LTspice and Darlington Pairs - Current values are not as expected.

Thread Starter

verdale01

Joined Feb 5, 2021
4
Hello all,

I'm attempting to simulate the behavior of a general Darlington pair using LTspice. Each time I run my simulation as a steady state circuit, it appears that current is flowing from ground through the emitter to the voltage source that is placed on the collector. I'm led to believe this due to positive voltage values from the simulation output.

There may be some applications were that is possible, however LTspice uses conventional current flows. That said, based on my circuit the described behavior should not be possible. I have had situations where accidentally rotated resistors will read current flow as a negative value due to orientation issues. Is this one of those situations or is am I missing something conceptually?

I am a curious novice with circuits so please take it easy on me. I will attach an screen shot containing my circuit and the simulation output. Note that I did not change any of the Darlington pairs' settings. It is setup as an NPN Bipolar Transistor. I actually do not know how to customize a transistor to reflect desired beta gains or values...(I'm not sure how there parameters are typically referenced).

Thank you.
 

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Papabravo

Joined Feb 24, 2006
17,240
Sorry, I don't know what you are simulating, but with the base current equal to the emitter current it looks like a diode. A Darlington transistor cannot be simulated with a basic Spice model. It has to be done with a subcircuit. Just making a symbol and calling it an NPN will not do the job.

EDIT: Look in the folder C:\Users\%Username%\Documents\LTspiceXVII\examples\Educational\PAsystem
Substitute you user foldername in place of %Username% in the string above.

This folder has symbols and subcircuits for the TIP121 and TIP127 devices, and a working example of a PA amplifier. That should keep you busy for a bit
 
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Thread Starter

verdale01

Joined Feb 5, 2021
4
I wanted to confirm that current would flow from collector to emitter only when the base has been provided with the correct amount of voltage. I will try to simulate them individually.

Thank you.
 

Papabravo

Joined Feb 24, 2006
17,240
Yes if you build a Darlington pair from individual transistors you can get some results. Using the default NPN model may or may not give you understandable results. The default values of each parameter are given in the LTspice help pages, but using the default model provides results of dubious quality. You can slightly modify the default values as follows:

Include the following .model statement as a spice directive on you schematic
.model NP NPN(BF=125 Cje=.5p Cjc=.5p Rb=500)

Then give each transistor the "model name" NP (instead of NPN). Sorta like this. Notice the separate base-emitter voltage drops on Q1 & Q2.

1625756499258.png
 

ericgibbs

Joined Jan 29, 2010
14,130
I have had situations where accidentally rotated resistors will read current flow as a negative value due to orientation issues. Is this one of those situations or is am I missing something conceptually?

hi 01,
The Net list determines the 'apparent' direction of current thru the resistor
eg R1 3 0 1E9 // the Label, Node 3, Node 0, Value.

E
 
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