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I'm using a circuit simulation app called EveryCircuit, and for the npn transistor settings it has forward beta, reverse beta, base resistance, collector resistance, and emitter resistance. I'm a bit confused because I've only heard of alpha gain, beta gain, and voltage drop as parameters of a transistor. I'm assuming that forward and reverse beta are referring to the gain, but I'm not sure why they give settings for the resistance?

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  • \$\begingroup\$ There is Ohmic resistance between the leads, through to the wire bond and into the die. The silicon doping prior to the active regions adds more. Typically, the base has a somewhat higher resistance (about an order or two higher) than the other two leads. It's not a lot. Maybe \$10-20\:\Omega\$ for the base for a small signal device. The earliest Ebers-Moll model didn't include them. It wasn't until version 2 (EM2) that these parasitics were added. \$\endgroup\$ – jonk Sep 20 '19 at 14:43
  • \$\begingroup\$ so if I'm writing a schematic should I add these in as resistors? \$\endgroup\$ – buckithed Sep 20 '19 at 14:54
  • \$\begingroup\$ A spice program includes parameters for these values, so you do not need to add them. If you writing your own sim program and schematic editor and your transistor model is just a version 1 of the Ebers-Moll model, then you may want to make them explicit. The level 1 version of 3 equiv. Ebers-Moll models doesn't include them. It wasn't until the level 2 modifications that they were added. It may also help you to read the different regions (I, II, and III) of operation for a BJT here. \$\endgroup\$ – jonk Sep 20 '19 at 15:03
  • \$\begingroup\$ @jonk, your comments should be posted as an answer. \$\endgroup\$ – The Photon Sep 20 '19 at 15:29
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    \$\begingroup\$ @ThePhoton Then I'd have to actually work. (I'd need to draw out the behavioral diagram, the original level 1 model, the additions of the level 2 model [and probably what was added in the level 3 model to delineate the boundaries of the level 2 model], and some further explanatory text.... I don't have time for that right now.) Maybe later? \$\endgroup\$ – jonk Sep 20 '19 at 15:32
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Rbe and Rce are useful parameters that are an indication of power handling of the device. My Rule of thumb is Rce = 0.5 / Pmax rated when used as a switch but less relevant when used as a current sink/source. ( CE,CB). Forward beta is usually rated at some current unsaturated then as a switch Vce-sat is rated at Ic/Ib=10 to 50 for superBeta devices >500.

The Rce as Jonk indicated is due to bulk resistance and thus size of semiconductor and thus its thermal resistance and thus max power dissipation improved for this size so Rce is inverse with Pmax, just as it is with all diodes and LED’s with the same correlation of Rs=0.5/Pmax (+/- 50%).

Rce is low as the CB junction has less doping (than BE) and as Vce saturates CB begins to conduct as Vce approaches 0. Then Rce becomes a current controlled conductance which is useful for “linear active loads” or switched loads.

Emitter resistance is usually (Rbe + source Z) / hFE. I think. But will neglect the source impedance just for the transistor characteristic.

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  • \$\begingroup\$ If they are Rbe and Rce, then why does it say base, emitter, and collector resistances individually? \$\endgroup\$ – buckithed Sep 20 '19 at 15:01
  • \$\begingroup\$ Resistance is a 2 port parameter. Is there no reference given? \$\endgroup\$ – Tony Stewart EE75 Sep 20 '19 at 15:03
  • \$\begingroup\$ It seems they are using a non-std approach to make you translate datasheet specs into these values. The 1st example I chose used 1 Ohm for each b,e,c which seems rather useless or ideal. Reverse beta is usually 1. \$\endgroup\$ – Tony Stewart EE75 Sep 20 '19 at 15:15
  • \$\begingroup\$ A power transistor or diode rated for 10W will have a Rce or saturated resistance of 50 mOhm approx thus 10A causes a rise of 500mV for example above threshold at say 100mA \$\endgroup\$ – Tony Stewart EE75 Sep 20 '19 at 15:20

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