Transistor Biasing for Set Gain

Discussion in 'Homework Help' started by Dannyhimself, Jan 22, 2013.

  1. Dannyhimself

    Thread Starter New Member

    Jan 6, 2013
    5
    0
    Hi guys.

    I'm trying to build a gain stage for a small pre-amplifier that has a gain of 5. I am using a common emitter configuration with an NPN transistor.

    The input source is a 10mV peak to peak signal from say a microphone, for examples sake feeding in a 1kHz signal.
    It is running of a 12v supply and i want to bias the transistor to run at its Quiescent point of 6v.

    I've used the following guidlines to come up with this resistor values as follows:
    (http://www.rason.org/Projects/bipolamp/bipolamp.htm)

    Vcc = 12v
    Ic = 2 mA
    hFE = 5

    Ib = Ic/Hfe = 0.002/5 = 0.0004 (.4 mA)

    R1 = half Vcc / IC = 6/.002 = 3000

    R2 = 1 (chosen voltage over emitter) / (Ib + Ic) = 1 / (.002 + .0004) = 416.7

    R4 = 1.7 (base voltage due to volt drop Vbe) / 10 x Ib = 1.7 / .004 = 425

    R3 = needed R3 Voltage drop / R4 current + Ib = 10.3 / 0.0044 = 2340

    summarised: R1 = 3K, R2 = 416, R3 = 2340, R4 = 425.


    The problem:

    When i simulate this using Multisim i experience absolutely no gain. I've attached a picture to show the circuit and how i have hooked up the oscilliscope.

    Can someone please help me with this? I am a beginner to electronics. In case i've confused you my goal is to achieve a gain of 5 while also biasing the transistor at 6v.

    Thank you.
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I stopped reading at this point. Hfe is the forward current gain of the transistor, and is typically 100-200 or more. The voltage gain you're designing for is 5, and that is determined by your design. Look up the forward gain for your transistor. Your equation should be something like:

    IB = IC/Hf = .002/100 = 20uA.

    There might be other problems, like I said I stopped reading...
     
  3. Dannyhimself

    Thread Starter New Member

    Jan 6, 2013
    5
    0
    Hi brownout,

    Yes, i've literally just been reading up on hFE and consequently realized what i've posted is gibberish. Like i say i'm still learning.


    The spec for the transistor in use has a Forward current transfer ratio (hFE), min-max: 200MIN. Should i be assuming a value of 200 for recalculations?

    Thanks for your reply. :)

    EDIT:

    I recalculated the resistor values use a hfe of 200:

    R1 = 3000
    R2 = 497
    R3 = 93636
    R4 = 17000
     
    Last edited: Jan 22, 2013
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    3 of your 4 resistor values are not sold anywhere.
    Notice that my schematic is not covered with chicken-pox dots like yours.
     
    Dannyhimself likes this.
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