Transistors, why is it here saturated ?

Thread Starter

Xenon02

Joined Feb 24, 2021
500
I gave you an exercise but it seems that you ignored it.
No I didn't, I still remember.
I thought of using photodiode as a part of Voltage divider. Maybe not a good plan.

But I had a problem with saturation. Also I need to exercise with transistors it self. From some example in Books or something. I just need to catch the flow of calculating transistors. Because now I am struggeling with resistors. With photodiode that changes its Voltage can make it a bit harder especially when I can't use Thevenin equivelant like jony did.
 

WBahn

Joined Mar 31, 2012
29,978
Oh I thought it was relevent because you mentioned it at PASS 1.
So I thought it must be 2,5V. But It seems it doesn't have to.
Please go back and read PASS #1 more carefully. I did NOT claim that that HAD to be the correct result. Quite the opposite. I said that I was making certain assumptions. Assumptions that might or might not be valid. After analyzing the circuit according to those assumptions, the conclusion was that at least one, possibly more, of those assumptions were NOT valid, and therefore NONE of the results can be considered correct. Any that happen to be correct or correct by coincidence. The circuit must be re-analyzed with according to a different set of assumptions.

I do understand that Vce is the factor I could say that tell me whether it is in saturation or not. Vce = 0,2V or Vce =0,1V.
Vce is primary indicator. If Vce < Vbe, then the device is starting to move out of the active and into saturation. When Vce gets below about 300 mV to 400 mV, you are starting to really make the shift. By the time Vce gets down below about 250 mV you are pretty firmly into saturation for most silicon NPN BJTs.

Also I see small difference. Can I call it "fully" saturated transistor when voltage on Rce is close to Vcc ?
For example Vce = 0,1 V and V_Re = 0.03 V, so V_Rc = 4.87 V.
No! The voltage across Rc may not be anywhere near Vcc. There may be significant voltage dropped across the emitter resistor.

Consider the following:

Q op2.png

The voltage across Rc is about 25.8 V compared to a Vcc of 50 V. Yes this circuit is in hard saturation. What is Vce?

Vce = Vc - Ve = 24.22 V - 24.15 V = 0.07 V

By the way does it really need that much calculation ? Isn't there a shortcut ?
I thought that it is more simple.
As I've said several times, we often make simplifying assumptions and WHEN those assumptions are valid, the analysis can often be done in your head.

But a lot of this is up to the designer. A good design, when possible, should be done so that these assumptions ARE valid.

Are there any exercises on the internet that I can use to practice ? I want to calculate more of those circuits to understand more about them. Because I know now something but I don't feel to confident. Especially when I add additional load like in common emiter or collector or other configurations.
There are millions of exercises on the Internet. Don't insist that others go find them for you.

Make up your own. Make them simple enough that you can do them. If you can't do them, make them simpler. When you can do them confidently, make them more complex.

You have a simulator (although not a particularly good on, in my opinion) to check your results against.
 

WBahn

Joined Mar 31, 2012
29,978
No I didn't, I still remember.
I thought of using photodiode as a part of Voltage divider. Maybe not a good plan.
That is almost certainly going in the wrong direction. You are having serious problems analyzing circuits like this, and you want to go making them even more complex before getting comfortable working with these?
 

Thread Starter

Xenon02

Joined Feb 24, 2021
500
No! The voltage across Rc may not be anywhere near Vcc. There may be significant voltage dropped across the emitter resistor.
I thought it was also a point because saturation is often used as a "switch" on and off so I thought that Rc will be close to Vcc (On).


There are millions of exercises on the Internet. Don't insist that others go find them for you.
I thought that maybe someone has some site that can recommend.


Make up your own. Make them simple enough that you can do them. If you can't do them, make them simpler. When you can do them confidently, make them more complex.
I didn't want to create my own example because usually they are to complex.


That is almost certainly going in the wrong direction. You are having serious problems analyzing circuits like this, and you want to go making them even more complex before getting comfortable working with these?
I know. I wanted to just reply to MrChips post.
I was thinking of using photodiode to his example he gave me. But as I mentioned before. I don't know how to calculate other stuff in transistor. Because the calculations we made was just using resistors. So photodiode as a part from "divider" on the base might be problematic. Because right now I don't know how to calculate it :D All I know is that I could use Thevenin equivelant but I can't because there is photodiode.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi X02,
Does this simulation give you an idea of the way the circuit works.?
E

Spróbuj zwizualizować działanie obwodu, zanim go przeanalizujesz;)

EG 1384.png
 
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