Transistor basics

Discussion in 'General Electronics Chat' started by Resist10k, Feb 20, 2012.

  1. Resist10k

    Thread Starter New Member

    May 7, 2011
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    I have some questions about how to properly setup a basic common-emitter circuit. What are the calculations In order to get the resistance for full saturation? And how is the gain value calculated in data sheets it ranges so much with tiny voltages and tiny currents.i don't see how to get an exact amount is it just a average gain? Is "full saturation" / fully on operation just allowing near maximum IC current through?
    lastly the equation i have for base resistor is
    Rb=Vb-Vbe / IC/Hfe
    not sure if that is correct :(
    [​IMG]
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Your calculation for the base resistor is completely WRONG.

    The hFE (DC current gain, hfe is its AC current gain) of a transistor is used to calculate its base resistor only when it has plenty of collector to emitter voltage so it is a linear amplifier and it is not saturated.

    The datasheet for every transistor shows its maximum saturation voltage when its base current is 1/10th or 1/20th its collector current with no mention of hFE.
     
  3. Resist10k

    Thread Starter New Member

    May 7, 2011
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    so how would i find the right base resistor? when using it as a switch?
     
  4. Felo

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    Feb 20, 2012
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    Personally over-saturation is my flavor, for popular BJT I use any res from 4k7 to 15k never had any problem and I use them heavily as a relay driver
     
  5. Audioguru

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    Simply make the base current 1/1oth the collector current like I said before and like it is shown on the datasheet for the transistor.

    Here is from the datasheet for a 2N2222A transistor:
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Resist10K, please feel free to skip this post. I'm going to commit All About Circuits heresy and tell you you don't have to follow the Ib=Ic/10 "rule."

    First off, your equation Rb=Vb-Vbe / IC/Hfe is correct if you put the parenthesis where I think you wanted them:

    Rb=Vb-Vbe / (IC/Hfe)

    There is nothing wrong with that, and there are many times it applies, especially when the transistor is in the "active" region (meaning on but not saturated).

    The 2N2222 is an old established product first introduced by Motorola in 1962. As such, if you do a little digging you can find what's known as the "curves," or graphs of the device for different conditions. Here is such a graph for one modern version:
    [​IMG]
    There is a lot of information packed into that graph. It is showing 4 different cases of saturation for collector currents of 1mA, 10 mA, 150 mA, and 500mA. Vertical is collector voltage, or "saturation" voltage, and horizontal is base current.

    Look at Ic=1.0mA: with only .01mA of base current the collector voltage is still a very low 0.1 volt, and the "gain" there is 1/.01 = 100! So much for "always" needing a base current of 1/10 the collector; that case it can be as low as 1/100.

    Now go to the high end for Ic=500mA. Things get tougher. Say I can allow 0.3V on the collector: that crosses at 15mA of base current, or a gain of 33.3. At that point the deice is burning off real power: .5A * 0.3V = .15W, and there is also another 0.1 watts in the base for a total of .16 watts. Still, the device is rated for .65 watts (room temperature) so maybe that is just fine.

    Here's an example: Say you drive a transistor from a 5V logic circuit, how big is the resistor you need?

    Well, our gain is 33.3, our collector current is 0.5A, and the Vbe is 0.7V. The resistor is thus:

    Rb= Vb-Vbe / (IC/Hfe)
    = (5V - 0.7V) / (0.5/33.33)
    = ( 4.3 ) / ( .015 )
    = 286.7 ohms

    All you told me was your load was "large" but if "large" is less then half an amp this should work. Personally, I look for the part of the curve "before the knee," or then end of the flat line to the right of the rise. I would not use this device for a half amp load, but would for a 150mA load, and I wouldn't try to drive it harder then Ib=5mA.

    Yes, using a gain of 10 at saturation works, it is also very very conservative in many cases.
     
  7. Audioguru

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    Dec 20, 2007
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    The curves in a datasheet are for a "typical" transistor. But transistors have a wide range of gain and you can't buy a "typical" one. You get whatever they have.
    Maybe they had a bad day and all their transistors in stock have low (but passable) gain.

    Therefore I always design circuits using "worst case" spec's like using a base current that is 1/10th the collector current to guarantee that a transistor will satuate at least as well as the worst one shown in the datasheet. That way ALL my circuits work perfectly.
    Some people design with "typical" spec's then replace all the transistors that have a gain less than "typical" to make their circuit work. Or maybe those people are lucky.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yep, blow back, as I predicted.

    I've worked on and designed circuitry that has literally been from the depths of the oceans to outer space. It is against my inclination to assume a circuit will operate over a temperature range of less then -55 to +125°C. To achieve that specs need to be carefully reviewed, slight tidbits extracted, and only considered complete after the circuit has been tested inside a temperature chamber to verify all the important parameters. I'm well familiar with worst case analysis.

    For this case, with an unstated project from someone apparently starting out an assumption of a steady +25°C environment is probably correct, and if not when the device fails the OP can repost and learn a little more.

