; typical values.
we see quite linear plot in Ic < 10mA

So. we can expect that hfe is "quite linear" in that area until we are coming
near saturation area.
Fig.4 Collector-emitter saturation voltage as a function of collector current; typical values.

so in circuit we must check value Ic together with Vce

Regards KAK

 

[Miguel Gaspar]

The answear is no, Hfe depends of Temperaure, Ic, Vcc and the resistor in the collector.

 

[kunilkuda]

@Miguel : Thanks. I saw the VCE vs Ic diagram for other transistor, and I thought it won't be the same as well. I wonder how do I get the hFE that's close enough for my calculation.

 

[Miguel Gaspar]

First start by calculate the Ic (DC).
Second you can use a electronic simulator using a DC sweep for IB



R1 must be Rc+Re, and V1 the voltage source that you are using

Regards

 

[kunilkuda]

I tested 4 different BC846A. Each device has different hFE and VBE. Although it's still within the datasheet limit, it's impossible to recreate the same result with the same circuit.

So, my lesson is not to depend on transistor characteristic, and deploy negative feedback to make sure that these hFE and VBE difference won't affect the circuit amplification performance.

Regarding VCE effect, the bigger the VCE is, the less the VBE forward voltage. The less the VCE is, the bigger the VBE forward voltage. Hence, with the same VB, the Ib tends to go up, if VCE increased.

For hFE, the higher VCE is, the higher the hFE will be. The lower VCE is the lower hFE is.