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As we know in JFET or MOSFET there is a triode or Linear region, Where for a fixed gate-source voltage, increasing drain source voltage is equivalent to applying drain source voltage across a resistor (approximately) . My question is whether there is a similar region for BJT in which the bjt would work as if its a resistor, though I know only about cut-off, active, and saturation region in BJT.

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    \$\begingroup\$ No a BJT does not have such a "linear" region. That's why in some circuits where an adjustable resistor component is needed, often a JFET is used. \$\endgroup\$ Mar 20, 2021 at 13:17
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    \$\begingroup\$ @Bimpelrekkie comment -> answer? \$\endgroup\$ Mar 20, 2021 at 13:18
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    \$\begingroup\$ The BJT saturation region has the same role, it's just not all that linear though. \$\endgroup\$
    – user16324
    Mar 20, 2021 at 13:57

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As @Brian says, in the saturation region this is true. The transistor has a dynamic resistance (for very small voltage changes) of approximately Vt/Ib.

So, for example, the collector of an NPN transistor with 50uA of base current will behave approximately like a 500 ohm resistor to ground for very small voltage changes (mV or less) that are AC-coupled. Remember that dynamic resistance is \$r_d=\Delta V/\Delta I\$.

schematic

simulate this circuit – Schematic created using CircuitLab

For larger voltage changes the nonlinearity will become very obvious.

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  • \$\begingroup\$ So you are assuming this is independent of hFE.? \$\endgroup\$ Mar 20, 2021 at 14:33
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 Yes, it is. For hFE >> 1 \$\endgroup\$ Mar 20, 2021 at 14:33
  • \$\begingroup\$ Over what range of Ib? \$\endgroup\$ Mar 20, 2021 at 14:34
  • \$\begingroup\$ Should be pretty broad so long as the transistor is well into saturation. Similar to the dynamic resistance of a diode. \$\endgroup\$ Mar 20, 2021 at 14:38
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    \$\begingroup\$ Yes saturation is the keyword here where hFE drops rapidly and becomes less sensitive using hFE/10 then Rce is the limiting factor as the 500 Ohms drops to Vce(sat) /Ic or 2 Ohms (bulk resistance)for a PN2222A which is a function of <1/Pmax rating and super beta types of the chip. (Stewart’s law) \$\endgroup\$ Mar 20, 2021 at 15:14
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No - the "quasilinear" region of the BJT cannot be used as a (grounded) resistor (as it is the case for the FET). Two main reasons:

  • The Ic=f(Vce) set of curves does NOT cross the origin

  • Even in the vicinity of the origin in the 3rd quadrant, these curves are NOT symmetrical to the "normal" first quadrant (for positive values of Ic and Vce).

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If the load voltage is fixed and there is an emitter resistor greater than Rbe to linearize the input impedance hFE*(Rbe+Re) then using a fixed input voltage and variable Rb will amplify conduction to appear as Rb/hFE as a common emitter amplifier.

This is one way to make an Active Load . The other way is to use a fixed Rb and vary the Vin to control load current as a fixed resistance Ic ~= Ib * hFE, where Ib = (Vin - 0.7(~) )/Rb. So the load resistance is proportion to Vin or Rin. Current feedback can make this even more linear.

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