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enter image description here

When I was trying to bias an NPN transistor I noticed that the base current Ib is negative in LTSpice. So for the sake of simplicity I only apply voltage to the base-emitter junction. As you see in the figure above LTSpice shows this current negative. Infact when you bring the cursor on R3 the arrow points +V2.

Isn't the direction of Ib is from +V2 to GND in an NPN transistor? Here is one example in tutorials: http://www.zen22142.zen.co.uk/Design/cct/sw1.gif

I'm confused..

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    \$\begingroup\$ What is the current direction when you place a probe? \$\endgroup\$
    – Eugene Sh.
    Dec 14, 2015 at 17:24
  • \$\begingroup\$ the current direction arrow points +V2 \$\endgroup\$
    – user16307
    Dec 14, 2015 at 17:26
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    \$\begingroup\$ Run the sim and place the probe close to the transistor's base and you'll see a clamp-on ammeter cursor. Left-click it and you'll see the current going into the base, not out of R3. Alternatively, hit ALT and at the same time move the probe to the wire between R3 and Q1 base. You'll see the clamp-on ammeter cursor; click it and you'll see the current into Q1 base. \$\endgroup\$
    – EM Fields
    Dec 14, 2015 at 19:54
  • \$\begingroup\$ @EMFields I think the trick is when you hold the ALT key it always shows the real current direction? Thanks \$\endgroup\$
    – user16307
    Dec 14, 2015 at 20:09
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    \$\begingroup\$ Yup. Another trick with ALT is that if you hold it and move the cursor over a component, you'll see a thermometer, and if you left-click it it'll you'll see the power that component is dissipating. \$\endgroup\$
    – EM Fields
    Dec 14, 2015 at 20:19

1 Answer 1

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You didn't measure the base current, you measured the current through R3.

Even though the symbol doesn't show it, LTSpice is keeping track of which pin is 1 and which is 2, and defining the sign of current with some fixed convention relative to those pin numbers. Either current is defined as going in to pin 1, or into pin 2, though I don't know which one.

To get the current through R3 to have the same sign as the current in to the BJT base, you just need to turn the resistor around 180 degrees.

Edit: User W5VO made some images showing how the cursor shows the reference direction when you hover over the resistor:

enter image description here

And how it changes after you rotate the resistor by 180 degrees:

enter image description here

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  • \$\begingroup\$ This saved my life! LTSPICE! If you are going to keep track of pins...at least show them near the symbol....lol \$\endgroup\$
    – Dumbo
    Aug 22, 2017 at 18:28

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