amplifier

Discussion in 'General Electronics Chat' started by napos, Mar 18, 2006.

  1. napos

    Thread Starter New Member

    Mar 3, 2006
    5
    0
    Hello!

    Most of the amplifier circuits in Internet are built on BPT. So I would like to now what is the reasons that there are so few (or non) amplifiers built on FET. FET-s take less energy to produce the amplification (you control amplification with voltage not with current like in BPT).

    One reason could be this:

    "...the control effect of gate voltage over drain current is nonlinear. Notice how the drain current does not decrease linearly as the gate-source voltage is increased. With the bipolar junction transistor, collector current was directly proportional to base current: output signal proportionately followed input signal. Not so with the JFET!..."

    So if there would be no problem with this linearity then the FET-s would be more popular in amplification circuits? Because of the nonlinearity FET is mostly used as a switch?

    The next question is: So if the FET-s are nonlinear, why there still are these amplifier circuits which are built on FET-s? Are these amplifiers not so demanding on this distortion issue, or these amplifiers are not used in sound amplification where the distortion is more noticeable?

    Next reason why I imagine, that FET-s are not so popular because of the fancy prize and the fear of electrostatic charge?


    Are there any more reasons why the FET-s are not so popular in amplifiers?

    Another concer with amplifiers is that, if we have a input signal which needs to be amplified, then we lead the signal to BPT amplifer and signal will be amplified. But if we want to amplify the same signal with FET then we just can just connect the source and the amplifier and the amplification will also work? Because in BPT the amplification is driven on current and now if we connect source to FET then there should be no current, because this will ruin our FET? Is this issue resolved so that if the input is always signal of voltage and in BPT there is a resistor which converts voltage to current and then will amplify it, but in FET amplifer there cannot be any resistor because no current is allowed? So both amplifiers are almost the same but the only difference is in this resistor?

    Also I've seen amplifier circuits which are built so that some of stages are on BPT and some are on FET-s. So why this is useful, why not built all stages on BPT?

    napos
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    First off I would like to point out that a it is a BJT. I haven't heard it called a BPT. BJT stands for Bipolar Junction Transistor.

    Now moving on. Amplifiers are not my forte but I will give it a go. You are pretty much right in what you are assuming. BJTs are more linear than a FET in basic configuration. But, and there is always buts, FET amps can be made just as linear as a BJT amp. It all comes down to the biasing. What maybe able to be achieved with a single stage BJT amp may take 3 stages with a FET amp.

    FETs can also be much less noisy which is why they are used a lot in output stages. Your concern about the current blowing up a FET is correct in a way. If you plug a FET directly in place of a BJT you may well destroy it. And visa versa.

    And again you are correct in the price. Decent power FETs can be very expensive.
     
  3. napos

    Thread Starter New Member

    Mar 3, 2006
    5
    0
    Sorry for the typo I'm too used to with my language where BPT means the same as BJT
     
  4. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    I think the main reasons might be price, and stability. Power FETs up to the task can be pricy (I had to replace the FETs in mine a while back) and from what I understand, some care must be taken to ensure the amp is stable into most loads.
     
  5. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Another thing that you have to be careful with when playing with FET O/P audio amps is the speakers. NEVER unplug or plug the speakers while the power is on. The sudden change in load can instantly pop the FETs. And as gadget says some of the big fancy ones can also pop your wallet.
     
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