Noob questions about Transistor BJT

[spid3rx]

This probably would have asked somewhere, may I ask when we pick a transistor, or when we want to study the reasoning of a transistor in a circuit,
in the transistor spec, what parameters, digit that you will first look for ? lets say the one in the attached. I know I need to check the bias voltage, is it the Vbe (sat) ?

BJT is voltage control transistor, JFET is current control transistor ? how and when do we choose which one to use everytime?

thanks

 

[LvW]

JFET is current control transistor

Who told you this ?

 

[mister_rf]

It's a typo error there, a vice versa situation… :grin:
1. The BJT is a current-controlled device since its output is determined on the input current, while FET is considered as a voltage-controlled device, because it depends on the field effect of the applied voltage.
2. The BJT (Bipolar Junction Transistor) uses both the minority and majority carriers (holes and electrons), while FETs, which are sometimes called unipolar transistors, uses either holes or electrons for conduction.

---------- Post added at 10:33 ---------- Previous post was at 10:28 ----------

Read more:

Bipolar Junction Transistors and Field Effect Transistors

**broken link removed**



Typical transistor ratings:
Power dissipation: maximum allowable power dissipation on a sustained basis.
Reverse voltages: maximum allowable VCE , VCB , VEB .
Collector current: the maximum allowable collector current.
Saturation voltage =VCE voltage drop in a saturated transistor.
Beta: β=IC/IB

Bias design can be simplified using transistor characteristic curves. The characteristic curves predict the performance of a BJT. There are an input characteristic curve, a transfer characteristic curve and an output characteristic curve. The most useful for amplifier design is the output characteristics curve, a graph displaying the output voltages and currents for different input currents. The linear part of the curve needs is utilized for an amplifier or oscillator. For use as a switch, a transistors biased at the extremities of the graph, these conditions are known as "cut-off" and "saturation".

 

[spid3rx]

sorry my typo, BJT for current controlled and FET for voltage control. My question is what do you look for first in the transistor spec and why ?

 

[mister_rf]

In order to choose parameters we need to know the applications where the BJT to be used.

 

[LvW]

..................
1. The BJT is a current-controlled device since its output is determined on the input current,

Hello, Mister_RF.

This question was discussed already earlier - but I don't remember if it was in this forum or another.
However, I must disagree with your above statement!
It is physically not true that the output current of the BJT is determined by the input current.
I know that everybody uses the simple relation Ic=beta*Ib.
But this formula says nothing else that the base current (that cannot be avoided, unfortunately!)
is a - more or less constant - percentage of the collector current.
Physically, the BJT is clearly also a voltage controlled device, see the classical Shockley exponential
formula that descibes the voltage-current relation across the pn junction.
We shouldn't mix physical facts with simplified all-day rules of thumb.
Regards
LvW

 

[mister_rf]

We are here on this section trying to make a very long story short, so please consider as it is.

 

[LvW]

We are here on this section trying to make a very long story short, so please consider as it is.

Hello, Mister_RF.

If you prefer "to make a very long story short" (even when the short story is wrong) you may, of course, continue with this approach.
However, you certainly will agree that that we are here also on this section to answer questions correctly - with the aim to improve the understanding for the physical function of circuits as well as the meaning of the respective formulas (instead of the blind usage of simplified rules of thumb).
By the way: In our case under discussion - the answer is not much longer by saying correctly "voltage-controlled" rather than "current-controlled".
Best regards
LvW

 

[mister_rf]

So, next time I’ll need to start with Ebers-Moll, correct? :roll:

 

[LvW]

So, next time I’ll need to start with Ebers-Moll, correct? :roll:

Yes, thank you.
LvW