Transistors, why is it here saturated ?

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Hello !

Does someone know what I am missing here in calculations ?

I know that Vbe = 0,7V. but I couldn't get the information about UR and UE. but it somehow saturated even though the current on collector circuit isn't maximum ? Ic_max =5V/1k = 5mA ?

Here it is not saturated :

Or something like this as well :

I don't know why it is like that. Did I miss something ?

LvW

Joined Jun 13, 2013
1,561
Because the base divider resistors are very low you can (as a first step) neglect the unknown base current and find the base voltage: Vb=2.5 V.
From this, the voltage at the emitter can be estimated (exact enough) to be Ve=1.8 volts.
Therefore, Ie=1.8/40=45 mA.
The voltage at the collector node is below the base voltage (large voltage drop across Rc) - and the CB pn junction is forward biased with Vcb>0.
Therefore, both junctions are forward biased (definition of saturation) and the circuit is in deep saturation.
For Rc=10 Ohms a current of 45mA will produce a collector voltage of Vc=5-0.45 V only (no saturation).

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Because the base divider resistors are very low you can (as a first step) neglect the unknown base current and find the base voltage: Vb=2.5 V.
From this, the voltage at the emitter can be estimated (exact enough) to be Ve=1.8 volts.
Therefore, Ie=1.8/40=45 mA.
The voltage at the collector node is below the base voltage (large voltage drop across Rc) - and the CB pn junction is forward biased with Vcb>0.
Therefore, both junctions are forward biased (definition of saturation) and the circuit is in deep saturation.
For Rc=10 Ohms a current of 45mA will produce a collector voltage of Vc=5-0.45 V only (no saturation).
I think I understand the first part where you calculated the Ve voltage.
Although I don't understand the part with saturation. How do I know when I saturated it or not ?
Or maybe a better question is why 1k ohm makes it saturated.

Also isn't the transistor saturated when Ic_max is crossed ? Like here Ic_max = 2mA, and current Ib on active is 156 uA, so on the collector Ic = 1,5 mA. So if I have Ib> 200uA then it should be saturated. But changing the resistor to 21,5k will make is still saturated even though the current close Ib = 200 uA.

Jony130

Joined Feb 17, 2009
5,419
Although I don't understand the part with saturation. How do I know when I saturated it or not ?
In your original circuit, the emitter resistor is setting the collector current value.
Ie = Ve/Re ≈ (2.5V - 0.7V)/40Ω ≈ 45mA
The voltage at emitter is around 1.8V thus If we assumed that Vce(sat) = 0.2V we have Rc_max = (5V - 1.8V + 0.2V)/45mA ≈ 76Ω Any larger value than this will saturate the BJT.

Thread Starter

Xenon02

Joined Feb 24, 2021
355
In your original circuit, the emitter resistor is setting the collector current value.
Ie = Ve/Re ≈ (2.5V - 0.7V)/40Ω ≈ 45mA
The voltage at emitter is around 1.8V thus If we assumed that Vce(sat) = 0.2V we have Rc_max = (5V - 1.8V + 0.2V)/45mA ≈ 76Ω Any larger value than this will saturate the BJT.
Hmmm interesting.
Isn't that I should cross the Ic_max on the collector? Like in the example above? Although it didn't work exactly like I wanted.

Jony130

Joined Feb 17, 2009
5,419
Isn't that I should cross the Ic_max on the collector? Like in the example above? Although it didn't work exactly like I wanted.
But you have crossed Ic_max. For Rc = 1kΩ and Vcc = 5V we have Ic_max = 5mA.
But your circuit "wanted to pass" 45mA of collector current, thus Ic > Ic_max = saturation.

Thread Starter

Xenon02

Joined Feb 24, 2021
355
But you have crossed Ic_max. For Rc = 1kΩ and Vcc = 5V we have Ic_max = 5mA.
But your circuit "wanted to pass" 45mA of collector current, thus Ic > Ic_max = saturation.
But how do I know the current on Ib?
To determine if Ic passed Ic_max I need to know Ib.

