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How to Understand and Use Transistors (BJTs) in Circuits

If you are able grasp how to use and connect various transistors in circuits, it might easily enable you to comprehend how most electronic circuits and its principles work. In this post we make an effort in this direction.


Transistors (BJTs specifically) are the building blocks of all semiconductor devices found today. If there wouldn’t be transistors there wouldn’t be any ICs or any other semiconductor component. Even ICs are made up of millions of closely knit transistors which constitute the features of the particular chip.

New electronic hobbyists usually find it difficult to handle these useful components and configure them as circuits for an intended application.

Here we’ll study the functions and the way of handling and implementing bipolar transistors into practical circuits.

What are Transistors

Bipolar transistors (BJTs) and Field effect transistors are generally three terminal active electronic components which fundamentally work like switches for either switching ON or switching OFF power to an connected load or with some electronic circuit stage.

Transistors are normally recognized by their external package in which the particular device may be packaged.

The most common types of package in which these useful devices are enclosed, are the T0-92, TO-126, TO-220 and TO-3. We will try to understand all these specifications of transistors and also learn how to use them in practical circuits.

Understanding Small Signal TO-92 Transistors:

Transistors like BC547, BC557, BC546, BC548, BC549, etc all come under this category.

These are the most elementary in the group and are used for applications involving low voltages and currents. Interestingly this category of transistors is used most extensively and universally in electronic circuits due to their versatile parameters.

NPN transistor BC547, BC548, BC546 pinouts

Normally these devices are designed to handle voltages anywhere between 30 to 100 volts across their collector and emitter.

The base voltage specs is normally not more than 6, but they can be easily triggered with a voltage level as low as 0.7 volts across their base/emitter. However the current is dependent on the load for the BJTs. An extensive explanation can be found HERE.

How Transistors Switch ON/OFF a Load

Transistors (BJTs) are mainly of two types, an NPN type and a PNP type, both are complementary to each other. Basically they both behave the same way but in the opposite references and directions.

For example an NPN device will require a positive trigger with respect to the ground while a PNP device will require a negative trigger with reference to a positive supply line for implementing the specified results.

The three leads of the transistor explained above needs to be assigned with specified inputs and outputs for making it work for a particular application which obviously is for switching a parameter.

The leads need to be assigned with the following input and output parameters:

The emitter of any transistor is the reference pin out of the device, meaning it needs to be assigned the specified common supply reference so that the remaining two leads can operate with reference to it.

An NPN transistor will always need a negative supply to be connected at its emitter lead for functioning while for a PNP, a positive supply line.

The collector is the load carrying lead of a transistor and the load which needs to be switched is introduced at the collector of a transistor (see figure).

how to connect NPN, PNP transistor pinouts

The base pin of a transistor is the trigger terminal which is required to be applied with a small voltage level with reference to its emitter pin.

This allows the internal collector/emitter junction of the BJT to open and allow a heavier current to pass across the collector pin to the emitter pin and finally to the ground line. Since load is supposed to be connected with the collector pin, this load is thus switched ON in response to the base voltage.

The removal of the trigger supply to the base immediately switches OFF the load or simply the current across the collector and the emitter terminals.

How to Configure a TO-92 Transistor into Practical The Designs

The three leads of a TO-92 transistor may be identified in the following manner:

The pin out are identified in the following manner:

Holding the device with its printed surface facing you, the right side lead is the emitter, the center lead is the collector and the left side lead is the base.

The functioning and the triggering principle is exactly similar to what is explained in the previous section.

The device is operated with loads anywhere from 100 mA to 2 amps across their collector to emitter.

The base trigger can be anywhere from 1 to 5 volts with currents not exceeding 50 mA depending upon the power of the loads to be switched.

Understanding TO-126, TO-220 Power Transistors:

These are medium type of power transistors used for applications which require switching of powerful relatively powerful loads like transformers, lamps etc. and for driving TO-3 devices, typical BJTs are BD139, BD140, BD135 etc.

pinout details for BJTs like BD139, BD140, BD135

Understanding TO-3 Power Transistors:

These can be seen in metallic packages as shown in the figure. The common examples of TO-3 power transistors are 2N3055, AD149, BU205, etc.

pinout details emitter, collector, base for power transistors like 2N3055, AD149, BU205

The leads of a TO-3 package can be identified as follows:

Holding the lead side of the device toward you such that the metal part beside the leads having larger area is held upward (see figure), the right side lead is the base, the left side lead is the emitter while the metallic body of the device forms the collector of the package.

