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Transistor Uses

Discussion in 'General Electronics Discussion' started by dondon, Jan 24, 2015.

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  1. dondon


    Jan 24, 2015
    Hello! Good Day! It's my first time here so please forgive me for whatever mistakes I will be going to make.

    I became interested in electronics because I think that it will be good to me. So therefore I am new to electronics.

    I searched about Transistors especially those BJT and I found out that it's most common uses are being as a switch and as an amplifier.

    I am just simple type and below average type of person and my knowledge in English language is very limited. I've poorly understand those stated in internet about its uses. It takes time for us (together with my friend) to write this Thread using good grammar.

    Can someone here discuss me the HOW, WHY, and WHEN does a transistor became switch and HOW, WHY, and WHEN does a transistor became an amplifier.

    Please discuss it using very common words and discuss it in a way you are discussing to a child (because I am slightly below average person). Please Please Please Teach me about this. I want to make some simple electronic circuits soon.

    Note :
    I understand that diode has forward and reverse biasing.
    I don't want some biasing of transistor is to be discussed here. I will come to that part as soon a understand how a transistor became switch and amplifier
    I know about Ohm's Law
    Most of all, please respect. You all started in nothing so please appreciate.

    Thank You So Much Everyone!
  2. LvW


    Apr 12, 2014
    I do not intend to go into details for this first answer. Therefore, as a starting point, some general points only:
    1.) Usage as a switch: The transistor cannot be used as a signal switch (like a relay) but only as a switch that can provide two FIXED ouput voltages (high/low) depending on a smaller control signal (voltage high/low) at the controlling input (base node).
    2.) Usage as a voltage amplifier: The transistor is in fact a device which acts as a voltage-controlled current source.
    This means: A small voltage change (signal voltage) between the controlling input nodes (base-emitter: Delta Vbe=Vin ) cause a corresponding output current change (Delta-Ic). This signal current at the output node (collector) can produce an output voltage Vout across an external resistor, which can be larger than the input signal voltage.
    This is equivalent to voltage amplification A=Vout/Vin.

    I hope this can be a good starting point for further discussion.
    Arouse1973 likes this.
  3. Laplace


    Apr 4, 2010
    The bipolar junction transistor has three recognized operating regions: Active, Saturation, Cutoff.

    Active Region -- The collector junction is reverse-biased and the emitter junction is forward-biased.

    Saturation Region -- Both the collector and emitter junctions are forward-biased.

    Cutoff Region -- Both the collector and emitter junctions are reverse-biased.

    Note that in order to cutoff the transistor, it is not enough to reduce the base current to zero. Instead, it is necessary to reverse-bias the emitter junction slightly (0.1 V for germanium, ≈0 V for silicon) so in cutoff the emitter current is zero, and the collector-base current equals the collector reverse saturation current, Ico.

    A transistor becomes a switch when it is driven between cutoff and saturation. For silicon transistors in common-emitter configuration, cutoff is when Ib=0, Vbe=0. Current flowing through the load in cutoff is just the reverse saturation current. When the transistor turns ON there should be enough base current supplied to drive the transistor into saturation. The datasheet may give values for current gain at several values of collector current. The minimum base current necessary will be the load current at the collector divided by the current gain. The actual current forced into the base will typically be 200% of the minimum.

    A transistor becomes an amplifier when it is biased halfway between saturation and cutoff and output is taken from the load resistor. (But biasing is another topic.) Once the transistor is properly biased it is common practice to use a transconductance model of the transistor to analyze the performance of the amplifier. However, the transconductance of the physical transistor is highly non-linear and temperature dependent. To assure accurate results and low distortion it is necessary to keep the signal variation of Vbe within a 10 mV slice of voltage around the bias point.
  4. BobK


    Jan 5, 2010
    I think you would be best off to read a book covering electronics theory and practice and come to the forum when you don't understand something. It is not reasonable to ask us to teach you electronics, one paragraph at a time.

    See the "Book Reviews" sticky thread in this forum. The first part is about a very good book that you could learn from.

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    When a BJT is used as a switch, it is always in one of two states: OFF and ON. Just like a light switch.

    A BJT is controlled by its base. Applying a voltage between the base and emitter causes current to flow, and this makes the transistor conduct. It allows current to flow through its collector-emitter path. This current can be a lot higher than the current flowing into the base.

    When a transistor is OFF, no voltage is applied to the base, and no current flows in the base-emitter circuit. The transistor does not conduct current in its collector-emitter circuit. No current flows; the circuit is inactive.

    To turn a transistor ON, a voltage must be applied to the base, usually through a resistor. This causes a small current to flow into the base and out the emitter, and the transistor conducts current through its collector-emitter path. Therefore the transistor switches the current in its collector circuit ON and OFF, depending on whether base voltage is applied.

    The current in the collector-emitter circuit can be many times higher than the base current that controls the transistor. So the transistor still behaves as a kind of amplifier, but it is only ever used in two states - ON and OFF.

    When a BJT is used as an amplifier, both the base current and the collector current vary over a certain range. In other words, the transistor is never fully OFF, and it is never fully ON; it is always somewhere in between.

    When a signal is applied at the base, the transistor produces an amplified signal, usually at the collector. This amplified signal has the same shape as the input signal, i.e. it is not greatly distorted, but it has more voltage, or more current, or both.

    There are three standard "configurations" for a BJT switch or amplifier: common emitter, common collector (also called emitter follower), and common base. These different configurations cause the BJT to amplify in slightly different ways, and are used in different situations.

    These descriptions are really just to satisfy your curiosity. You will not be able to use them to design or create a circuit using BJTs. There are a lot of other things you will need to learn - about voltage, current, resistance, and other support components - before you will be able to properly understand and design BJT circuits.
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