• Register
  • Login
  • looking for something?

    Transistors are inevitable parts of Electronic circuits. The success of a circuit design lies in the selection of proper transistor type and calculation of voltage and current flowing through it. A small variation in the voltage or current level in the transitor will affects the working of the whole circuit. Here explains how a transistor works.

    Voltage and Current in the Transistor design

    The Fig.1 explains how voltage and current are flowing through a bipolar transistor. Input voltage to the circuit is 12 volt DC. The base of T1 is connected to a potential divider R1-R2. If they have equal values, half supply voltage will be available at the base of T1. Here the value of R1 is 3.2 Ohms. If the value of R1 is three times greater than R2, then three quarter of 12V drops by R1 and allow one quarter to pass through R2. Therefore the base voltage of T1 will be 12 / 4 = 3 V.

    Thus the voltage provided by R1 to the base of T1 is 3 volts. The emitter voltage of T1 will be 0.7 volts less than 3 volts since T1 drops 0.7 volts for its biasing. Thus the emitter voltage appears as 3-0.7 = 2.3 volts. If the value of the emitter resistor R4 is 1K, then if 2.3 volt passes through it, emitter current will be 2.3V/ 1 = 2.3 mA. Collector current also remains same. If the value of the load resistor R3 is 2K, two times higher than that of R4, then the voltage drop across it will be 2 x 2.3V = 4.6 volts. There fore the collector voltage of T1 remains as 12 – 4.6 = 7.4 volts.

    Load current

    In the circuit shown in Fig.2 ,6 volt DC supply is provided. T1 is a general purpose NPN transtior like BC 548. A potential divider comprising R1 and R2 bias the base of T1. Minimum base voltage necessary for biasing T1 is 0.7 volts.
    The potential divider R1-R2 drops 6-0.7 = 6.3 volts.

    If the load takes 4 volts,then the collector voltage will be 2 volts. 6-4=2 volts.

    Value of the collector current depends on the base voltage. When the base voltage increases, collector current also increases. This results in more volts in the load. In short, 0.1 volt increase in base voltage causes 1 Volt increases in the load.

    Current in the Transistor Amplifier

    Normally when a High volt is present at the collector and Low volt in the base, Base-Emitter junction of T1 will be reverse biased.If the collector remains open, collector voltage will be 0 and hence the base current will be 100 mA. If collector of T1 is connected to the Vcc, 98 mA current flows through the collector and 2mA to the base.That is

    Emitter current = Base current + Collector current = 2mA + 98mA = 100mA
    Collector current = Emitter current – Base current = 100mA – 2 mA =98 mA
    Base current = Emitter current – Collector current = 100mA – 98mA = 2 mA

    The condition is just reversed in the case of a PNP transistor as shown in Fig.4. The base-emitter junction of T1 forward biased and the base-collector junction is reverse biased. In this state,T1 remains non conducting. If we makes the base more negative, T1 conducts and collector current appears.

    Transistor as a Signal Amplifier

    Through a small capacitor AC signals of small volt can be given to the base of a signal amplifier as shown in Fig.5. This changes the load voltage. A large change in base voltage – Amplified Signal – gives output signals from the collector of T1.

    Amplification = Value of Output signal / Value of Input signal.

    Amplification usually lies between 10 and 100. That is the Output signal is 10 to 100 times higher than the input signal.

    Transistor Biasing

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
    Find more projects
    

    23 Responses to "Transistor Circuits Design"

    subscribe to newsletter
    1. Thanks, very nice article, a must for newly introduced to electronics

      Like or Dislike: Thumb up 0 Thumb down 0

    2. thank you sir for this very useful information abt designing transistor ckt..i’ve done this b4 in university,,its a refreshing witdh full details..more power

      Like or Dislike: Thumb up 0 Thumb down 0

    3. sir one thing more…on the voltage and current design FIGURE,,,Vc is all respected to the ground?that only if R4 is connected ryt???
      so from this we can create a REGULATED DC VOLTAGE frm Vc-collector output..
      this is more usefull in STEPPING DOWN VOLTAGE REGULATOR…pls i need ur reply,i wna knw more to mastered in transistor design,tnx once again!

      Like or Dislike: Thumb up 0 Thumb down 0

    4. the gain of the amplifier should added in the calculation of

      Like or Dislike: Thumb up 0 Thumb down 0

    5. vasanthkudva says: on June 1, 2011 at 10:27 am

      Thank u Sir for such a useful information. . . . .

      Like or Dislike: Thumb up 0 Thumb down 0

    6. This article is completely incorrect. It is the worst description I have ever seen.

      It should be removed from the web immediately.

      All D. Mohankumar’s articles are poorly written. He has absolutely no idea about electronics.

      Colin Mitchell
      TALKING ELECTRONICS
      talking@tpg.com.au

      See my website for the correct way to describe how a transistor amplifier works.

      Like or Dislike: Thumb up 0 Thumb down 0

      • Colin Mitchell WHERE is your site? The best skipper is always ashore.

        Like or Dislike: Thumb up 0 Thumb down 0

      • Click on his name to visit his website.

        Like or Dislike: Thumb up 0 Thumb down 0

      • C.Mitra Chinmoy Mitra says: on July 23, 2012 at 8:38 am

        Colin is correct, the article is all rubbish. Has anybody ever heard of a base biasing resistor of 3.2 ohms? This resistor (R1) and the lower resistor (R2) are in series directly across the 12V battery! The resistor will blow in milliseconds! Please don’t publish such misleading circuits for the sake of the poor beginners who are naive enough to follow your directions and try to construct transistor projects.

