Transistor Circuits Design

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.

Related product: Diodes, Transistors and Thyristors

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


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  • Colin Mitchell

    All the diagrams above are incorrect and don’t teach you anything about electronics.
    Nigon Kouk is messing up two different areas of discussion.
    Of course electronics is about learning from mistakes, but the article above is trying to pretend the discussion is FACTUAL. It is full of mistakes. The first diagram will not work because the 3R2 and 1k should be swapped and R2 is not identified.
    The second circuit does not work because the base sees less than 0.3v
    The third and fourth circuits have no load resistor and will blow up.
    The fifth circuit does not turn ON.
    These are just junk circuits from Professor D Mohankumar who has now been banned from publishing in Electronics For You Magazine.
    It has taken me 16 months to get him banned.
    You cannot have the web filled with incorrect information.

  • Nigon Kouk

    Even though this circuit is backwards :[) this Colin dude seems like a real ass, just like a cop who would shoot you down in the street for merely walking :[) but as that it be it may” electronics is about discovery, and experimenting, you have to make mistakes to learn from them 🙂 Happy Electronicking Y’all

  • Mesmeric

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

  • Eliminator

    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.

  • Popescu Marian

    I do not think will lose visitors because of Colin’s comments. You can see our stats in this article 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.

  • Colin Mitchell

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

  • Eliminator

    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 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 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.

  • Colin Mitchell

    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.

  • Eliminator

    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…… 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 to delete all those comments where the person has tried to divert the readers into his or her website.

  • Clive Grant

    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.

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