# NPN transistor modeling

Discussion in 'General Electronics Chat' started by lorenzoalari, Jun 30, 2017.

1. ### lorenzoalari Thread Starter New Member

Jun 30, 2017
8
0
Hi everyone,
I am a mechanical engineering student and I need to develop an electric circuit. I designed the circuit with Pspice, but then I needed to implement it in Simulink. In the circuit, I have npn and pnp transistors (tip31c, tip32c) and in Simulink I cannot pick up the model of the transistor directly, but I need to specify some properties from the datasheet.
• the forward current transfer ratio h_fe: in a datasheet 3 different values of h_fe are specified, and I do not know the difference among them
• Ic and Vce at which h_fe is specified: I found them in the datasheet
• output admittance h_oe: I guess it is Ic/Vce with the values of point 2
• Voltage Vbe: I found it in the datasheet
• Current Ib for voltage Vbe: in the datasheet I found Ic for Vbe, thus I divided Ic by h_fe. Is it correct?
• Reverse current transfer ratio: no idea about what it is, set to 1
• Collector and Emitor resistances: I have no idea where to find them

2. ### freesoftDX New Member

Feb 6, 2017
4
0
it would help if we had a schematic. what is the circuit supposed to do? do U want small-signal parameters, or DC measurements (the two are quite different)? is it possible for U to get the needed parameters from Pspice?

3. ### lorenzoalari Thread Starter New Member

Jun 30, 2017
8
0
Hi, thank you for your answer. The circuit is shown in the attached picture. The input is a current source, sinusoidal in this case. Everytime the voltage has a minimum or a maximum, the circuit let the current flow through the inductor, so to reverse the voltage and increase the magnitude of the voltage on the load. I simulated it with PSpice and it works, but with PSpice I just chose from a catalogue the transistors I needed, I did not need to specify anything.
Honestly, I do not know the difference between DC parameter and small-signal parameters.
I tried to get those values from PSpice but honestly I did not find anything.

Lorenzo

Last edited: Jun 30, 2017
4. ### freesoftDX New Member

Feb 6, 2017
4
0
i'm guessing by the name of your drawing that U are studying piezoelectrical effects; piezoelectric crystals are used in electrical circuits that need humungous inductors (with L > 1 H, etc.) so as to get a very high Q for the circuit; hence the current source as a driver, and using the inductor to convert the input current signal to a voltage. the problem U are having is that the parameters U need aren't necessarily directly available on the datasheet, if at all; instead, they need to be calculated from curves. in the real world, I would suggest measuring the values yourself, but even that isn't always practical for a student who isn't well-versed in electronics.

since U are defining a custom device in Simulink, U might want to look at the corresponding function in Pspice. in other words, check Pspice to see if there is a way to define a custom device; it almost certainly does have such a function. with any luck, it will not only let U define a custom device, it will also let U start with an already-defined device (namely, a TIP32C or TIP31C) as a base from which U can work by modifying its characteristics. it has been over 20 years now, but i once worked with a circuit simulator that had such functionality; the problem i had with it was that i was not allowed to "copy" some devices because of software licensing issues (gotta love proprietary software!). if that isn't available, U might be stuck with googling for the parameters.

5. ### lorenzoalari Thread Starter New Member

Jun 30, 2017
8
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I tried to follow your advice but the new situation is like the one in the picture:

I would like to use my transistors to have the situation described by the dashed blue line, whereas with simulink the situation I got is the one in the black-yellow picture: the circuit opens at every maximum as I wish, but at some point the voltage stops dropping and starts increasing again. I really have no clue what the problem can be.

6. ### crutschow Expert

Mar 14, 2008
21,422
6,137
Post the circuit you are simulating.

7. ### lorenzoalari Thread Starter New Member

Jun 30, 2017
8
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This is the circuit. The simulation with PSpice work, but I am not able to get a proper result in Simulink.. I don't know whether it is because of the trainsistor or because of the diodes. Appereantly, since the voltage is reversed at every max/min the transistor work properly, but I am not sure

8. ### crutschow Expert

Mar 14, 2008
21,422
6,137
Since it's not apparent where those various waveforms in post #5 are from on the circuit it's difficult to say what the problem is.

What is the purpose of that circuit?

9. ### lorenzoalari Thread Starter New Member

Jun 30, 2017
8
0
The purpose of the circuit is to let the current flow on the inductance every time the voltage reaches a maximum, so to create an electrical oscillator through the inductance and invert the voltage value. The original voltage is sinusoidal; once it is inverted after each max/min the transistor open again, and the voltage increases/decreases in a sinusoidal trend again.

10. ### crutschow Expert

Mar 14, 2008
21,422
6,137
That's what the circuit does.
But what's the purpose of doing that?
Why do you want that odd current flow?

11. ### Bordodynov Well-Known Member

May 20, 2015
2,086
632
Hi.
It seems strange to me to include the diodes D7 and D8 (direction).

Jul 13, 2015
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