Mosfets

Discussion in 'General Electronics Chat' started by Pim, Jun 16, 2015.

  1. Pim

    Thread Starter New Member

    Jun 16, 2015
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    Halo forum Members,

    I am new here.

    my question is about a diagram of an amplifier input stage where little gain must come from approximately 6 times amplification.
    Now I wondered how the input stage can link a mosfet output amplifier steps.

    I've been working with transistors at the output.

    But see no way to do the same with mosfets.

    Who has ideas.

    Sincerely,

    Pim
     
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,800
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    First off MOSFETs are transistors (i.e. MOSFET abbreviates; "Metal Oxide Semiconductor Field Effect Transistor" )--- I will assume the intent of your (above quoted) statement was distinction of BJTs from MOSFETs ?-- Apologies for the 'nit picking' howbeit words really do matter:)


    I apologize that the 'language barrier' is hindering communication:oops:
    1)Do you mean a weak signal must be amplified sixfold?
    2)Are you inquiring about interstage impedance matching?

    Clarification of points 1 & 2 (above) will greatly assist me to assist you:):):)

    Best regards
    HP
     
  3. Pim

    Thread Starter New Member

    Jun 16, 2015
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    0
    Hello, HP

    Fine you want to help me.

    It involves the following.

    The preamplifier with NPN and PNP transistors in the electrical diagram 1 works well.

    Now I'm wondering if the same could be used now with MOSFETs, electrical diagram 2.

    The voltage on the collector of the driver Q 1 + 28Volt and Q 8 stage - 28 volts.

    When a mosfet is now connected to the points Collector Q1 colector and Q 8.

    There is the gate of Q 12 + 28 volts and Q 13-28 volts.

    But since the voltage between gate / source must not exceed 15 volts it's there fault.

    Normally placed, a 9 volt zener diode of between gate and source, electrical diagram 3.

    But in this case, therefore, will the voltage at Q1 and Q8 also go to 15 Volts, and that is not the intention.

    How to solve this problem, that is the question.

    Sincerely,

    Pim
     
  4. Pim

    Thread Starter New Member

    Jun 16, 2015
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    I forgot to mention.
    The gain is approximately 6 ×.
     
  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    1,237
    In General:

    Please be advised that (most significantly) BJTs and MOSFETs exhibit disparate bias requirements, and, hence, are not suited to 'drop in' substitution...

    You may find it useful to regard MOSFETs as electrostatically, and, hence, EMF controlled while BJTs (with some qualifications) might be seen as current controlled... While this is, of course, a gross oversimplification, I feel it is, nonetheless, illustrative of the basic concept at hand...

    It is not sufficient to merely protect the gate 'junction' from 'out of range' conditions -- maintenance of gain, linearity, noise figure, etc... requires the redesign to 'embrace' the fundamental differences in device operation...

    Best regards
    HP:)

    PS I apologize that I was unable to load/view your attachments (owing to so called 'security issues' with Adobe:mad:) -- my problem, not yours...:oops:
     
    Last edited: Jun 17, 2015
  6. Bordodynov

    Active Member

    May 20, 2015
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    I have a question for what this amp (Schema 3.pdf). Because the resistors in the transistors (R18 = 1.5 kOhm and the like) transistors are triode region. In fact, the transistors can not provide a large current. Very strange scheme. Detail of the purpose?
    The scheme is not properly constructed. The first stage on the non-inverting. The output transistors are included as repeaters of. Where negative feedback? I think you have to swap the output transistors! Nmos<-->Pmos. And accordingly move the zener. Transistory enable the scheme common source.
     
    Last edited: Jun 17, 2015
  7. Pim

    Thread Starter New Member

    Jun 16, 2015
    4
    0
    You're right.
    These are equally selected to complement the schedule, these resistors will be about 240 ohms in practice.

    But that was not the problem.
    Thanks
     
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