simple BJT question

Thread Starter

MikeJacobs

Joined Dec 7, 2019
226
I have simulated the left part of the circuit only (BC177):
Collector voltage: 0.597 volts
Emitter voltage: 0.604 volts
Vce=7 mV only (unrealistic)
Emitter current: 1.140mA
Collector current: 59.7µA
Base current: 1.08mA
(The majority of the emitter current goes through the base node)
I can sim the circuit also but the point was to gain intuition into the circuit
 

Papabravo

Joined Feb 24, 2006
21,225
Thanks to everyone for the effort. This circuit is just trying to help understand what parts of a bjt constrain the others. That was the whole point. I saw some questions about what kind of signal and such. There is no signal. Its just a dc circuit for the purpose of learning. Thats all. It serves no actual purpose
No problem -- you wanted to approach the (re)learning task in a different way. It was hard for me to see how it would be helpful, but that is my problem and not yours. I apologize if my attempt to migrate to a place I was familiar with actually ended up throwing you a curve ball. That was not my intention.

One of WBahn's critically important points is that you cannot depend on actual values for transistor parameters. Any design that you use must take that into account and work regardless of the specific values of a device's parameters. The standard form circuits from the textbooks pretty much all have that feature, That is why we study them.
 

LvW

Joined Jun 13, 2013
1,759
The same is true for Ohm's Law. Don't try to think of it dogmatically as a voltage causing a current versus a current causing a voltage. Think of it as merely as a device in which the ratio of the voltage across it to the current,through it, however they happen to come about, exhibits the property that it's a value that we call the resistance.
@WBahn.....I think your contribution is very clear and precise and helpful for the questioner. Of course, I can agree to everything.
More than that, your discussion about Ohms law can serve as a very good example to demonstrate two different views:
Practical aspects vs. physical laws.
During design and analysis of circuits with resistors we are allowed to treat the resistor as a part which can produce a voltage across its nodes when a current goes through the device (V=I*R).
However, we know that this is - physically spoken - not correct.
The current I through the resistor is always the result of a voltage across the resistor (resp. the corresponding E-field within the resistor). No continuous current without a driving voltage.
And - as you have mentioned, something similar applies to the BJT as far as the currents are concerned.
In some cases, we can treat the BJT as a current-controlled device (knowing that this is - physically spoken - not the case).
Such an approach may be somewhat confusing to the beginner - however, this will appear not as a contradiction as long as we know what we are doing - and why ! (see application of Ohm`s law).
 
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