I have confusion about biasing circuit to FET (Field Effect Transistor) amplifier.

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Jan 31, 2016.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    Hello.

    Please see the following images.

    This is JFET amplifier.
    JFET amplifier.jpg

    And this is only biasing circuit to the amplifier.

    JFET amplifier biasing circuit.jpg

    Let's say V_DD is 12 V and R1 = R2 = 1 kΩ. Without V_IN, V_G is clearly 6 V, no doubt at all. good.

    However, when V_IN applies, how can I determine V_G? Some signal current from V_In should flows along the path from + GND through C1 and R2 so that current times R2 gives V_G. But...V_G is also determined as 6 V from voltage divider. This is contradiction and if V_G is fixed, whole circuit means nothing to do useful job.

    There is something I can't see right now. Could you please fix my vision to this circuit?
     
  2. David Knight

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    Aug 4, 2015
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  3. Jony130

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  4. Picbuster

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  5. crutschow

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    Mar 14, 2008
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    Assuming C1 is large enough to have an impedance much smaller than the parallel value of R1 and R2 at the input frequency, then Vg will be the DC bias (6V) plus and minus the input AC voltage.
     
  6. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    Hello.

    I think C1 can be fairly small as DC biasing current from VCC power source will not enter even with small C1, biasing voltage is still 6 V.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Adding to Crutschow's contribution, if C1 is small it will have a high AC impedance so the signal will have only a small effect on the gate voltage.
     
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  8. crutschow

    Expert

    Mar 14, 2008
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    Certainly the size of the capacitor has no effect on the DC bias.
    But the capacitor has to be large to pass the AC voltage without significant attenuation.
    The AC signal will be attenuated by -3dB when the capacitance reactance at the input frequency equals the parallel value of R1 and R2 and rolloff at -6dB/octave below that frequency.

    Since the purpose of this circuit is to amplify AC signals, you don't want to attenuate the AC input signal.
     
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  9. hp1729

    Well-Known Member

    Nov 23, 2015
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    Not homework, right?

    VGS of 6 Volts? Or <0.6 V including signal. Should you avoid forward biasing the gate? It will be conducting as VGS gets down to about -0.7 V. The more negative VGS the more the JFET turns off. Depletion mode device, right? As voltage is applied to the gate it turns off.
    6 V on the gate, yes, as long as > 6.7 V is dropped across the Source resistor.
    Jeeez, JFETs has been a long time ago for me. :)
    (edited to correct my math and thinking)

    So what kind of signal is being applied?
     
    Last edited: Jan 31, 2016
  10. bertus

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  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    JFET exercises
    One exercise is just the JFET with a Drain resistor to +5 V. V in is varied and the output voltage is noted.
    The other is similar to the circuit in this thread.
    Included is the results in an Excel file (pdf format).

    No, biasing does not have to be at 1/2 of VDD. Many examples are around with the high side resistor being a 220K and the low side being 22K. I have never done an exercise with this configuration.
     
    Last edited: Jan 31, 2016
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    So what happens if we change the voltage divider to 220K and 22K?

    Our input voltage range shifts up.

    Does this help the question about biasing point?
     
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