# BJT Transistor Collector Current

• FAS1998
In summary, a textbook equation for the collector current in a bjt transistor is not applicable in this context because the junctions have to be biased correctly. There is a lot missing from your simulations, which makes it difficult to understand the underlying physics.

#### FAS1998

I’ve attached an image below of a section from my textbook and some circuit simulations. The textbook gives an equation for the collector current in a bjt transistor. The simulation shows two circuits (with the same bjt transistor) that hav equal values for all variables present in the textbook equation. Shouldn’t the two circuits be producing the same collector current? Why are thy different in the simulation?

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• C2259B94-5A74-451A-B2B8-4A83A0E9D808.jpeg
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It looks like there is a lot missing from your simulations. Correct biasing of the transistors in the Common Emitter (CE) configuration, for one.

Are you familiar with how to bias an NPN BJT in the CE configuration?

FAS1998
berkeman said:
It looks like there is a lot missing from your simulations. Correct biasing of the transistors in the Common Emitter (CE) configuration, for one.

Are you familiar with how to bias an NPN BJT in the CE configuration?
No, I’m not quite sure what that means. I’m just beginning to learn how bjt transistors work and assumed that the textbook equation generally described how the collector current is related to the voltage between the base and emitter. Why is the equation not applicable in this context?

Because the junctions have to be biased correctly in order for the equations to apply.

https://en.wikipedia.org/wiki/Common_emitter

FAS1998
berkeman said:
Because the junctions have to be biased correctly in order for the equations to apply.

https://en.wikipedia.org/wiki/Common_emitter
I’m not sure. Is this related to modes of operation? Does this equation maybe only work in the active mode when Vc > Vb > Ve? Most of the material preceding the equation is about modes of operation.

FAS1998 said:
Does this equation maybe only work in the active mode when Vc > Vb > Ve?

That is the normal biasing for CE operation, so I would say yes. The equations for the other modes of operation (like CB, etc.) are likely similar, though. In your original simulation screenshots there was no biasing at all, or at best parasitic biasing from the base current you were forcing into the base.

Have you been able to set up a better biased simulation based on the Google Images results yet?

FAS1998 said:
Shouldn’t the two circuits be producing the same collector current? Why are thy different in the simulation?
The diagrams assume saturation, not bias as a CE amplifier.
The second diagram has a 1k collector load.

berkeman
FAS1998 said:
Does this equation maybe only work in the active mode when Vc > Vb > Ve? Most of the material preceding the equation is about modes of operation.
Yes - the shown exponential relationship between Ic and Vbe applies only if Vc>Vb.
Therefore, you need an additional voltage source of some volts biasing the collector.
For the second circuit (with a collector resistance), this voltage must be large enough to cope for the voltage drop acrooss this resistor (caused by the expected collector current).

By the way: Who told you to bias the base with a voltage of 1V ? Instead, take 0.6...0.7 V only.

berkeman
Here is another good source. Look at the collector curves about half way down.
https://www.electronics-tutorials.ws/transistor/tran_2.html

berkeman