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[Newb question] Transistor types

Discussion in 'General Electronics Chat' started by Richardcavell, Jan 17, 2013.

  1. Richardcavell

    Richardcavell Member

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    Hi everyone. I'm quite knowledgeable about software and software requires that you think digitally. Since taking up basic electronic hardware, I've discovered that transistors are suitable for analogue applications like amplification of an audio signal, in addition to digital on/off scenarios.

    1. Is it the case that some types of transistors are more suitable for digital applications, while others are more suitable for analogue ones?

    2. Digital circuits tend to have many transistors that are switched in and out of series with each other, so the base current of a later resistor and the emitter current (after resistance of each) of an earlier resistor would tend to be similar, right?

    Richard
     
  2. Mike odom

    Mike odom Active Member

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    I would say most transistors are good for analog, digital is just a subset of how you use the transistor, put it into saturation to make sure it is full on. Darlingtons (coupled transistors) are mostly used for digital switching, although some high end amps use them as well. Again, it's how they are used, in their linear region, or saturated.

    2) It is generally called the collector current, the current that flows through the emitter/collector junction. But, yes, when you follow one transistor with a second one, the collector current is fed into the base of the latter. This is also used in amplifiers, to increase gain.
     
  3. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    You can get digital transistors, they usually have built in resistors
     
  4. dave

    Dave New Member

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  5. crutschow

    crutschow Well-Known Member Most Helpful Member

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    1. Bipolar transistors are usually used for low level audio circuits, since they are cheap and have generally good low noise performance. Virtually all audio ICs are built with BJTs. Power MOSFETS are often used in the output stages of high power audio amps since they require no drive current and are generally more resistant to momentary overload.

    All modern digital ICs use MOSFETs since they consume less power than equivalent performance BJT circuits. BJTs were widely used in some of the early ICs such as the 74xx series, but that gave way to CMOS versions such as the 74Cxx or 74HCxx.
     
  6. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    When I'm designing a circuit, I generally use BJTs for amplifiers since they are most often used in the linear region and require a fair amount of current to operate in saturation/cutoff mode. For switching circuits I usually use FETs, since they hardly require gate current, and they are generally better suited for high-power applications.

    Obviously each can be used for different applications, but this is usually how I select them.
     
  7. unclejed613

    unclejed613 Well-Known Member

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    bipolar transistors have various characteristics that make them good audio transistors or good switching transistors, but the fact is a good audio transistor can be used as a switch, and a good switching transistor can be used as an analog amp. many transistors have characteristics optimized for one role or the other (or for RF amps, or whatever), so a "switching" transistor (optimized for short switching time) will work better as a switch than an audio output transistor (optimized for ultralinear characteristics) would, and vice versa.
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I don't understand #2 in the first post.

    For #1. The key is optimization. Transistors are optimized for different tasks and we probably should be tailoring this discussion about "bipolar transistors", the NPN and PNP variety.

    The idea that a "digital transistor" does exist because internally it's an IC with a couple of resistors in it. It saves board space.
     

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