Also I tried also with the examples above but for resistor 22k It should still be active but it's saturated

Jony130

Joined Feb 17, 2009
5,419
But how do I know the current on Ib?
To determine if Ic passed Ic_max I need to know Ib.
You need to apply Thevenin's theorem or KVL, KCL.

So we have

Vth = Ib * Rth + Vbe + Ie*Re

Additional we know that Ie = Ib + Ic = Ib + Ib*β = (β +1)*Ib
Therefore
Vth = Ib * Rth + Vbe + Ib*(β +1)*Re

Ib = (Vth - Vbe)/(Rth + (β +1)*Re)

Or we can solve for Ie current Ib = Ie/( β +1)
Thus,
Ie = (Vth - Vbe)/(Re + Rth/(β +1))

Also I tried also with the examples above but for resistor 22k It should still be active but it's saturated
For which circuit?

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Thread Starter

Xenon02

Joined Feb 24, 2021
355
You need to apply Thevenin's theorem or KVL, KCL.
View attachment 274916

So we have

Vth = Ib * Rth + Vbe + Ie*Re

Additional we know that Ie = Ib + Ic = Ib + Ib*β = (β +1)*Ib
Therefore
Vth = Ib * Rth + Vbe + Ib*(β +1)*Re

Ib = (Vth - Vbe)/(Rth + (β +1)*Re)

Or we can solve for Ie current Ib = Ie/( β +1)
Thus,
Ie = (Vth - Vbe)/(Re + Rth/(β +1))

For which circuit?
From the post 3

Jony130

Joined Feb 17, 2009
5,419
Also I tried also with the examples above but for resistor 22k It should still be active but it's saturated
Because you have ignored Vce(sat) when you estimated your Ic_max current.

LvW

Joined Jun 13, 2013
1,561
Also isn't the transistor saturated when Ic_max is crossed ?
Xenon02 - when you speak about "saturation" it would be helpful to remember the corresponding DEFINITION !
As I have mentioned in my answer - all the currents (Ic and Ib) are only RESULTS of the condition of saturation and NOT its cause.
The definition is simple:
We have saturation as soon as the base-collector junction is forward biased. And this happens when the voltage drop across the resistor Rc is so large that the voltage Vc (collector) falls below the base voltage Vb (Vc<Vb).
As a consequence the base current (without saturation only between B and E) is drastically increased because of the additional current between B and C (both junctions forward biased).
That is the reason that the parameters B (resp. beta, h21, hfe) do not apply anymore.

crutschow

Joined Mar 14, 2008
30,788
Below is the LTspice simulation of your post #3 circuit going from cutoff, through the active region, and then to saturation (with a transistor Beta of 100 for the generic transistor model I have).
The horizontal axis is V2's voltage.
Saturation occurs when the collector voltage (red trace) drops to about 0.2V as the transistor turns fully on, where the collector current (green trace) is near the maximum value as determined by the value of V1 and R1.

Any increase in base current past that point only serves to reduce the saturation voltage [V(C)] slightly.

(Note: If you are serious about learning electronics, then you should really learn to use a better simulator, such as the free LTspice), since it will allow you to display more information of a circuit's operation, and thus gain a better understanding.
It does have a somewhat steep learning curve, but there are many example circuits and tutorials available, as well as people on this site to help you with the process.)

Last edited:
Thread Starter

Xenon02

Joined Feb 24, 2021
355
So we have

Vth = Ib * Rth + Vbe + Ie*Re

Additional we know that Ie = Ib + Ic = Ib + Ib*β = (β +1)*Ib
Therefore
Vth = Ib * Rth + Vbe + Ib*(β +1)*Re

Ib = (Vth - Vbe)/(Rth + (β +1)*Re)

Or we can solve for Ie current Ib = Ie/( β +1)
Thus,
Ie = (Vth - Vbe)/(Re + Rth/(β +1))
Ah so from those equations I can calculate Ib and Ie without knowing their value. As I know it only applies for active transistor. So from this I know what is the Ib current and I can calculate the Ic, and from it I can compare my Ic_max (for 1k ohm and for 10 ohm) and check in which it will saturate ?