The function and operating principle is just about the same as explained for the small signal transistor however the power specs increase proportionately as given below:

Collector-emitter voltage can be anywhere between 30 to 400 volts and current between 10 to 30 Amps.

Base trigger should be optimally around 5 volts, with current levels from 10 to 50 mA depending upon the magnitude of the load to be triggered. The base triggering current is directly proportional to the load current.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. hello , my name is sajeesh, i am from kerala. i am a cable tv technician. is it possible to work our optical receiving unit by the induction current produced from the television and the set top box from the customer end. optical receiving unit ratings 6volt 150mA . average co axial cable length to customer is 60 meters. now optical photo diode works well in this scenario but not sufficient signal gain so need a amplifier with optical photo diode with two fet in it. can you help me

  2. Helo, can you elaborate on the optical receiving unit? I have no idea what it is.

  3. optical receiving unit consists of a photo diode plus two fet . Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry
    There are several photodetector types:
    PhotodiOptical receivers convert optical signal (light) to electrical signal (current/voltage)
    Hence referred ‘O/E Codes, Phototransistors, Photon multipliers, Photo-resistors etc. Positive-Intrinsic-Negative (pin) photodiode
    No internal gain
    Avalanche Photo Diode (APD)
    An internal gain of M due to self multiplication
    Photodiodes are sufficiently reverse biased during normal operation  no current flow, the intrinsic region is fully depleted of carriers.The first receiver element is a pin or an avalanche photodiode, which produces an electric current proportional to the received power level.
    Since this electric current typically is very weak, a front-end amplifier boosts it to a level that can be used by the following electronics. normally five stage fet is used for amplification but ineed only two stage amplification

  4. OK, but where is this unit installed, what is the function of this unit in the whole application?
    Is it kept on the TV set? If not then how should the induction toggle the unit? through a wire link?

  5. how to control current and voltage to the base of a transistor and if your answer is Resistor then i would like to know how to determine the value of a resistor?

  6. you can refer to the following article for knowing the formula:

    you can also go through the following article for more info

  7. sir I made a generator self starter driving circuit. it was built around ic555 and 4017. 555 used in astable mode to drive 4017. pin 3 of 555 connected to pin 14 of 4017.
    I used pin 2,7 and 1 to drive a relay. each pin has its own resistor of 4k7 value. then i connected them to the base of a transistor (547/8050 I used both ) and connected a relay. I tested the circuit by connecting it to 12v 40Ah battery. it was all okay but when I connected the self starter wire to the relay(NO point) it started malfunctioning by tripping again and again disrupting the ic programming. I tried changing the base resistor and transistor as well. I observed that when high current pass through the relay circuit starts malfunctioning. why please solve

  8. Mujahid, what is the function of the astable in your circuit? I could not understand this....also why did you use 3 pins in parallel of the IC 4017 for connecting the relay driver transistor?

    By the way did you connect a capacitor across pin5 and ground of the IC555? Do this if not done yet.

  9. Sir,
    As you stated for an NPN transistor, its emitter will connect to (-) of voltage source and giving a positive trigger voltage (resistorised) at its base, the transistor now conducts between collector and emitter.

    similar states for a PNP transistor with its emitter going to (+) of volt.source and passing a neg.trig signal at its base, the transistor conducts between collector and emitter.

    The rule is: As long as a transistor gets an appropriate input signal at its base, the path between emitter to collector gets to to conduct freely within its rated parameters.

    The above all is clear to me.

    Now i have some doubts:

    1. if we take an NPN transistor and give (+) to its collector, giving a negative trig.sig at its base and take the (+) output from its emitter, is this CORRECT to do??
    please guide me here.

    2. what do you mean by "The base triggering current is directly proportional to the load current". please do explain with some example.


  10. Sherwin,

    your first question is not possible because an NPN will accept only a positive trigger with reference to its emitter ground connection, so your idea cannot be feasible.

    2) The base current for any BJT is dependent on its collector load current, as the load spec increases the base current requirement also proportionately increases, you can refer to the following example article for more info:

  11. Swag,

    Simple! Dead simple! Just by reading your lectures I find the connections I missed. Thank you very much for explaining the inner characteristics of these components.

    The lack of this knowledge made me afraid to build and test. You are giving us the ability to build in a safe manner so that we do not just blow things up. I have not seen any other electronics lectures where this is done so clearly.

    Without this all theoretical electronics knowledge is useless.

    Thank you again,

  12. Hi Christo, I really appreciate your involvement with this website, please keep up the good work, I am truly glad this site is helping you learn something useful. Please continue posting your queries, I'll be most happy to help!!


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