Transconductance in Transistors

Discussion in 'General Electronics Chat' started by lkgan, Jan 16, 2010.

  1. lkgan

    Thread Starter Member

    Dec 18, 2009
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    Hi everyone,

    Why bipolar transistors have fewer nonlinearity problems compared to JFET? It's a fact that JFET has a lower transconductance than a BJT for the same current. How would the transconductance, gm affects the stability of a transistor? And how would the temperature variations affect the transconductance?
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A transistor is almost always used with negative feedback to reduce its very high distortion. It has a high transconductance so reducing its gain with negative feedback still results in plenty of gain.

    A FET also causes high distortion but its transconductance is low so not much negative feedback can be used to reduce its distortion. So its distortion will be higher than a transistor.
     
  3. lkgan

    Thread Starter Member

    Dec 18, 2009
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    Do you know what's the relationship between temperature variations and transconductance in a transistor? In what way can I know whether a BJT is more stable compared to JFET?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    gm for BJT is equal:
    gm=1/re without external emitter degeneration resistor (Re=0).
    And with external emitter degeneration resistor gm=1/(re+Re)

    And re=Vt/Ic when
    Ic- collector current
    Vt- thermal voltage
    Vt=(K•T)/q
    K-Boltzmann constant k =1,3804•10^-23J/°K
    q - elementary charge q =1,6021•10^-19C
    T- absolute temperature in kelvin.
    So Vt=(K/q)•T=86.1619uV•T
    And for example now we can easy calculate Vt that for 25 ° C, ie T = 298.15 K
    Vt=86.17uV•298.15K=25.691mV but for practical calculation we assume Vt=26mV

    And gm for JFET for Rs=0Ω

    gm=\frac{2*I_{DSS}}{|V_{gs(off)}|}*(1-\frac{Vgs}{V_{gs(off)}})=\frac{2*I_{DSS}}{|V_{gs(off)}|}*\sqr{\frac{Id}{I_{DSS}}

    And with external Rs resistor
    gm_z = 1/( 1/gm+Rs )=gm/(1+gm•Rs)
     
    Last edited: Jan 17, 2010
  5. Audioguru

    New Member

    Dec 20, 2007
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    If you bias a Jfet at a certain current then its transconductance does not change when the temperature changes.
     
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