Below is the LTspice simulation of your post #3 circuit going from cutoff, through the active region, and then to saturation (with a transistor Beta of 100 for the generic transistor model I have).
The horizontal axis is V2's voltage.
Saturation occurs when the collector voltage (red trace) drops to about 0.2V as the transistor turns fully on, where the collector current (green trace) is near the maximum value as determined by the value of V1 and R1.

Any increase in base current past that point only serves to reduce the saturation voltage [V(C)] slightly.
What is it changing ? The value of V1 or V2 that the Vc changes ?

(Note: If you are serious about learning electronics, then you should really learn to use a better simulator, such as the free LTspice), since it will allow you to display more information of a circuit's operation, and thus gain a better understanding.
It does have a somewhat steep learning curve, but there are many example circuits and tutorials available, as well as people on this site to help you with the process.)
I was starting to learn it. Also does LTSpice has an option of visual current flow like in falstad ? I mean that where the current flows, in falstad there was that animation. And I really liked this feature.

Jony130

Joined Feb 17, 2009
5,419
What is it changing ? The value of V1 or V2 that the Vc changes ?
The input voltage (V2)- from 0V to 2V.

Also does LTSpice has an option of visual current flow like in falstad ?
No.
and from it I can compare my Ic_max (for 1k ohm and for 10 ohm) and check in which it will saturate.
Forget about Ic_max in this case. For this circuit with Re resistor, transistor win enter saturation when from analysis Vce (Vce = Rc *Ic) is greater than (Vcc - Ve - Vce(sat).

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Forget about Ic_max in this case. For this circuit with Re resistor, transistor win enter saturation when from analysis Vce (Vce = Rc *Ic) is greater than (Vcc - Ve - Vce(sat).
Hmm I don't understand this part.
I remember you said that when I want to get saturation then I have to cross Ic_max.

So how do I analize it for different cases ? Like with Re, without Re, With Rc and without Rc. Is there a pattern to know when I have a certain mode ? I'm asking it because I saw some circuits without Rc or without Rc but with Re. I know only the pattern with Ic_max.

Jony130

Joined Feb 17, 2009
5,419
So how do I analyze it for different cases ? Like with Re, without Re, With Rc and without Rc. Is there a pattern to know when I have a certain mode ? I'm asking it because I saw some circuits without Rc or without Rc but with Re. I know only the pattern with Ic_max.
They are all shortcuts. Do you know (understand) what causes BJT to be saturated in the first place?

Thread Starter

Xenon02

Joined Feb 24, 2021
355
They are all shortcuts. Do you know (understand) what causes BJT to be saturated in the first place?
To much current in Ib ?
That Ic_max is crossed ? I mean when I see Re then my pattern with Ic_max is a little bit disturbed so I don't know I need to check some examples. And how to approach them

Jony130

Joined Feb 17, 2009
5,419
I mean when I see Re then my pattern with Ic_max is a little bit disturbed so I don't know I need to check some examples. And how to approach them
This is why you need to forget about Ic_max. You need to back to basics and stop using the "shortcuts" (Ic_max).
Transition enters saturation region when Vc < Vb.

Can you solve for Ib, Ic, and Vce? And determine whether the BJT is working in the active region or in the saturation region.
Assume Vbe = 0.7 and β = 100.

crutschow

Joined Mar 14, 2008
30,788
What is it changing ? The value of V1 or V2 that the Vc changes ?
If you read my description, I stated that the horizontal axis on the graph was the (changing) voltage of V2.

(If you can't understand what I am saying, or understand the simulation graph, then what we have here, is a failure to communicate. )
Also does LTSpice has an option of visual current flow like in falstad ?
I mean that where the current flows, in falstad there was that animation.
Nope.
Serious simulators don't generally do that so. If you like the cute animation, you'll have to stick with Falstad.

LvW

Joined Jun 13, 2013
1,561
Jony130 said:
They are all shortcuts. Do you know (understand) what causes BJT to be saturated in the first place?

To much current in Ib ?
That Ic_max is crossed ? I mean when I see Re then my pattern with Ic_max is a little bit disturbed so I don't know I need to check some examples. And how to approach them
Xenon02 - may I remember you the DEFINITION of saturation?
Again: Ib is the result (an indication) for saturation but NOT its cause!