Transistor saturation

Discussion in 'Homework Help' started by dcepticon, Apr 29, 2009.

  1. dcepticon

    Thread Starter New Member

    Apr 29, 2009
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    Can someone please help me with this question

    Determine wether or not the transistor in figure Q.1(a) is in saturation
    Assume Beta = 50 and Vbe = 0.7v

    I just need to know the method of working it out, im not sure on how to form the equation for it

    Am i right in assuming that the transistor is in saturation when Vce = 0?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Where is figure 1?
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Vce(sat) is typically given as 0.2 volts.
     
  4. dcepticon

    Thread Starter New Member

    Apr 29, 2009
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    i will upload schematic when i get home
     
  5. dcepticon

    Thread Starter New Member

    Apr 29, 2009
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    Diagram as promised

    [​IMG]
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Determine current through Rb. With the transistor on what is the voltage at the base?

    What is the current throught the collector? What is the relationship of base current to collector current in a transistor?

    What is the voltage drop across Rc?

    Is Vce(sat) below 0.2 volt approximation for saturation?
     
  7. dcepticon

    Thread Starter New Member

    Apr 29, 2009
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    what is the formula to get that?

    im useless at electronics
     
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    V = i * R will get you through this one.

    I think I've given enough hints to solve this problem... maybe a review of simple transistor theory is in order.
     
  9. PRS

    Well-Known Member

    Aug 24, 2008
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    If Vbe is .7 volts and the source is 3 volts, then we have 2.3 volts across that 10k ohm resistor. And that means we have a base current, Ib, of .23mA. The collector current is beta*Ib=11.5mA. So the voltage at the collector is 10-1k*11.5mA =-1.5 volts. But this negative voltage is not available from the source. Therefore the transistor is saturated and the voltage at the collector is .2 volts.
     
  10. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Nah, you've been set an impossible question is all. Without knowing the characteristics of the transistor there's not enough information to predict saturation. You need to know the relationships between Ic, Ib and Vce, and for that you need the transistor datasheet. Check out the saturation section at http://www.allaboutcircuits.com/vol_3/chpt_4/9.html

    Your teacher has confused saturation with running out of headroom. The latter can be proved with S@HE's method: The base current will be (3V-0.7V)/10kΩ = 230 μA, and as the gain is 50 then the collector current wants to be 230μAx50 = 11.5 mA. With an adequate collector supply voltage the voltage across the collector resistor will be 11.5mAx1kΩ = 11.5 V. But there's only 10 V available, so it won't after all. But that doesn't necessarily mean the transistor is saturated, just that there isn't enough voltage compliance to supply the collector current demand.

    Imagine it another way - same circuit, except Vcc is now 20 V instead of 10 V. Now we can drop 11.5 V across the collector resistor, which means Vce=20V-11.5V = 8.5 V. That's definitely not saturated. But it does assume the transistor is operating in the linear region, which again can't be assumed without reference to the datasheet etc. etc.
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    Assume that the transistor is working in its linear region.

    Calculate the current through the base by assuming a Vbe of 0.7V:

    Ib=(3-0.7)/10K=0.23mA

    Then find the current through the collector:

    Ic=Ib*beta=0.23mA*50=11.5mA

    The voltage drop across Rc is 11.5mA*1K=11.5. This is greater than the supply voltage and thus the transistor works in saturation.
     
  12. dcepticon

    Thread Starter New Member

    Apr 29, 2009
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    [​IMG]

    Am i right in assuming

    Vbe = 0.7
    Ib = .13mA
    Ic = .93mA

    what i dont understand is how do i get IE? like how do i determine beta from alpha?
     
  13. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Alpha is Ic/Ie. You can work out Ie from Vbb and Vbe and R, and you can work out Ic from Ie and alpha. You don't need to know beta or Ib.
     
  14. steveb

    Senior Member

    Jul 3, 2008
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    Ie=Ic+Ib using the typically defined current directions
     
  15. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Anyhoo, going back to the transistor saturation question - I know I'm in a minority of one here, but I have to re-iterate. Yes that transistor circuit has run out of collector voltage headroom to supply the collector current as defined by βxIb, but no that doesn't mean it's saturated.

    If you don't believe it, try it. Make up that circuit, tweak the values to get a certain voltage across the collector resistor, and then reduce the Vcc supply to below the voltage you just measured. Now measure Vce - it won't be saturated, as we've just guaranteed that by biasing in the linear region from the start, all we've done is reduce the headroom to below the minimum voltage compliance needed to supply all the collector current.
     
  16. steveb

    Senior Member

    Jul 3, 2008
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    Actually, that does mean it is saturated. Transistor saturation occurs when both the collector-base and emitter-base junctions become forward biased. In the given context, this is just another way of saying that the collector voltage is too low.
     
  17. Darren Holdstock

    Active Member

    Feb 10, 2009
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    I was thinking of saturation as the transistor being turned hard on such that Vce is at a low value, which might explain a lot. But surely the c-b and b-e junctions are going to to forward biased in that circuit anyway?
     
  18. Darren Holdstock

    Active Member

    Feb 10, 2009
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    So are there other definitions of transistor saturation I'm not aware of? I was beginning to have doubts, so sought comfort in the bible, Horowitz and Hill, 2nd Edition. From Appendix G, "Transistor Saturation":

    "...at small values of collector-to-emitter voltage (typically 0.25 V or less) some of the base current will be "robbed" by conduction of the collector-base diode. This lowers the effective hFE and makes it necessary to supply relatively large base currents to bring the collector close to the emitter..."

    and

    "The measured data in Figure 4 [shows graph of Ic vs Vce for various Ib] shows that it is hard to define exactly when a transistor is "saturated"; you might use some arbitrary criterion such as Ic=10xIb."

    Maybe a good rule of thumb for saturation is when Vbe>Vce.

    Back to the OP circuit. Assuming a large enough voltage compliance at Vcc, the collector resistor wants to drop 11.5 V, and it does. Vce will be Vcc-11.5 volts. But now reduce Vcc to 10 V, and it doesn't follow that Vce now approaches Vce(sat) - it will probably still be a couple of volts. That's not saturation in my book, and besides it's impossible to predict Vce in the OP circuit without knowing the characteristics of the transistor.

    I could be wrong. I don't think I am, but if I am indeed labouring under a misapprehension, I could be be brought around with patient explanations.:)
     
  19. steveb

    Senior Member

    Jul 3, 2008
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    I'm agreeing with most of your post above, but lets step through the problem area.

    OK, I agree here.

    OK if Vce were really a couple of volts, it would not be saturation in my book either. However, the initial conclusion isn't right. If you reduce Vcc to 10V, then the desired 11.5 V is not available. The end result is that Vce does go to about 0.2V and the final collector current is about 9.8V/1000ohms=9.8mA, which implies a reduced value of beta.

    I agree with this statement if you qualify it as being "impossible to EXACTLY predict the value Vce". The saturation value does depend on the transistor, but the general rule of thumb that Vce is about 0.2V works in practice. You dont' need the exact value to know when the transistor is clearly in saturation. In this case assume any value, even zero, and the supply voltage is not available to drive the desired current.

    I could be wrong too. Have I misinterpreted your point in some way?
     
  20. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I believed the transistor would be saturated, but as a check, I ran a simulation on a 2N3904 with the beta modified to be β=50. Vce was 125mV.

    EDIT: Having said that, almost all transistor saturation specs are with forced beta=10. Good design practice says the designer should use this as a guideline.
     
    Last edited: Apr 30, 2009
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