Transistors, bjt, mosfet can't understand the values or what is needed to make it operate.

Discussion in 'General Electronics Chat' started by Dr.killjoy, Apr 29, 2016.

  1. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    I have a hard time understanding transistors, bjt, mosfets and such .. I already read several articles and can't understand the values or what is needed to make it operate .. Do you have any suggestion and also is there a trick to reading the datasheets ??
     
  2. Marley

    Member

    Apr 4, 2016
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    You question is a bit wide for a simple answer here. This site has some good articles in the education section.
    A good way to learn is to make a simple project and fully understand it as you build. Ask questions!
     
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  3. crutschow

    Expert

    Mar 14, 2008
    13,042
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    Both MOSFETs and BJTs are devices that basically control their output current (collector-emitter current in a BJT and drain-source current in a MOSFET) to achieve amplification of the input signal.

    The control is determined by the base-emitter current in a BJT. The base current is multiplied by the current gain (called hFE or Beta) and is typically in the neighborhood of 100 but can vary by a factor of three or more.
    The base-emitter input looks like a forward-biased diode so it's a low-impedance and has a typical voltage of about 0.7V when the transistor is on.
    Because of its low impedance you can't apply a voltage directly to the base, so you normally add a base resistor to limit the base current to the desired value.
    If you use the BJT as a switch, then the base current is usually set to be at least 1/10th of the maximum collector current to fully turn it on.

    In a MOSFET the current control is determined by the gate-source voltage.
    The MOSFET doesn't start to turn on until the threshold voltage is reached [as shown in the data sheet as Vgs(th)].
    Above that point the drain-source current is determined by the transconductance value (current vs. voltage as shown in the data sheet).
    The gate input of a MOSFET looks like a capacitor with a near infinite input resistance.
    If the MOSFET is used as a switch then you need to apply a minimum voltage to the gate-source to fully turn it on, as shown in the data sheet where the ON resistance [Vgs(on)] is specified.
    This voltage is typically 10V for a standard MOSFET and 5V or less for a "logic-level" type MOSFET.

    To convert the current change in the transistor caused by the signal change on the input to give a voltage change at the output, you add a resistor from the power supply to the collector/drain of the transistor with the emitter/source grounded.

    Data sheets can be a little intimidating when you first look at them, so it takes a little work to understand them.
    Look at each item in the data sheet and try to determine its meaning.
    If you don't understand a parameter, try Google for an explanation.
    Some of the values are called Absolute Maximum Ratings and those should never be exceeded in normal operation (ideally you want to operate well below them).
    Other parameters are those needed in normal design of the device such as gain, frequency response, ON resistance or saturation voltage, leakage current, rise and fall times, etc.
     
    Last edited: Apr 29, 2016
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  4. Papabravo

    Expert

    Feb 24, 2006
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    In another thread, the TS jogged my memory of how I came by my understanding of transistors and MOSFETs. I had to build test jigs to measure the characteristics of the device, and plot the results by hand using french curves. It was boring and tedious, but ultimately was the trick to achieving enlightenment. In the modern era you could use LTSpice to build the test jigs and plot the data. You won't have to suffer like I did and you may or may not reach Nirvana.
     
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  5. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    My plan is exactly that and I dont have many parts but a couple 2n2222 and 2n3904 I think, but will make it work for better enlightenment I hope..
     
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  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    I recall doing that with the J310 (notes still in my file). Helped to really understand things.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    To check ScottWang's blog and you make some examples which you want to know, if I know then I will explain the details for you.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    So true. You learn about base resistors best when you've touched a fiery hot transistor that didn't have one. You learn about the base-emitter voltage drop when you configure your first emitter-follower and plot out the output voltage against base voltage. You learn the importance of pinouts when you get enough of them wrong despite your best efforts. And finally you learn how incredibly useful these things are once you gain control over them.
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
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    Have you tried reading any text books?

