Transistor Saturation

Audioguru

Joined Dec 20, 2007
11,248
Hfe is not a factor for Vsat.
True.
HFE is not used for a saturated transistor. HFE is when the transistor is a linear amplifier with plenty of collector to emitter voltage (not saturated).
Many of the HFE spec's have a VCE of 10V.

HFE is current gain. It has nothing to do with voltage gain.
 

Thread Starter

vindicate

Joined Jul 9, 2009
158
So what is the best way to test when or to know when a transistor is saturated? By testing the Voltage across Vce? If it is less than 1V chances are it's saturated?
 

beenthere

Joined Apr 20, 2004
15,819
When an increase in base current does not produce more collector current, the transistor is saturated.

Do you have some application in mind?
 

Audioguru

Joined Dec 20, 2007
11,248
The transistor's datasheet shows the saturation voltage.

A 2N3055 has a max collector current of 15A.
With a collector current of "only" 10A and a base current as high as 3.3A its max saturation voltage is 3.0V.
 

Thread Starter

vindicate

Joined Jul 9, 2009
158
When an increase in base current does not produce more collector current, the transistor is saturated.

Do you have some application in mind?
I understand that, I was just wondering if there was a more mathematical approach to figuring it out, instead of inceasing base and monitoring the collector current.

The transistor's datasheet shows the saturation voltage.

A 2N3055 has a max collector current of 15A.
With a collector current of "only" 10A and a base current as high as 3.3A its max saturation voltage is 3.0V.
So what are you saying is the for a 2N3055 transistor if the voltage across CE is 3volts or less it will be saturated?
 

Thread Starter

vindicate

Joined Jul 9, 2009
158
No.
If the current is low then 3V is no where near saturation.
Not sure what you mean.

Let's try this again.

This is a section from a PN2222a


For the test they posted Current through the collector was 500mA and voltage through the Collector Emitter junction was 10V then Vce Saturation was a max of 1 volt. I don't understand how that can be. how can Vce be both 10V and 1V? If Vce Sat is a voltage drop wouldn't it be just 1V(or less) across Vce?
 

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Ratch

Joined Mar 20, 2007
1,070
vindicate,

I was just wondering if there was a more mathematical approach to figuring it out, instead of inceasing base and monitoring the collector current.
Sure there is. Remember what I said before? If both the emitter and collector diodes are forward biased, then saturation occurs. That is the cause, loss of Ic control with Ib is the effect. In a CE or CC configuration, all you have to do is figure what the total voltage drop across the emitter and collector resistance is. Then compare it to Vcc. If drop across the resistors is greater than Vcc, then you will have saturation because Vcc will not be able to maintain a reverse bias across the collector diode. Vsat is what the specs say it is, and there is nothing you can do about it except go or no-go into the saturation region.

So what are you saying is the for a 2N3055 transistor if the voltage across CE is 3volts or less it will be saturated?
No, not necessarily. The conditions for saturation have to be met. That is, both the emitter and collector diodes must be forward biased.

Ratch
 

ELECTRONERD

Joined May 26, 2009
1,147
A transistor is turned on by current, not voltage. It sets its own base voltage that could be 0.6V to 1.2V for a 2N2222A transistor with a collector current of 150mA and a base current of 15mA.

Electronerd,
You guessed wrong and added too many resistors.
The base needs a current, not a voltage.
The datasheet used a VCE of 10V to list its HFE. We are not using HFE, we are saturating the switching transistor with a base current that is 1/10th the collector current.
It is quite evident that the base current needs to have current, but almost certain that voltage at the base is a part of the saturation as well. I mean, what about Vce? The specs say that it has to be a 10V difference between the collector and emitter. Voltage at the base must make a difference!
 

Thread Starter

vindicate

Joined Jul 9, 2009
158
In a CE or CC configuration, all you have to do is figure what the total voltage drop across the emitter and collector resistance is. Then compare it to Vcc.
While a transistor is saturated isn't the voltage drop across Vce supposed to be near 0V though?

EDIT:

Here is an example of what I am referring to if it helps at all. This transistor is saturated, and the Vdrop across CE is 65.31mV.

 

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Audioguru

Joined Dec 20, 2007
11,248
Not sure what you mean.

Let's try this again.

