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help in designing circuit.

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allegro

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I would like to have something genreal clear (to me).
I have a NPN BJT (PN2222A), which its IC should be 80mA.
I would like to have minimal VCE, so that BJT's power consumption would be minimal and that it will have little effect (as possible) on the circuit.
What value then should be IB?
The graph that i miss in order to answer this question is VCE Vs hFE in saturation region.
 
The answer depends on what circuit the transistor is put into. For example, do you intend the transistor to operate as a switch, or as a linear amplifier or something else? The VCEsat in your circuit (assuming a common emitter switch configuration for now) is a function of the load resistor, the power supply voltage put to the load resistor, the base current, the HFE and to some degree the temperature. If you want to consume little power, then make your load resistance relatively high so that very little collector current needs to pass through the transistor. In this case you can find the HFE from the data sheet and interpolate or estimate from the values given or the graph given. If you intend your transistor to operate as a saturated switch, you should overdrive the base current to some extent to insure complete saturation for lowest VCEsat. I could suggest some values but this is a waste of time unless you post a schematic diagram.
 
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Its a simple diagram,
The BJT is a switch.
7V Vcc goes to collector through 87ohm resistor (this is the load you said).
3V inputs goes to base through resistor RB which i dont know what value to choose.
But i dont understand why cant it be a general rule?
I dont want particular values, i want to understand so next time i would know by myself.
 
At 80 mA load, the ideal PN2222A saturation voltage is a little bit less than 0.1 volt. The data sheet only specifies the saturation condition of Beta = 10. Using this number, you would want to provide 8 mA to the base to achieve this saturation. (Remember that when the collector is in saturation with 80 mA, at room temperature the base is sitting at about 0.8V.) If your supply to the base is 5V, then the resistor would be 4.2/0.008 = 525 ohms (or the closest common value).

Saturation is probably possible at higher assumed values of Beta, and you might find out more by running the spice model. But these would probably only be speculation.
 
First, I'll definitely use the spice for simulation, thanks for reminding me.
So you say that spice simulation is the only way to find out about VCE vs. hFE in saturation region?
How much can spice be trusted about it?
 
Spice simulation and the curves on the datasheet are only for "typical" transistors.
Some transistors have poorer spec's than typical ones.
If you want every passing transistor to saturate well then use the guaranteed specs of Ib= 1/10th Ic listed on the datasheet.
 
The graph that I miss in order to answer this question is VCE Vs hFE in saturation region.

You are not likely to find such a graph as the saturation region is a general region, not one specific point. As mentioned, you can determine saturation voltage from collector current, then determine Hfe from collector current, then determine required base current. Transistors have widely varying typical parameters. Use the maximums and minimum parameters for design.
 
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