    I happen to be quite aware of the saturation characteristics of the 2N2222A, especially in it's military version screened to the JANTX level. I spent several hours in the bowels of the USS Virginia (SSN-774) due to these devices failing due to an excessively high saturation voltage even running with a forced beta of 10.

    So much for "1/10th the collector current to guarantee that a transistor will saturate at least as well as the worst one shown in the datasheet" rule.

    What forced me to post was when Resist10K's perfectly good equation for computing his base resistor was discarded, without explanation and without correction.

    Resist10K, sorry for the thread hijack. I do want you to know you seem to be heading in the right direction.
     
  9. Audioguru

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    ErnieM,
    Why didn't your 2N2222A transistors meet their guaranteed worst case spec?
    Were they counterfeit?
     
  10. Resist10k

    Thread Starter New Member

    May 7, 2011
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    Thanks for clarifying a bit Ernie :) but im still confused about how to find the gain. If i wasn't using a collector current on the curves chart how would i get the right values? i was wanting to push a 2n4401 to 400ma IC with a 5v base voltage and i cant seem to find a similar curve sheet. is the only way to get an accurate beta value for your specified collector current is with curves or ball-park guesses? And is there a reason that we dont just force alot of base current in to make sure its always saturated?
     
    Last edited: Feb 21, 2012
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    They were quite genuine, and in fact as far as I could tell were still meeting the military spec for JANTX screening.

    It's just that whoever wrote the military spec for that device did not define what the saturation characteristics were. So parts with a 1.5V and above sat were still okie-dokie.

    We were not allowed to make any changes to the BOM, so first I got samples from every QPL manufacturer and ran the parts in a circuit forcing the same conditions they would see in our circuit, found two manufacturers that were acceptable. Those were given to purchasing and we added an incomming screening step.

    As far as the military was concerned we didn't make any changes so it was acceptable.

    Some 2N4401 curves

    You seem to now understand that the gain of a transistor is not just a simple number. Congratulations. That spec sheet has a section of ELECTRICAL CHARACTERISTICS which tells you to expect a gain of 50 when you put 500mA (closest to your 400mA) into the collector, but you still get a Vce of 2V, which is 1 watt of real power. What's worst, the spec only holds for "TA = 25°C unless otherwise noted."

    When picking any electronic device the first thing is to never push it to it's limit. If you have 1W of power buy a 2 watt resistor or more. For 400mA in a collector look for a device rated for at least 800mA.

    As far as pushing too much current into the base, well, you could pump in 1 amp for every amp of collector current you need, but that is a gain of 1. It will have an excellent sat voltage. :D

    (Plus transistors don't turn off as fast if you drive them too hard)

    When you buy a transistor you are buying current gain. I'm a cheap bastich, I want a lot of gain for my money.

    <sigh> I'm too tired to state how I pick out a number for gain tonight.
     
  12. Audioguru

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    Dec 20, 2007
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    The curves on a datasheet are only for a "typical" transistor.
    But you cannot buy a "typical" transistor, you might get one with a low current gain or one with a high current gain. Transistors also have different base-emitter voltages.

    Why don't you look at the datasheet for your 2N4401 transistor? It says that it is guaranteed to saturate when its base current is 1/10th the collector current. Then its max collector-emitter saturation voltage is about 0.7V but the curve for a "typical" transistor shows 0.25V.

    If you feed 5V to the base then you will blow up the transistor. The base emitter saturation voltage will be max about 1.15V or about 0.97V for a "typical" transistor.
    Since the base-emitter voltage is different for each transistor and it changes when the temperature changes then a transistor is turned on with a current, not with a voltage.
     
  13. Resist10k

    Thread Starter New Member

    May 7, 2011
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    Id absolutely love to believe the 1/10th Collector current for base rule, i am sure that it works but is there a more exact / energy efficient way? maybe we dont always need the whole 1/10th of collector current. that holds true for the curve that ernie posted the 1/10th rule would make everything alot easier but...
    EVERY site i go to has never mentioned that rule they all use a gain calculation the only thing they differ on is how to get that gain. this is the only place i have ever heard the 1/10th rule. every other site is wrong?
    http://www.kpsec.freeuk.com/trancirc.htm
    http://www.rason.org/Projects/transwit/transwit.htm
    http://www.electronics-tutorials.ws/transistor/tran_4.html

    Im still confused on how to determine the gain of the 4401 at any collector current :(
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    Neither is wrong. What Audioguru is saying is if you use the 1/10th rule, it will work every time.

    What you can do is get a variable resistor or a resistor substitution box and start with the resistor from the 1/10 calculation. Increase the resistor and observe at what point the transistor comes out of saturation. Then pick a resistor value somewhere in between the two values.
     
  15. Audioguru

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    Dec 20, 2007
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    Buy a few thousand transistors made at different times and made by different manufacturers, test them all then use only the few best ones.

    Or design a circuit busing "typical" specs, make your circuits and test them, then replace the transistors with better ones in the circuits that don't work.

    Or design the circuit worst case so that ANY passing transistor will work perfectly.
     
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