This is a section from a PN2222a


For the test they posted Current through the collector was 500mA and voltage through the Collector Emitter junction was 10V then Vce Saturation was a max of 1 volt. I don't understand how that can be. how can Vce be both 10V and 1V? If Vce Sat is a voltage drop wouldn't it be just 1V(or less) across Vce?
It should not show the VCE as 10V. That is the condition for HFE, not for saturation.
 

Ratch

Joined Mar 20, 2007
1,070
vindicate,

For the test they posted Current through the collector was 500mA and voltage through the Collector Emitter junction was 10V then Vce Saturation was a max of 1 volt. I don't understand how that can be. how can Vce be both 10V and 1V? If Vce Sat is a voltage drop wouldn't it be just 1V(or less) across Vce?
Good question. If you look at the parameter column, you will see a asterisk. Following the asterisk to the bottom you will see that they are using a 10V pulse voltage with a specified width and duty cycle. Averaged out, it probably comes out to the value in the max column.

Ratch
 

Ratch

Joined Mar 20, 2007
1,070
vindicate,

Here is an example of what I am referring to if it helps at all. This transistor is saturated, and the Vdrop across CE is 65.31mV. While a transistor is saturated isn't the voltage drop across Vce supposed to be near 0V though?
Isn't 65 millivolts rather "near zero"?

The 2N2222 Vsat spec lists 0.3V and 1.0V for low and high current respectively. Isn't that close to zero too?

Ratch
 

ELECTRONERD

Joined May 26, 2009
1,147
Audioguru, you haven't answered my question. Please check post 29, I don't understand why voltage at the base has nothing to do with the saturation. Voltage at the base determines voltage at the emitter too.
 

Ratch

Joined Mar 20, 2007
1,070
ELECTRONERD,

It is quite evident that the base current needs to have current...
Circular statement.

...but almost certain that voltage at the base is a part of the saturation as well.
The data sheet specifies the Vbe at saturation.

I mean, what about Vce?
Yes, what about it?

The specs say that it has to be a 10V difference between the collector and emitter.
No they don't. As I pointed out above, those are pulse voltages used for testing, and not the average Vce saturation voltage.

Voltage at the base must make a difference!
Once the transistor enters saturation, Ib and Vbe loose control of the transistor until the transistor comes back into the active or cutoff region again.

Ratch
 

ELECTRONERD

Joined May 26, 2009
1,147
ELECTRONERD,



Circular statement.



The data sheet specifies the Vbe at saturation.



Yes, what about it?



No they don't. As I pointed out above, those are pulse voltages used for testing, and not the average Vce saturation voltage.



Once the transistor enters saturation, Ib and Vbe loose control of the transistor until the transistor comes back into the active or cutoff region again.

Ratch
Ratch,

If you look at the specs, for the "Base-Emitter saturation voltage" on page two, you notice that it says "Ic=150mA, Vce=10V." Vce is calculated by the following equation: Vce = Vcc/Ve. So I made 5V at the emitter for my design with a 15V supply, 15V-5V=10V for Vce. You said:
Yes, what about it?
So according to you, Vce shouldn't even be mentioned on the specs.
 

ELECTRONERD

Joined May 26, 2009
1,147
Ok, I went ahead and tried to saturate the 2N3904 transistor, let me know if I did it correctly. My questions are where the input and output is of the "switch"? Also, I noticed that with the flashing LED cicuits they use two transistors that switch two LED's on and off. Now, they also have capacitors to limit the time they turn on and off. If I need a capacitor, where would I have it? Let's say I only wanted to blink a single LED on and off, where would I put this? In addition, what value should Re be? I would think that it determines the gain of the transistor, but you've all said that gain isn't important for saturation. Please help!
 

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Audioguru

Joined Dec 20, 2007
11,248
It is quite evident that the base current needs to have current, but almost certain that voltage at the base is a part of the saturation as well. I mean, what about Vce? The specs say that it has to be a 10V difference between the collector and emitter. Voltage at the base must make a difference!
A saturated transistor does not have 10V from collector to emitter.
I looked in 25 datasheets and could not find one that said that.
They all listed the base current as 1/10th the collector current.
Some datasheets show the transistor being pulsed to keep it cool.

Transistors set their own base to emitter voltage. It is the base current that turns on the transistor.
 

Audioguru

Joined Dec 20, 2007
11,248
A switching transistor usually does not have an emitter resistor. The emitter is usually grounded. The load is between the positive supply and the collector.
 
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