    Once you know which parameters are important for your circuit, the datasheet should be easier to read.
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    The following pages of the EDUCYPEDIA will have theory and other usefull info on:
    JFET
    Mosfet's
    Transistor
    UJT

    Bertus
     
  11. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    Yes, a test jig for sure. Many data sheets show the test jig set-up. Device Under Test, I think they call it. Some manufacturers will supply test jigs. i.e. evaluation boards. Even for single transistors. At least they use to.

    You can compare the results of your unit with the data sheet. Also the manufacturers application notes have very valuable information.

    The character, parameters and measurements are reliable and usually well within tolerance.

    As for the theory of operation, if you are hip on the Standard Model of atomic structure, you should be ok.
     
  12. fastlingo

    New Member

    Apr 29, 2016
    5
    1
    mosfets are easier to understand I think, because they can be viewed as switches
    for n type mosfets, the source has to be more negative than the drain, for p type mosfets the source is more positive
    in practical terms you will connect a
    -n type mosfet usually with the source to ground, the load will be connected between vcc and the drain of the mosfet
    -p type mosfet usually with the source to vcc and the load will connect between the drain and the ground

    you will turn on a mosfet (close the switch) by
    -for n type by raising the gate voltage above the source by a value specified in the datasheet, called vth (threshold voltage)
    -for p type by lowering the gate voltage below the source by the minimum threshold voltage

    to turn off a mosfet (open the switch)
    -for n type you bring the voltage of the gate to ground
    -for p type you raise the voltage of the gate to vcc

    the gate of the mosfet is like a capacitor, meaning when you try to change its voltage you will have a lot of current for a short period of time. power mosfets cannot be turned on and off effectively from a microcontroller pin for example (they cannot sink or source a lot of current). but once turned on or off, the mosfet will draw almost no power, it's the switching that draws power

    bjts are a bit more complicated to understand (at least it was for me)
    if you want you can view the transistor as a variable resistance that is controlled by the current that flows between the base and the emitter (corresponding to the gate and source in mosfets)
    unlike a mosfet, the base (gate) of a bjt is not a capacitor, meaning it will draw current continuously, in practical terms if you apply let's say 5 volts to the base of a npn bjt,without a resistor, you will destroy the transistor because you will have a short circuit to the ground
    the more current you have flowing through the base, the lower the value of the variable resistor
    in fact the resistance of the transistor will adjust itself so that the current flowing between the collector and emitter (drain and source) will be a multiple of the base current
    another difference from the mosfet is that bjt only need about 0.7 volts difference between the base and the emitter to start conducting
    because a bjt is controlled by current on the base, there will usually be a resistor connected between the signal that controls the base and the base, and in this way you vary the current flowing by varying the voltage applied (ohms law).
    bjts, like mosfets are of n and p type
    if you want to use the bjt as a switch (not its main strength) you have to
    -for a npn, connect the emitter to the ground, the load between vcc and the collector, and then raise the voltage of the base 0.7 v above the emitter (ground). what you will have between the collector and emitter (ground) is in essence a resistor that you can lower by increasing the base voltage
    -for a pnp, connect the emitter to vcc and the load between collector and ground. then you must apply a voltage to the base that is at least 0.7 volts lower than the emitter (vcc). as opposed to the npn, where current will flow through the base resistor into the ground, here current will flow from vcc through the base to ground

    unlike mosfets, transistors react linearly to changes in voltage on the base (at least for a certain region), which means they can be used to amplify analog signals. mosfets are hard to use this way because the region linearity is too small, they are almost like a switch, open or closed, whereas bjts are more like variable resistors
    the differences between them make them suitable for different applications
    in digital circuit for example, where voltages are mainly either high (vcc) or low (gnd) mosfets are more useful, since they are easier to set up.
    bjts are more useful on the other hand when you want to amplify weak signals, as they are very sensitive to minute changes in current.
    hope this was helpful
     
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