Help requested to identify common emitter, collector base bjt circuits

[iouae]

I skimmed through the thread and did not see this: Back to the original post, #5 is incorrect. #5 is common collector, just like #2.

There are pedantic, semantic, and qualifying issues with each statement, but I see the kind of truth table you are trying to construct. All of the subtleties of real circuit design notwithstanding, the table is ok for guidance.

Something you might add are the (low / medium / high) input and output impedances of each of the three circuit classes. Very handy in figuring out someone else's design decisions.

ak

Thanks for the correction AnalogKid. Yes #5 is wrong. My new general rule which replaces all the ones in the original post is the pin of the transistor with the least changing voltage is the common pin. If it is an out pin it would be changing.

 

[iouae]

My understanding, and I may need correcting here, BUT....as your diagram shows the load below the transistor emitter, as the transistor starts to turn on, it's emitter voltage would start to rise, turning the transistor off again.

These days, it's simpler, easier, quicker, just to use a mosfet.
No real calculations for base current, wattage dissipation etc. etc. as long as there is sufficient current handling capacity.
If using with uC use logic level.

I have hundreds of bjt's and only a handfull of mosfets, because they are more expensive. But I want to live in a mosfet world and plan to use a lot more mosfets in future.

And I do believe that using a load like a motor which keeps changing, if this is attached to an emitter, would keep changing the emitter voltage and in so doing turning the transistor on and off, because it is the voltage of the base RELATIVE to the emitter which turns a transistor on. The voltage of the base relative to the collector does not matter, therefore the motor is attached to the collector. I think that's so.

I wonder if Spice can accurately simulate a motor's changing load, and whether it would accurately simulate the differences between common e and common c when connected to a motor?

 

[Harald Kapp]

I wonder if Spice can accurately simulate a motor's changing load

See e.g. here.

 

[iouae]

See e.g. here.

So do-able thanks Harald.

 

[aleckpellitier1]

I worked my way through the paper you recommended. Probably a first for me to bother with the mathematics surrounding a transistor. I award myself a Vc (Victoria cross) that I did it and survived. I had to look up what KVL means and am quite pleased with myself that I was able to work out V(RE) for myself in 4.7.2

But I know that I have missed the point the paper was trying to convey. For instance in 4.7.1 the LED and resistor are together, and in 4.7.2 they are on either side of the transistor. Is this important at all?

In 4.7.1 there is a base resistor, and 4.7.2 there is none. Is there some point to that?

As a personality flaw, I despise details, which probably explains why I hate maths. I just need someone to tell me, does it matter when placing a motor and transistor together in series whether the motor is on top (common emitter) or on the bottom (common collector). I will take their word for it (without mathematical proof) if they say yes it matters, one generally places a motor in common collector mode.

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The following is what shako.ai said in answer to my question "which is a better way to connect a transistor and motor in series, common emitter or common collector mode?". "In general, common emitter mode is better for driving low-current loads, such as LEDs or sensors, while common collector mode is better for driving high-current loads, such as motors or speakers."

Yet when I do a Google search for "dc motor and transistor circuit" almost every circuit has the motor in common emitter mode.

Thanks again, it was a new experience for me.

As for the placement of the motor and transistor in series, whether in common emitter or common collector mode, the choice depends on the requirements of the load and the desired characteristics of the circuit. Common emitter mode is typically better for low-current loads like LEDs or sensors, while common collector mode is more suitable for high-current loads like motors or speakers. However, there can be exceptions and different design choices based on specific circumstances.