Do the capacitors always charge exponentially?

[Femme_physics]

Low pass filters, wiki article on logic gates...general official references to have with me, since I can't use google during the test :(

 

[I like Serena]

Yes. It's best to be prepared!Tbh the circuit doesn't look as if it makes much sense.

Since Vb=0, I think Ve will become 0.7V always.
At first the capacitor would charge, up to 0.7V (exponentially), and then it would stop.

So no, not a constant current source.

 

[Femme_physics]

In that case I'll just print two of your replies

It depends on how the capacitor is charged.

If the capacitor is charged from a constant voltage source, it will be charged exponentially as you say.

If the capacitor is charged from a constant current source, it will be charged linearly (as in your recent thread).
The current source will have to work harder and harder to keep its current constant though. ;)

If the capacitor is charged in another way, it will yet be different.

If you have a symbol in your circuit with the text "current source" next to it, it is also a current source. ;)

Note that your mediating transistor also needs constant voltage between the base and the emitter, before it will behave like a current source.
In your previous threads you have seen a couple of examples of that (for instance a zener diode).

What you need is that the current is the same whether the capacitor is full or empty.
If you can calculate the current in both those cases and it is the same, you have a current source.

Should do the job :) thanks

 

[I like Serena]

Don't forget to trust your visual cues! ;)

 

[Femme_physics]

Good point!

 

[I like Serena]

Not points - cues!

If you see points, you have to connect the dots.

 

[sophiecentaur]

How to recognise a constant voltage source? The volts are the same for a big range of load resistance values. i.e. you have a low (enough) source resistance. What does "same" mean? It means small enough not to matter in the context that the source is used. Most electronic measurements are 'good enough' if they are within 1% (more than enough for many purposes). If the source resistance is 1/100 of the load resistance, you are in the right ball park. (And electronics is very often a 'ball park' topic) It would be adequate for charging many types of battery at constant voltage, for instance.

You can apply precisely the same sort of criterion to determine whether you have a constant current source. Does the current vary significantly over the range of load resistances you plan to use? If you are after 1% accuracy, a high enough voltage source and a series resistor 100 times that of the load will also give you a 'useful' current source.
Those are examples of "rules of thumb". On some occasions, they would be entirely inadequate, of course.

Once you include some gain and feedback, your accuracy can easily be much better and you wouldn't actually need a vast voltage and high series resistance.