Transistor Saturation

Discussion in 'General Electronics Chat' started by vindicate, Jul 28, 2009.

  1. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    on a PN2222A the min saturation voltage is 0.6volts. So does the mean if the voltage on base is .6V it's fully on? As in no restriction in current through C-E?

    Also what happens when you go over the B-E Satuarion Max? According the the PN2222A datasheet the max is 1.2 or 2V depending on Ic.
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
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    I think that the 0.6V pertains to Vbe (Voltage from the base to emitter, usually 0.7V for a silicon transistor). That means if I supplied 4.5V to the base, I would get 3.8V at the emitter; a 0.7V drop. If you go over Vbe saturation max the transistor will be damaged.
     
  3. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    So Vbe is more of a voltage drop then? so technically I would have to have just a little over 0.7volts going into the base for it to be saturated and full current through C-E?

    Also, does current(on base) matter for "switching on" a transistor or is voltage all that really matter?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    If the transistor will go into saturation depends on the load and the base current. Always use a resistor in series with the base to limit the current and not destroy the transistor.
     
  5. ELECTRONERD

    Senior Member

    May 26, 2009
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    0.6V is the minimum voltage required to make the transistor saturated; or let the transistor act as a switch. If you go below this voltage the transistor won't be able to turn on. Notice that you can go over 0.6V a little bit; as they list the maximum voltage to be 1.2V. So a typical value would be 0.7V.

    Example: According to the specs, Vce should equal 10V, Ic should equal 150mA. So we'll have 15V as Vcc. Now, first we need to have Vce as 10V and Vce = Vcc - Ve. So 15V - 5V would equal a Vce of 10V. So the emitter voltage needs to be 5V. A voltage divider will do, R1 = 10K and R2 = 6K. This will produce a base voltage of 5.625V but if we have resistors with a 5% tolerance we'll probably get 5.7V. Now according to the voltage drop we chose 0.7V and we'll subtract 5.7V - 0.7V and get 5V at the emitter. Perfect for Vce! That's it! I think that the emitter resistor will determine the gain. In addition to that, it will also determine the impedance of the base. So that can be calculated by (R1 * R2/ R1 + R2) / β I'm not an expert on transistors so if someone recognizes a mistake please correct me.
     
  6. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    When they say a transistor is "fully on" is that just when the base current x Gain is Higher than the current source is able to put out?

    Or is there actually a state at which the transistor doesn't restrict current in any way across the CE junction
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    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.
     
  8. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Any chance you could edit Electronerd's example and make it correct. Because I have that in my head now :p
     
  9. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Ok lets start with the Basics.

    1. What is saturation?

    2. What is saturiation Voltage?

    3. What is Vce?

    4. Is Vce considered a voltage drop?
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    1. A transistor is in saturation when an increase in base current does not produce a change in collector current.

    2. That depends on the configuration of the transistor circuit. A transistor breaks into conduction when the B-E voltage reaches some value, typically .7 volts for a silicon transistor. After that, conduction is controlled by base current.

    3. Vce is the voltage between the collector and emitter. In the spec sheet, it defines the maximum voltage the transistor can withstand before being destroyed by breaking down.

    4. Only if the transistor is conducting.

    More material - http://www.allaboutcircuits.com/vol_3/chpt_4/4.html
     
    Last edited: Jul 28, 2009
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Just a few figures to backup beenthere's statements. I needed to know the C-E saturation V of a 2N3904, so ran a short test; conduction started @ 6μA base, increased I to32μA, B-E @ 687mV,C-E 11.12V; middle of linear region, I 104μA, C-E 7.95V B-E 696mV; 1 mA,C-E 169mV, B-E 817mV; getting closer; 3mA,115mv.837mV;4.7mA,103mV,847mV;8.6mA,93mV,864mV;terminated test.C-E 100mV @ 2Ω. transisister will do.
     
  12. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Thank you, that was very helpful.

    So when a datasheet says Collector-Emitter Saturation voltage that means how much voltage drop there is across CE when the Transistor saturated?
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Yes and no.
    The 2N3904 datasheet from Fairchild lists the maximum saturation voltage of VCE. It is for a poor but barely passing transistor.
    The graphs show the typical saturation voltage which is much less.

    You don't know if your transistors are barely passing, typical or best.
     
  14. ELECTRONERD

    Senior Member

    May 26, 2009
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    Thanks for informing me Audioguru! How would you correct my ignorance?
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Maybe you should learn about how transistors work.
    And also learn about what is said on a datasheet.

    My neighbour also prays all the time. She never gets anything done.
    She goes to church twice per day plus "talks" to her long dead mother.
     
  16. Ratch

    New Member

    Mar 20, 2007
    1,068
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    vindicate,

    No, it means that when the transistor in in saturation, the Veb will be a minimum of 0.6V .

    How can that happen when the transistor is in saturation? According to your specs, the max Veb in saturation is between 1.2V and 2.0V . The transistor determines that. Your worry is to make sure that the Ic max and Ib max is not exceeded.

    You really need to find out what saturation means. You don't enter saturation by just driving the base unless the collector circuit is designed to saturate.

    You can't have one without the other in a forward biased base circuit, can you?

    mik3,

    Actually, the collector voltage, Ic, and Rload.

    vindicate,

    No.

    Deep saturation is the lowest impedance the transistor can have from emitter to collector. And it is not zero.

    1) Saturation occurs when BOTH the emitter and collector diodes of the BJT are forward biased.

    2) The voltage of Vce and Vbe when the BJT is in saturation.

    3) The voltage between the emitter and collector.

    4) Of course. A transistor is not a solid piece of copper wire. It has an impedance and a voltage when charge flows through it.

    Yes.

    If the Ic and Rload are high enough, they can drop enough voltage so that Vcc cannot supply the reverse bias to keep the transistor in the active region. Then the BJT will saturate. If you apply enough Vcc and lower the Rload, you can make the transistor saturation proof. In other words, you will exceed the Ic max before the transistor saturates. You should realize that saturation is activated by the base drive, and determined by the collector circuit.

    Ratch
     
  17. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Thank you everyone, and Ratch for the detailed post.

    Last question is about determining HFE. The datasheet shows:
    [​IMG]

    If I were going to try to calculate how much current I needed at base for a load or just wanted to know the how much curren I could pass though CE how would I know. I know you do base current x gain, but the gain looks like it is different at diff levels.

    I read on the net alot that most common transistors are 100 but this shows many different numbers.
     
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  18. Ratch

    New Member

    Mar 20, 2007
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    vindicate,

    Different topic, same thread. That's supposed to be a no no.

    In general, you design negative feedback into the circuit so that variations in beta and other parameters are minimized. For instance, if beta is a typical 100, you design the amplification to be 10 by inserting enough emitter resistance. In a common emitter circuit, both the emitter and base current pass through the emitter resistor causing negative feedback. That make parameter variations reduce to 10/100 or 10% of what they would have been without negative feedback. Of course, your gain is now limited, so you will need another amplifier stage for more gain. Negative feedback always trades gain for stability.

    Now you are getting into circuit design, which you should study using text books and the web. It is too large a topic for a thread. But if you have a specific question about a limited subject...

    You can pass the amount of current up to Ic max through a transistor. That is very easy to accomplish including frying the transistor.

    Ratch
     
  19. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    It's the same topic isn't it? Don't you need to know Hfe to help determine saturation voltage?
     
  20. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    vindicate,

    No, you look it up in the spec sheet or measure it directly in the circuit. The transistor's design determines its saturation voltage, not the technician. He only observes it or knows what its design range is supposed to be. Hfe is not a factor for Vsat.

    Ratch
     
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