        Like or Dislike: Thumb up 0 Thumb down 0

      • Eliminator says: on March 1, 2013 at 3:10 pm
          member

        Everybody please check out the following forum post to see what other electronic experts have to say about Mr. Colin Mitchell:

        http://forum.allaboutcircuits.com/showthread.php?t=62887

        Like or Dislike: Thumb up 0 Thumb down 0

    7. Can I use a series transistor as a modulator for an AM transmitter without dropping 1/2 of the supply voltage acros the series modulator?

      Like or Dislike: Thumb up 0 Thumb down 0

    8. is there any one willing and able to help me design a switching circuit
      i have a basic design idea(schmatic)
      and other info just not sure how to choose correct transisors and other related parts yes i am transistor illiterate only basic knowledge

      Like or Dislike: Thumb up 0 Thumb down 0

    9. See my website for the correct way to describe how a transistor amplifier works.
      See The Transistor Amplifier article

      Like or Dislike: Thumb up 0 Thumb down 0

    10. how do I display the pervebial “bird” finger with gravatar.com? if all is as Collin says it seems nothing is as it seems.

      Like or Dislike: Thumb up 0 Thumb down 0

    11. Clive Grant says: on November 17, 2012 at 1:58 pm

      I confirm Colllin Mitchel’s assessment of this article. Doesn’t this guy know that voltage does not flow, it is stationary and is measured across circuit elements – potential difference between the two points. It is the current that flows through an element that creates this potential difference!
      Please focus and if, as seems necessary, go back to school before you attempt to subject students to further nonsense.

      Like or Dislike: Thumb up 0 Thumb down 0

    12. Eliminator says: on February 28, 2013 at 12:09 pm
        member

      Mr.Colin, are you trying to promote your website here?

      because I can see many affiliate and other types of advertisement links in your website….

      Your criticisms are valid but are put in a harsh way so that the readers get instantly influenced by them……..it seems you have come solely for promoting your website and your business.

      If you are intentions are honest enough you should provide the full explanation here itself.

      I would like to request electroschematics.com to delete all those comments where the person has tried to divert the readers into his or her website.

      Like or Dislike: Thumb up 0 Thumb down 0

    13. Eliminator!! It has taken you 3 months to read my last discussion. I have never seen you before.

      I have already had a discussion with P Marion who runs this website and he has removed D Mohankumar from this site due to the number of faults in his circuits and discussions.
      No-one else has complained about my website and I get over 6,000 visitors EACH DAY.
      This site does not allow me to include a circuit diagram and it is much easier to let readers come to my site where I have hundreds of pages of discussions and articles.
      You are the first to whine, out of hundreds of thousands of visitors.
      If you knew how damaging D Mohankumar’s articles are, you would be in total agreement that this type of incompetent person should be removed from the web.
      He is also putting projects in Electronics For You magazine and because they do not have a technical editor to scrutinise the articles, the are publishing faulty projects with glaring mistakes.
      I have already made millions out of Talking Electronics and retired over 10 years ago. I don’t need any further income so don’t waste your time on the money issue.

      Like or Dislike: Thumb up 0 Thumb down 0

    14. Eliminator says: on March 1, 2013 at 4:30 am
        member

      Mr. Colin nobody has ever complained because nobody could ever understand about the nice business tactics that you are employing here.

      Out of the 6000 visitors 2000 might be coming from this site, because electrschematic.com has a very good reputation and a huge traffic statistics.

      Mr.Collins you might have made millions, but you have no right to divert traffic from any site into your website.

      I am sure electroschematic.com is LOSING a lot of had earned traffic due to Mr.Collins links.

      Don’t worry about posting the images….you can do one thing, there are many free image hosting sites online, you can upload the corrected images there and provide the link to the readers here.

      Like or Dislike: Thumb up 0 Thumb down 0

    15. The number of people reading these discussions would be less than 50 per day.

      Like or Dislike: Thumb up 0 Thumb down 0

    16. I do not think electroschematics.com will lose visitors because of Colin’s comments. You can see our stats in this article http://electroschematics.com/7462/electroschematics-com-2012-stats/ and we have around 7000 to 8000 visits per day. I think the visitors are looking for info about a certain circuit and then compare it with other sites. Some will choose the circuit that is simpler, some will choose one depending on the available components, some on other factors.

      I do not comment on Mr Colin’s knowledge, he seems to have read a lot, practice and even contribute a lot on an electronics magazine. I will post soon a video about him. I actually like him, he seems to be a good man, but his way of criticizing is kind of harsh.

      Like or Dislike: Thumb up 0 Thumb down 0

    17. Eliminator says: on March 1, 2013 at 6:45 pm
        member

      The point is that he is creating an impression that he is too concerned about the readers.

      If he so much concerned about the electronic enthusiasts he should start an open forum in his websites, and allow the visitors to ask specific questions related to electronics, and should make sure that each of the questions are answered.

      Actually he speaks as per his own convenience, he will never entertain specific questions, and won’t bother to see whether they are solved or not.

      Like or Dislike: Thumb up 0 Thumb down 0

    18. Mesmeric says: on March 23, 2014 at 4:39 pm

      How do find the solutions for problems involving potential devider and bias transistors?

      Like or Dislike: Thumb up 0 Thumb down 0

    What do you think about this article? Leave a comment!